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We make them tougher in Texas

Katarina Anthony — Tue, 08 Sep 2026 09:17:45 +0000

We make them tougher in Texas

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Tim Andeen

Inner Tracker ITk

inner detector

The most extreme radiation environments on Earth aren't hidden in some classified military bunker—they're inside a 17-mile ring buried on the Franco-Swiss border. It's called the Large Hadron Collider, and it’s the world's most powerful atom smasher.

Right now, scientists and engineers are undertaking a massive upgrade that will dramatically boost the LHC's collision rate. This upgrade, called the High-Luminosity LHC, will give us a data “goldrush” and allow us to peer into the heart of matter more deeply than ever before.

But that new window into the universe comes at a price: higher luminosity means far more radiation. Some regions of the upgraded detectors will face conditions harsher than anywhere in our solar system outside the Sun itself. Even the "gentler" regions exceed what standard space-qualified hardware can handle.

That means we can't just order new electronics off the shelf. The technology literally doesn’t exist.

But my research group at the University of Texas at Austin runs on a simple philosophy: if you can't buy what you need, you build it yourself. And that’s exactly what we’re doing. Texas has one of the best ecosystems in the country for chip design and advanced manufacturing, which makes it the ideal place to push this technology to its absolute limit.

But building radiation-hard electronics isn't just about making them tougher—it's about making them smarter.

Need electronics that can survive the harshest radiation on Earth? Texas has got y’all covered.

Professor Tim Andeen (center), graduate student Natalie Gray (left) and research scientist Michael Himmelsbach (right) fine tune their robotic testing setup. This setup will receive 40,000 custom designed chips, which can withstand some of the most intense radiation environments in the solar system. (Image: Nolan Zunk / UT Austin)

Here's why: radiation doesn't just slowly degrade electronics. It can also flip a single bit, like a microscopic hacker quietly implanting a virus. Nothing visibly breaks, and the device keeps running. But the numbers it reports are wrong. For an experiment built on exquisitely precise measurements, that silent corruption is more dangerous than a chip that fails outright.

To meet that challenge (and many others), we built a collaboration between physicists and electrical engineers at UT Austin and Columbia University in New York. Our pitch to the engineers was simple: We need electronics that are fast, low-power, sensitive across a wide range of signals, radiation hard, and most importantly, high-fidelity. Basically, cutting-edge in every way. Our engineers’ response? Game on.

But to build some of the world’s toughest and smartest chips, we couldn’t just start with a standard design: we had to rethink everything from the ground up. We started with radiation-resistant components and clever circuit designs that minimize the damage caused by radiation and allow the electronics to keep working as they age. However, the fidelity requirement was trickier. How could we know for sure if a single bit had been corrupted? We landed on the solution of triple redundancy, which means every circuit is built three times over. That way, if radiation corrupts one circuit, the other two can fact check and outvote the error.

The design was completely new—so how could we know if it would work? One thing was certain: We needed physical prototypes. Unfortunately, prototypes are expensive and take roughly one year to build. Luckily, we have access to the Texas Advanced Computing Center, which is funded by the National Science Foundation, and were able to create, test, and refine digital mockups of the key parts of our chips before ever touching silicon. We had budgeted for five physical prototypes. Thanks to the computing resources at the Texas Advanced Computing Center, we managed to arrive at the final design in just four.

The toughest obstacle, though, turned out to be something we didn't expect: the global supply chain. Like most chips, ours are manufactured by TSMC in Taiwan, and supply disruptions there can mean months of delay. These delays ripple straight into our project timelines, which are already ambitious and cannot afford silly setbacks. That's why initiatives like the Texas Electronics Institute—a planned foundry right here in Austin—matter too. Domestic fabrication could cut our design-to-product lag from years to weeks, and dramatically lower costs.

Over the next four years, we will install these radiation-hard electronics directly into our detector. And when the HiLumi LHC launches in the 2030s, these chips will help us make major new discoveries about the subatomic world. But the payoff isn't just scientific. Industry has taken notice, and an aerospace company has already approached us about licensing our radiation-hard, high-fidelity chip technology.

What started as solving one of the hardest problems in modern physics instrumentation has turned into a budding Texas-driven industry that will not only help us understand the universe at a subatomic scale, but also strengthen the US domestic industrial base and power the next generation of space and aerospace innovation.

Published in collaboration with Beyond Standard. Banner illustration by Sandbox Studio, Chicago with Ana Kova.

ATLAS publishes first paper on Public Engagement with Science

Katarina Anthony — Wed, 02 Sep 2026 11:04:42 +0000

ATLAS publishes first paper on Public Engagement with Science

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Experiment Briefing

ATLAS Collaboration

outreach

You can’t exactly take the ATLAS experiment on tour. At 7,000 tonnes and buried 100 metres underground at CERN, it’s not easy to move. So, for more than a decade, the ATLAS Collaboration has found another way to bring the experiment to people around the world: live, interactive virtual visits with the scientists who work on it.

In a recent paper published in the European Physical Journal Plus, the ATLAS Collaboration documents the evolution and impact of its flagship Virtual Visits programme. The paper is the Collaboration’s first publication focused specifically on science education, communication and outreach. It traces the programme from its beginnings as an experimental outreach effort in 2010 through its growth into a global programme – connecting classrooms in the Amazon, schools in the Himalayas and even a research station in Antarctica. Drawing on data collected between 2019 and 2025, the paper examines the programme’s reach and impact.

Global reach, local connection

Figure 1: World map showing the number of virtual visits across participating countries from 2019 to 2025. Cooler colours (purple and blue) are used for countries with fewer visits, and warmer colours (red and orange) for countries with more visits. (Image: ATLAS Collaboration/CERN)

The core mission of the ATLAS Virtual Visit programme is to connect audiences worldwide with particle physicists and engage them in accessible discussions about cutting-edge research. While visiting CERN in person is an unforgettable experience, financial and logistical barriers can make such a trip difficult for many students and public groups. Video conferencing has allowed the Collaboration to break down these barriers: between 2019 and 2025, the Collaboration hosted 698 virtual visits, engaging tens of thousands of participants across nearly 70 countries (see Figure 1).

Interactivity is central to the programme. Far from simple one-way broadcasts, Virtual Visit audience members are encouraged to engage in conversation with the scientist(s) hosting the visit. Most hosts are volunteers from the ATLAS Collaboration, who bring their diverse scientific expertise, language skills and cultural backgrounds to each visit. Where possible, audiences are paired with volunteer scientists who share their native language and cultural background, making it easier to engage, ask questions and connect with the hosts. Between 2019 and 2025, Virtual Visits were held in 19 different languages.

The core mission of the ATLAS Virtual Visit programme is to connect audiences worldwide with particle physicists and engage them in accessible discussions about cutting-edge research.

Constant evolution

Figure 2: The number of ATLAS Virtual Visits organised annually since 2019. (Image: ATLAS Collaboration/CERN)

The ATLAS Virtual Visit programme has continuously evolved to enhance the visitor experience. Early sessions were hosted directly from the ATLAS Control Room before transitioning in 2014 to a dedicated space in the ATLAS Visitor Centre, equipped with modern videoconferencing technology. In 2019, the ATLAS Collaboration expanded the programme’s reach, introducing live Virtual Visits from the experimental cavern during year-end maintenance stops and long LHC shutdowns. These exclusive sessions take participants beyond the ATLAS visitor platform, offering real-time, up-close views of the towering detector.

When the COVID-19 pandemic disrupted in-person gatherings worldwide, demand for virtual engagement surged. The ATLAS Collaboration adapted rapidly by introducing Open Virtual Visits streamed via Zoom and YouTube, allowing individual science enthusiasts and families to join live guided tours. Figure 2 shows that the programme reached a peak of 155 visits in 2021 and has since maintained around 100 visits per year, demonstrating sustained engagement beyond the pandemic-driven increase.

More than just a tour

Beyond participation numbers, the paper highlights the positive impact on participants and educators. Teachers report successfully integrating Virtual Visits into their physics curricula, sparking sustained curiosity and in many cases inspiring students to pursue STEM studies and careers. Feedback received also highlighted how the direct dialogue with ATLAS scientists has given students an authentic and personal perspective on particle physics, helping them connect what they learn in the classroom with the people behind ATLAS research.

Looking ahead, the programme will continue to evolve with new technologies and communication platforms. The goal is simple: to give even more audiences the opportunity to engage directly with particle physics and make fundamental science open to all.

Learn more

Breaking barriers: the impact of ATLAS Virtual Visits in science communication (Eur. Phys. J. Plus 141, 874 (2026), arXiv:2601.22457, see figures)
ATLAS Virtual Visit Dashboard
Register your group for an ATLAS Virtual Visit or join the upcoming open Virtual Visit (for individuals, to be held 24 September 2026).

Your phone is smarter than the world’s biggest particle detectors (but not for long)

Katarina Anthony — Tue, 01 Sep 2026 10:25:26 +0000

Your phone is smarter than the world’s biggest particle detectors (but not for long)

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David Miller

trigger

Einstein probably wasn’t thinking about helping you find Starbucks when he wrote down his theories of relativity, but Google Maps wouldn’t work without them. In fact, these fundamental laws of nature—special relativity, general relativity, and quantum mechanics—are the foundation of all modern technology.

As a physicist, I’ll let you in on a little secret: Even though we use these laws almost every day, we don’t understand them very well. For instance, we can use models of gravity to send astronauts to the moon, yet have no clue how gravity works at smaller scales.

That tension between everyday use and deep ignorance is why we built the Large Hadron Collider. We study how matter behaves at the smallest scales by colliding protons at close to the speed of light and then capture the remnants with particle detectors the size of cathedrals. And then we do something astonishing: We immediately throw away 99.999% of the data. Why? Because most of the data is “boring,” and we don’t have nearly enough computing resources to keep it all.

To understand this concept, imagine that the LHC is a soccer game, and our detector is a sports photographer. Most of the gameplay is totally uninteresting, but even so, our “photographer” keeps clicking away hoping to get that million-dollar shot. Even if our photographer does catch something amazing, there is always the risk that her photo is overexposed, blurry, or poorly framed. We struggle with these same kinds of problems (just the particle detector version.)

But there’s a big difference between a photographer and our detectors: A human photographer can learn from her bad photos and adjust in real time. Our detectors cannot: they just keep on collecting data, and we have to wait months—sometimes years—before we can evaluate if the saved data is truly as “good” as it could be. (Or worse: realizing that the data we thought was “boring” might have held some valuable scientific information.)

I don’t know about you, but I really don’t like waiting and hoping for the best.

Smart detectors will let us learn from “boring” data and could dramatically reduce the time between groundbreaking discoveries.

Scientists performing an alignment check inside the new ATLAS inner tracker. (Image: A. Barr/ATLAS Collaboration)

The solution I and others are pursuing is to build intelligence directly into the instrumentation: what we call smart detectors. As we take data, our smart detector will automatically notice when conditions change: when noise increases, when a region of the detector starts misbehaving, when the proton beam drifts. And in response it should be able to adjust voltages, refine calibrations, or re-balance which kinds of events it saves.

This is far from a novel idea: the camera on my phone automatically adjusts to ensure that I’m always getting the best possible pics. So why can’t our particle detectors?

This brings us to the crux of the problem: We are not just toggling the exposure on a camera up and down. We are talking about letting algorithms influence voltages and calibrations on delicate, expensive hardware, and shape which data we keep and which we discard. We worry that a runaway feedback loop or a misjudged pattern could cause us to miss a Nobel Prize–worthy discovery or even damage parts of the detector.

That is why we are proceeding cautiously. We are adapting commercially available reinforcement learning methods and testing them inside digital twins of our experiments. From this, we can compare the smart detector performance with our more traditional methods for data collection and evaluate the risks versus the benefits. Our near-term goal is to build a physical prototype to see if this can be done safely and effectively before anything is deployed more broadly.

If we succeed, the benefits are not abstract. It would shorten the time between taking data and understanding what that data really means. It would free students and researchers from spending their nights hand-tuning voltages and chasing down noise, so they can focus on the physics itself. Most importantly, it could compress the timelines of discovery. In the past, it took decades for ideas like relativity or antimatter to turn into practical tools like GPS and PET scans. By building real-time learning directly into our instruments, we may be able to accelerate the time between paradigm shifting discoveries from a human lifetime to a single PhD thesis.

Published in collaboration with Beyond Standard. Banner illustration by Sandbox Studio, Chicago with Corinne Mucha.

More than physics: transferable skills in ATLAS

Katarina Anthony — Wed, 19 Aug 2026 15:17:04 +0000

More than physics: transferable skills in ATLAS

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Harriet Watson

ATLAS Week

There is a particular energy to an ATLAS Collaboration Week, and this June it came to Glasgow. Early-career scientists are the engine room of ATLAS: the students and postdocs who build the software, run the shifts, wrestle the data and, more often than not, drive the analyses that make headlines. As a member of the ATLAS Early Career Scientist Board, I was keen to make sure this week reflected that. Alongside the usual meetings and results, we worked to boost early-career involvement through a high-capacity poster session (this time with a whisky tasting, which did wonders for the atmosphere), a hands-on grant-writing workshop, and a careers-focused panel Q&A.

I had the pleasure of chairing one of the ECS focused sessions: a panel discussion on the transferable skills we build while working on ATLAS. Five panellists*, a room full of early-career researchers, and a conversation that ranged well beyond particle physics into persuasion, conflict management and, at one point, the value of amateur dramatics.

Chairing it gave me a front-row seat to some genuinely useful advice, the kind that changes how you see the work you already do every day. Here are the points that stood out.

The skills you don't realise you're learning

I opened with a deceptively simple question: which skill has had the biggest impact on your career so far? Tellingly, almost nobody named a technical one.

For Sarah Boutle, who now works in government, it was persuasion, meaning the ability to build an argument and defend it under scrutiny. Coming from a field where every result is picked apart by hundreds of colleagues, she said, you learn to anticipate the difficult questions before they are even asked. Craig Sawyer picked up the thread with listening: on an experiment spanning dozens of countries and working styles, the real skill is gathering all those perspectives and stitching them into something coherent.

Anna Sfyrla argued that trying to single out one "hard skill" misses the picture entirely. It is the sheer number of skills we juggle, from technical operations to data analysis to interviews, and our ability to combine them clearly, that sets us apart. Sara Alderweireldt added that much of our work happens at the edge of what has been done before: not making things simply run, but finding the bottlenecks and thinking our way around them. Flavia de Almeida Dias offered the most candid observation of the session, which was that careers rarely follow a plan. There is a lot of luck, and a lot of not quite knowing what you are doing.

I opened with a deceptively simple question: which skill has had the biggest impact on your career so far? Tellingly, almost nobody named a technical one.

Our panellists take questions from early-career researchers during the transferable skills panel in Glasgow. (Image: Alice Reed/ATLAS Collaboration)

The skills we wish we'd learned sooner

When the discussion turned to what the panellists wished they'd developed earlier, a clear pattern emerged, and it wasn't about physics.

Time management and prioritisation came up repeatedly. Sarah, once a physicist now senior civil servant for the UK government, said learning to prioritise ruthlessly would have saved her years. Flavia agreed: with so many interesting things competing for your attention, focusing on the one that matters is a discipline in itself. Sara framed it gently, noting that you don't have to do everything at once, and you'll often get further by doing fewer things really well.

Anna wished she'd learned conflict management sooner: how to work with colleagues when you disagree, so that different opinions lead to better outcomes rather than a fight. And Craig made a plea many PhD students would recognise, which was that we should build these soft skills alongside our technical training, rather than being handed a coordination role and left to figure it out.

Selling ATLAS to the outside world

This was the part the audience had really come for, and the advice was practical and, at times, refreshingly blunt.

Yes, "CERN" is a brilliant brand, and Sarah joked that nobody ever doubts you're smart. But that only gets you so far. The trick, she said, is to flip your CV upside down. The specific analysis you're proud of means little to an outside employer; what impresses them is the scale of what you've been part of. How often does someone apply for a job having worked in an organisation of six thousand people, across every timezone, with no shared boss, and still deliver? That is project management, software engineering and collaboration on a scale most industries never see.

Two qualities came up as especially prized: resilience and comfort with uncertainty. Employers actively want people who can cope when they don't know whether the result will come or the paper will be published. We live with that uncertainty every day, and it turns out to be genuinely desirable.

The panel also acknowledged a common challenge: we often struggle to translate terms such as "convener" or "trigger operations" into language a recruiter understands. Encouragingly, work is underway on a "skills dictionary" and supporting documents that translate ATLAS roles into transferable skills and responsibilities recognised beyond the collaboration.

Failure, and unexpected training

One of the more valuable exchanges was about things going wrong. Flavia put it plainly: you learn far more when things don't go well. Anna reminded us that the path to a published analysis is essentially one problem after another, and the success is just the visible bit at the end.

And the "training” the panellists had found most unexpectedly useful? Rarely anything on a physics syllabus. Sarah credited drama and public-speaking training for teaching her how to hold a room, and recommended simply asking colleagues, "how do you think that went?". Anna found that training as a CERN guide transformed her ability to give talks. Flavia highlighted how much of our work is actually writing (think theses, grants, reports), and how vital it is to explain your excitement to someone who isn't a physicist. She talks to her mum about her work; if you can get her excited, you can get anyone excited.

The takeaway

I came away from chairing the session with a clearer sense of something many early-career researchers underestimate: our skills are broad, resilient and genuinely valued, and we just need to learn to describe them. As Sara pointed out, moving into industry isn't so different from moving between experiments - the language changes but the skills underneath travel with you.

Whether you stay in physics or head somewhere new, the message from Glasgow was clear. Learn to listen, learn to prioritise, get comfortable with uncertainty, and don't underestimate what working on ATLAS has already taught you.

With thanks to the five panellists, and to everyone who joined us in Glasgow, and Katie Walkingshaw-Pass for help editing this blog.

*Sarah Boutle, Craig Sawyer, Anna Sfyrla, Sara Alderweireldt, Flavia de Almeida Dias

ATLAS breaks its own record with new search for double-Higgs production

Katarina Anthony — Wed, 05 Aug 2026 06:28:00 +0000

ATLAS breaks its own record with new search for double-Higgs production

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Producing two Higgs bosons in a single proton-proton collision is among the rarest processes at the LHC. Yet these elusive events hold the key to one of the biggest unanswered questions: how does the Higgs boson interact with itself? The answer will help reveal the shape of the Higgs potential, deepening our understanding of electroweak symmetry breaking and the evolution of the early Universe.

To hunt for these double-Higgs events, the ATLAS Collaboration has targeted the distinctive HH→bb̄τ⁺τ⁻ decay channel, in which one Higgs boson decays into two bottom quarks and the other into two tau leptons. Their latest search, presented at the 2026 International Conference on High-Energy Physics (ICHEP 2026), is the most sensitive ATLAS analysis of this channel to date. It uses 196 fb⁻¹ of proton–proton collision data recorded between 2015 and 2023, combining the full LHC Run-2 dataset (140 fb⁻¹ at 13 TeV) with 56 fb⁻¹ of Run-3 data collected at 13.6 TeV.

The bb̄τ⁺τ⁻decay mode is one of the most sensitive ways to study double-Higgs production. It benefits from both the Higgs boson's dominant decay to bottom quarks – accounting for over half of all Higgs decays – and the distinctive experimental signatures provided by tau leptons. In their new analysis, ATLAS researchers considered two complementary tau signatures: events in which both tau leptons decay into hadrons (τ_hadτ_had), and those in which one tau decays into hadrons while the other decays into an electron or a muon (τ_hadτ_lep).

Separating these exceptionally rare events from the background is a significant challenge. Tau lepton decays always produce neutrinos, which escape ATLAS undetected and make the events more difficult to reconstruct. To overcome this, the ATLAS team developed a new transformer-based machine-learning classifier, and took advantage of improved bottom-quark jet identification and additional trigger chains introduced during Run 3.

Figure 1: Fitted values of the (HH) signal strength (μ_HH) for the individual analysis channels and their combination (left), together with the discovery significance on HH signal (right). (Image: ATLAS Collaboration/CERN)

The new result demonstrates the experiment's growing sensitivity of the channel to double Higgs-boson production, bringing physicists one step closer to observing this rare process.

Figure 2: Combined event yields for all channels and run periods plotted against the predicted signal-to-background ratio, derived from the machine-learning discriminant score. The top panel compares data (black dots) to the best-fit signal and background models. The lower panel shows how much the data deviates from the fitted background (the statistical significance of the difference between two), with statistical uncertainties only. The solid line represents the expected difference using the best-fit signal model, while the dashed line shows the Standard Model prediction.

As shown in Figure 1, the double-Higgs production rate was found to be 2.6 ± 1.4 times the Standard-Model prediction, corresponding to a signal significance of 2.6 standard deviations relative to the background-only prediction. This measurement lies about 1.65 standard deviations above the Standard-Model expectation – for which a signal significance of 1.2 standard deviations is expected – and is therefore compatible with it. Compared with the previous ATLAS search for this decay mode, the expected sensitivity improves by about 60%. Figure 2 compares the data to the best-fit signal and background models, as a function of the signal-to-background ratio derived from the machine-learning discriminant. As shown in Figure 3, researchers also placed tight bounds on two critical parameters: the Higgs self-coupling multiplier, κ_λ, which measures how strongly a Higgs boson interacts with other Higgs bosons, and κ_2V, the coupling modifier governing the interaction between two Higgs bosons and two vector bosons.

In a novel extension to this analysis, the ATLAS Collaboration also searched for two Z bosons (ZZ) and a Z boson with a Higgs boson (ZH) decaying into bb̄τ⁺τ⁻. Using the same analysis strategy and swapping machine-learning models to hunt for ZZ and ZH, researchers found the first evidence from ATLAS for ZH production in the bb̄τ⁺τ⁻ final state, with a significance of 3.5 standard deviations. The ZZ and ZH production rates were found to be compatible with those predicted by the Standard Model, bolstering confidence in the analysis strategy used for the double-Higgs search.

This new HH→bb̄τ⁺τ⁻ result demonstrates the growing sensitivity of the channel to double Higgs-boson production, bringing physicists one step closer to observing this rare process. It joins the recent search for HH→bb̄γγ, which has a similar expected sensitivity. Using the remaining Run 3 data and future datasets from the High-Luminosity LHC, ATLAS will continue to sharpen its study of the Higgs self-interaction and illuminate the origin of electroweak symmetry breaking.

Figure 3: Constraints on the Higgs-boson self-coupling modifier (κ_λ) (left), and the interaction-strength modifier between two Higgs bosons and two vector bosons (κ_2V) (right). The observed results are shown in orange and the Standard Model expectations in blue. (Image: ATLAS Collaboration/CERN)

About the banner image: Display of an HH→bb̄τ⁺τ⁻ candidate event. The two b-tagged jets are highlighted by green cones and the two hadronically decaying tau lepton candidates arr highlighted by purple cones. The missing transverse momentum is shown by a dashed white line. Yellow lines indicate tracks of charged particles in the inner detector. Green and yellow boxes indicate energy deposits in the electromagnetic and hadronic calorimeters respectively.

Learn more

Improved analysis of non-resonant Higgs boson pair production in the bb̄τ+τ− final state with 196 fb−1 of data collected at 13 TeV and 13.6 TeV with the ATLAS detector (arXiv:2607.26879, see figures)
ICHEP2026 presentation by G. D'Anniballe: Study of HH→bb̄τ+τ− Process in ATLAS
Combination of searches for Higgs boson pair production in pp collisions at 13 TeV with the ATLAS detector (Phys. Rev. Lett. 133 (2024) 101801, arXiv:2406.09971, see figures)
Combination of ATLAS and CMS searches for Higgs boson pair production at 13 TeV (arXiv:2602.23991, see figures)
Transforming jet flavour tagging at ATLAS (Nature Commun. 17 (2026) 541, arXiv:2505.19689, see figures)
Two Higgs bosons are better than one, Physics Briefing, July 2021

Polarised bosons: a window into the Higgs mechanism

Katarina Anthony — Tue, 04 Aug 2026 08:46:39 +0000

Polarised bosons: a window into the Higgs mechanism

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While the Higgs mechanism is best known for breaking electroweak symmetry and giving fundamental particles mass, it can also leave a distinctive imprint on the polarisation of certain particles. A massless particle such as the photon only has two transverse polarisation states. By contrast, the massive W and Z bosons – the carriers of the weak force – have an additional longitudinal polarisation state, in which their spin is oriented perpendicular to their direction of motion. The existence of this extra state is a direct consequence of the Higgs mechanism. Measuring how often W and Z bosons are produced in different polarisation states thus offers a unique window into this fundamental process.

Polarisation studies of W and Z bosons began at the Large Electron–Positron (LEP) collider experiments, the predecessor of the Large Hadron Collider (LHC). Taking full advantage of the LHC dataset and novel analysis techniques, the ATLAS Collaboration pioneered measurements of polarised boson scattering and observed the joint polarisation of W and Z bosons. Now, with yet more data and refined methods, physicists are tackling even rarer processes. Such measurements would either provide support for the Higgs mechanism or hint new physics beyond the Standard Model.

In new results presented at a CERN seminar, the ATLAS Collaboration reported the first observation of ZZ events containing two longitudinally polarised Z bosons, together with evidence for longitudinally polarised Z bosons in electroweak WZ production accompanied by two particle jets – a process that is particularly sensitive to boson scattering.

Observing two longitudinally polarised Z bosons

Figure 1: The observed profile likelihood ratio as a function of the signal strength, which is defined as the measured production rate of two longitudinally polarised Z bosons divided by the theoretically predicted rate. The dashed horizontal lines indicate the thresholds corresponding to the 3 sigma and 5 sigma intervals. (Image: ATLAS Collaboration/CERN)

Among the rare diboson processes, the simultaneous production of two longitudinally polarised Z bosons is one of the rarest. It is also one of the cleanest to study experimentally. Z bosons can decay into pairs of charged leptons (electrons or muons), whose decay kinematics can be fully reconstructed in the ATLAS experiment. The ATLAS Collaboration analysed 164 fb⁻¹ of proton–proton collision data collected at a centre-of-mass energy of 13.6 TeV between 2022 and 2024, selecting events where the Z bosons decay into two pairs of oppositely charged electrons or muons.

The angular distributions of these leptons retain information about the polarisation of their parent bosons. To maximise sensitivity, researchers trained a multivariate classifier using these angular observables to distinguish events containing two longitudinally polarised Z bosons from those with other polarisation combinations. They then combined the Run-3 analysis with the corresponding Run-2 measurement to achieve the first observation of the simultaneous production of two longitudinally polarised Z bosons, with an observed (expected) significance of 6.5 (5.6) standard deviations (Figure 1).

The team also measured the Z boson polarisation fractions both for the full dataset and in different ranges of the four-lepton invariant mass. Since the Standard Model predicts such events become increasingly rare at higher energies, these measurements provide an important test of the theory. The measured results agree well with Standard-Model predictions.

Taking full advantage of the LHC dataset and novel analysis techniques, the ATLAS Collaboration are tackling extremely rare measurements of polarised bosons. Such studies would either provide support for the Higgs mechanism or hint new physics beyond the Standard Model.

Longitudinal polarisation in boson scattering

Figure 2: Measured polarised fractions of the W and Z bosons in electroweak WZjj production (WZjj-EW). The cross section for the WZjj-EW process (black line) is compared to the most detailed theoretical predictions (coloured dots). The dashed black line represents the uncertainty in the measurement. The fractions “f00” represent the case where both bosons are longitudinally polarised, “f0X” and “fX0” where either the W or the Z is longitudinally polarised, regardless of the polarisation of the Z or the W bosons, respectively. The ratio “μ_WZjj-EW” between the measured and the predicted production cross section of WZjj-EW is shown. (Image: ATLAS Collaboration/CERN)

Longitudinally polarised bosons also offer a powerful way to test the Standard Model. When two such bosons scatter, they can exchange a Higgs boson. Without this contribution – or if the Higgs boson behaved differently from Standard Model predictions – the scattering rate would grow uncontrollably with energy, leading to a “resonance catastrophe”. Measuring the fraction of longitudinally polarised bosons provides a sensitive test of this mechanism.

For the first time, the ATLAS Collaboration measured the boson polarisation fractions in electroweak production of a W and Z boson in association with two jets (electroweak WZjj production). This rare Standard Model process contains events where two quarks radiate weak bosons that subsequently scatter from one another. The analysis uses the full Run-2 dataset (recorded between 2015 and 2018) and Run-3 data collected from 2022 to 2024. As in the ZZ analysis, only decays of the W and Z bosons into electrons and muons were considered.

This measurement was particularly challenging due to significant backgrounds from other processes, especially WZjj production via the strong interaction. In the signal region, only about 13% of selected events are expected to originate from electroweak WZjj production. Out of the total 1860 predicted events containing signal and background events, approximately 65 are expected to be electroweak WZjj events, which contain at least one longitudinally polarised W or Z boson, and just 20 are expected to contain two. Researchers employed advanced machine-learning techniques to isolate the tiny signal and identify the different boson polarisation states. A statistical fit extracted the overall electroweak WZjj production rate and the fractions of events containing one or two longitudinally polarised bosons. The measurements were compared with state-of-the-art theoretical predictions developed in close collaboration with theorists.

The ATLAS Collaboration found evidence for longitudinally polarised Z bosons with an observed (expected) significance of 4.0 (2.9) standard deviations, corresponding to an observed (expected) longitudinal Z polarisation fraction of 32% ± 9% (27.45% ± 0.08%). The measured electroweak WZjj cross section agrees with predictions. No evidence was found for longitudinally polarised W bosons, regardless of the Z boson polarisation. The team set upper limits on these fractions. All measurements remain consistent with Standard-Model predictions within 2.4 standard deviations (see Figure 2).

With the much larger datasets expected from the High-Luminosity LHC, physicists will be able to measure these rare polarisation states with new precision – providing increasingly sensitive tests of the Higgs mechanism and new opportunities to uncover physics beyond the Standard Model.

About the banner image: Candidate event display of the pair production of Z bosons, with one Z boson decaying into an oppositely-charged electron pair and the other decaying into an oppositely-charged muon pair. Tracks of charged particles in the inner detector are shown as orange lines. Electron tracks are shown as green lines, with their deposited energy in the calorimeter represented as green rectangles. Muons are shown as red lines, with their “hit” information in the muon spectrometer represented by blue boxes around the muon track. Finally, energy deposits in the liquid-argon and tile calorimeter are shown as green/teal and yellow/orange boxes, respectively. (Image: ATLAS Collaboration/CERN)

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Observation of pair production of longitudinally polarized Z bosons in four-lepton final state with the ATLAS detector (arXiv:2607.12879, see figures)
Evidence for longitudinally polarised Z bosons in electroweak WZ production in association with two jets from pp collisions at 13 TeV and 13.6 TeV with the ATLAS detector (STDM-2024-03)
CERN Seminar presentation by L. Xu: No Time to Be Transverse: Probing Electroweak Symmetry Breaking with Longitudinal Weak Bosons at ATLAS

ATLAS deepens the search for long-lived particles with Run 3 data

Katarina Anthony — Tue, 04 Aug 2026 05:43:53 +0000

ATLAS deepens the search for long-lived particles with Run 3 data

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Long-lived particles (LLPs) are among the most compelling targets in the search for physics beyond the Standard Model. Unlike most particles produced in proton–proton collisions at the Large Hadron Collider (LHC), which decay almost instantaneously, LLPs can travel a measurable distance before decaying. This delay can leave a striking signature in the ATLAS experiment: “displaced” tracks away from the interaction point (see event display). While the Standard Model contains a few LLPs – bb-hadrons, for example, travel measurable distances before decaying and muons travel through the entire ATLAS detector before decaying – many theories extending the Standard Model predict additional LLPs. These new particles could help explain outstanding mysteries in physics, such as the nature of dark matter.

Figure 1: Distribution of the predicted background yields and observed data in the far signal region (i.e. where the displaced vertex is more than 4 mm from the beam line) as a function of the reduced mass of the displaced vertex. The vertex’s mass is scaled by how tightly clustered its associated tracks are (see publication for definition). The data points (black) are consistent with the background prediction. An example benchmark new-physics scenario – supersymmetric Higgs-boson (“Higgsino”) pair production whose decay produces the displaced vertex and displaced muon signature this search targets – is overlaid for illustration. (Image: ATLAS Collaboration/CERN)

To search for these hypothetical particles, researchers typically look for the experimental signatures left behind when an LLP decays, which often occur at some distance from the interaction point. When several displaced tracks originate from the same location, they can be reconstructed as a displaced vertex. In addition, some LLP models predict decays that produce displaced muons. The ATLAS Collaboration has released a new search for massive, long-lived particles at a collision energy of 13.6 TeV, targeting events containing at least one displaced vertex and one displaced muon. This is the first ATLAS search for LLPs to use LHC Run-3 data collected between 2022 and 2024, corresponding to an integrated luminosity of 164 fb^-1.

The analysis benefits from several important improvements in tracking, vertex reconstruction and real-time event-selection tools (“triggers”). In 2022, the ATLAS Collaboration introduced a dedicated trigger for displaced muons, capable of identifying muons with transverse momentum as low as 20 GeV, thus extending the experiment's sensitivity to LLPs with masses at the electroweak scale. Improvements in displaced tracking and vertex reconstruction further enhanced sensitivity to LLPs with mean decay lengths ranging from one millimetre – comparable to the typical decay length of a b-hadron – to tens of centimetres, covering a wide range of possible new-physics scenarios.

Leveraging Run-3 ATLAS detector capabilities, the ATLAS Collaboration has set competitive model-independent limits on events with a displaced vertex and displaced muon, and world-leading limits on multiple benchmark models of R-parity-violating supersymmetry.

Figure 2: Observed and expected limits at 95% confidence level for supersymmetry top-quark (“stop”) pair production, whose decay yields a displaced vertex and displaced muon signature, as a function of stop mass and proper lifetime. Previous limits from ATLAS and CMS are shown for comparison. (Image: ATLAS Collaboration/CERN)

To estimate the number of background events that could mimic the LLP signal, researchers developed a fully data-driven approach that accounted for unconventional sources, including cosmic rays, fake tracks (i.e. tracks reconstructed from unrelated hits that do not correspond to a real particle) and decays of b-hadrons. Their method used discriminating variables such as muon displacement and displaced-vertex mass to distinguish these backgrounds from the signal directly in collision data, as simulations do not accurately model these processes.

No significant excess above the expected background was observed (see Figure 1) and the results were interpreted in two complementary ways. In a model-independent approach, researchers set limits as low as 0.018 fb on the visible cross-section of events containing at least one displaced vertex and one displaced muon. In a model-dependent approach, the team set world-leading limits on several benchmark models of R-parity-violating supersymmetry (see Figure 2), improving previous limits on the production cross section by up to two orders of magnitude for some benchmark scenarios.

Future analyses will continue to explore displaced signatures using the full Run 2 and Run 3 datasets, further developing these techniques to enhance sensitivity to LLPs at the High-Luminosity LHC and beyond.

About the banner image: Event display of a candidate signal event featuring a displaced vertex (blue circle) reconstructed from four displaced tracks, located 47 mm from the primary vertex (pink circle) in the transverse plane and with an invariant mass of 32 GeV. The event also contains a displaced muon candidate (red line on the left, orange line on the right) with a transverse impact parameter (the shortest distance in the transverse plane between the particle's track and the primary interaction vertex) of approximately 3 mm. Primary tracks originating from the collision point are shown in yellow, while displaced tracks with large impact parameters are shown in cyan – all with a transverse momentum of at least 2 GeV. (Image: ATLAS Collaboration/CERN)

Learn more

Search for massive, long-lived particles with displaced vertices and displaced muons in proton-proton collisions at 13.6 TeV with the ATLAS experiment (Phys. Lett B 878 (2026) 140509, arXiv:2603.01991, see figures)
ICHEP 2026 presentation by Cristiano Sebastiani: ATLAS searches with unconventional signatures and analysis workflows
LLP 2026 presentation by Laura Bruce: Search for massive, long-lived particles with displaced vertices and displaced muons using ATLAS 2022-2024 Data
La Thuile 2026 presentation by Emily Duden: Search for massive, long-lived particles in events with displaced vertices and displaced muons at 13.6 TeV with the ATLAS experiment

DeParTing from tradition: AI sharpens quark vs gluon tagging

Katarina Anthony — Thu, 30 Jul 2026 10:21:19 +0000

DeParTing from tradition: AI sharpens quark vs gluon tagging

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Quarks and gluons are the most common particles produced in the ATLAS experiment – and some of the hardest to tell apart. When produced in collisions at the Large Hadron Collider (LHC), they instantly dress themselves in a spray of hadrons known as a jet. At first glance, these jets can seem nearly identical. Look closer, however, and small but persistent differences emerge.

Gluon jets tend to be wider, busier and more uniform in how they share their energy among their constituents. Quark jets, by contrast, are typically narrower and more focused. Those small differences have a big impact. Distinguishing between quark and gluon jets is essential for many ATLAS measurements, from studies of the Higgs boson, notably produced via vector-boson fusion with associated quark jets, to searches for new particles that preferentially decay into quarks, as well as precision measurements that rely on accurate jet-energy calibration.

The ATLAS Collaboration has now introduced DeParT, a new quark/gluon discriminator (or “tagger”) built on the same transformer architecture that powers today's most familiar AI tools. Previous ATLAS quark/gluon taggers reduced each jet to a handful of summary variables, like its width or charged-particle multiplicity, before inputting them into a neural network for classification. DeParT skips that step and examines the jet directly. Using information about the jet's constituent particles – including momenta, angular position and and their relationship with each other – it decides for itself which features most effectively distinguish quark jets from gluon jets.

DeParT was trained on more than 100 million simulated jets and achieves sensitivity across an unusually wide phase space. It can identify differences between quark and gluon jets with transverse momenta as low as 20 GeV, where the two types of jets look most alike because they carry few constituent particles. It is also able to identify jets that fly out almost parallel to the LHC beam pipe, where detector information is sparse.

DeParT joins a wave of transformer-based tools now reshaping how the ATLAS Collaboration reconstructs and identifies the signals recorded by the experiment.

Knowing how well DeParT performs in simulation is one thing; knowing how well it performs on real data is another. Measured jets, of course, don't come stamped with "quark" or "gluon". To get around this, ATLAS researchers harnessed a feature of Standard Model multijet production: up and down quarks dominate at high momenta. As a result, jets emerging closer to the beam pipe (with higher pseudorapidity) are statistically more quark-like, while central jets contain a larger fraction of gluon jets. Treating these samples as different mixtures of quark and gluon jets allowed researchers to "unmix" them, extracting the underlying tagger-score distributions directly from data (Figure 1).

Figure 1: DeParT score distributions for quark and gluon jets in simulation, compared with Run 2 data, for jets with 800 < p_T < 1100 GeV in the central detector region (|η| < 1.2). Quark jets pile up near a score of 1, gluons near 0 — the visual signature of a tagger that has learned its job. (Image: ATLAS Collaboration/CERN)

Figure 2: Comparison of data-to-simulation scale factors for central quark jets extracted with the new jet topics method (green) and the established matrix method (pink). The lower panel shows the relative total uncertainty for each, illustrating the precision gain from jet topics in the 300–1000 GeV range. (Image: ATLAS Collaboration/CERN)

Two demixing methods were used: the established matrix method, which relies on simulation to set the quark and gluon fractions in each sample; and the new jet topics method, a technique originally proposed for collider physics in 2018 and introduced here for the first time at ATLAS. Similar to how the “topics” of a text arise directly from the text itself, the jet topics method determines the quark and gluon fractions directly from data, with only minimal reliance on simulation. Both methods gave consistent results, but the jet topics approach reduced systematic uncertainties by up to a factor of 2 in some regions of phase space (Figure 2).

At a working point that selects 50% of quark jets while rejecting up to 95% of gluon jets, correction factors used to account for differences between data and simulation land between 0.88 and 1.30 for quark jets and between 0.61 and 1.05 for gluon jets, where a value of 1 corresponds to perfect agreement between data and simulation. The associated uncertainties range from ten to several tens of percent depending on momentum and pseudorapidity. Run 2 and Run 3 results agree within their uncertainties, demonstrating that DeParT behaves consistently across data-taking conditions and detector upgrades.

DeParT joins a wave of transformer-based tools now reshaping how the ATLAS Collaboration reconstructs and identifies the signals recorded by the experiment. Alongside GN2 for heavy-flavour tagging, these new tools provide a sharper view of one of the LHC’s most ubiquitous signatures and expand the sensitivity of the many future analyses that will depend on them.

About the banner image: Graphic representing a neural network transforming data in the ATLAS experiment. (K. Anthony/ATLAS Collaboration)

Learn more

Performance and efficiency of a transformer-based quark/gluon jet tagger in the ATLAS experiment (Submitted to EPJC., arXiv:2512.03949, see figures)
Performance and calibration of quark/gluon-jet taggers using 140 fb⁻¹ of proton-proton collisions at 13 TeV (Chin. Phys. C 48 (2024) 023001, arXiv:2308.00716, see figures)
Constituent-Based Quark Gluon Tagging using Transformers with the ATLAS detector (ATL-PHYS-PUB-2023-032).
Metodiev & Thaler, On the Topic of Jets: Disentangling Quarks and Gluons at Colliders (Phys. Rev. Lett. 120, 241602 (2018), arXiv:1802.00008)
ICHEP26 presentation by A. Sopio: Classifying hadronic objects in ATLAS with ML/AI algorithms
BOOST26 presentation by Samuel Jankovych: Calibration of a Transformer-Based Quark/Gluon Tagger in ATLAS
ATLAS enters a new era of jet flavour tagging – powered by AI, Physics Briefing, July 2025

Summary of new ATLAS results from ICHEP 2026

Katarina Anthony — Thu, 30 Jul 2026 06:01:05 +0000

Summary of new ATLAS results from ICHEP 2026

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Every two years, the International Conference on High Energy Physics (ICHEP) brings together the world's leading theorists and experimentalists to present the latest advances in particle physics. The 2026 edition, hosted in Natal, Brazil, will welcome the global particle physics community from 30 July to 5 August for a week of presentations, discussions and collaboration.

The ATLAS Collaboration will present a portfolio of new results at ICHEP 2026, showcasing the breadth and strength of its research programme. Among them, first observations of polarisation in Z boson pairs and innovative new studies of double Higgs boson processes, as well as the latest searches for physics beyond the Standard Model. These results are based on the extensive datasets collected during both Run 2 (2015–2018) and the recently completed Run 3 (2022–2026) of the Large Hadron Collider (LHC).

Many of these results will be featured in dedicated physics briefings and news pieces. Follow the ICHEP 2026 tag to stay up to date with the latest ATLAS announcements, and explore the full list of new ATLAS results below as it grows during the conference.

Latest Physics Briefings and News

DeParTing from tradition: AI sharpens quark vs gluon tagging, Physics Briefing, 30 July
Polarised bosons: a window into the Higgs mechanism, Physics Briefing, 4 August
ATLAS deepens the search for long-lived particles with Run 3 data, Physics Briefing, 4 August
ATLAS breaks its own record with new search for double-Higgs production, Physics Briefing, 5 August

New results presented at ICHEP 2026

Exotic New Physics

Higgs boson

Scalar boson and diboson searches

Standard Model

Heavy Ions

Measurement of J/ψ-jet correlations in pp and Pb+Pb collisions at 5.02 TeV with the ATLAS detector (arXiv:2606.31375, see figures)
Observation of centrality-dependent dijet transverse momentum imbalance in O+O and Ne+Ne collisions at 5.36 TeV with the ATLAS detector (arXiv:2606.20463, see figures)
Measurements of charged-particle pseudorapidity and transverse momentum distributions in O+O and Ne+Ne collisions at 5.36 TeV with the ATLAS detector (arXiv:2606.20257, see figures)
Measurement of isolated photon plus two-jet correlations in Pb+Pb and pp collisions at 5.02 TeV with ATLAS (arXiv:2606.17760, see figures)
Jet azimuthal anisotropies in O+O collisions at 5.36 TeV with the ATLAS detector (ATLAS-CONF-2026-006)
Measurement of forward jet suppression in Pb+Pb collisions at 5.02 TeV with the ATLAS detector (arXiv:2608.03261)

Combined Performance

Measurement of the b-jet identification efficiency in dileptonic tt¯ events using proton-proton collision data at 13.6 TeV collected with the ATLAS detector (arXiv:2607.05322, see figures)
Simultaneous efficiency measurements of b- and c-jets in tt¯ events from 13.6 TeV pp collision data collected with the ATLAS detector (arXiv:2607.06199, see figures)
GN3X: Improved Transformer-based Tagger for Boosted Higgs Bosons in ATLAS (ATL-PHYS-PUB-2026-013)
The calibration of large-radius jets using the Run 2 dataset with the ATLAS detector (arXiv:2607.25893, see figures)

ATLAS and CMS recognise longstanding industrial partner Hamamatsu Photonics K.K.

Katarina Anthony — Thu, 16 Jul 2026 01:19:05 +0000

ATLAS and CMS recognise longstanding industrial partner Hamamatsu Photonics K.K.

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News

ATLAS and CMS Collaborations

awards

ITk

Hamamatsu Photonics K.K. was announced as the 2026 Industrial Award winner by the ATLAS and CMS Collaborations at a dedicated ceremony on 15 July 2026.

Hamamatsu Photonics K.K. Representative Director Takayuki Suzuki is presented the Industrial Award by CERN Director of Research and Computing, Gautier Hamel de Monchenault, together with ATLAS Spokesperson Stéphane Willocq.

With this award, the ATLAS and CMS Collaborations recognise the crucial role Hamamatsu Photonics K.K. has played in the development and production of advanced technologies, as well as the ongoing, deeply rooted partnership that has been established with both experiments over many years. Through this close collaboration, Hamamatsu Photonics has made exceptional contributions to the construction of the strip and pixel detectors for the ATLAS Inner Tracker (ITk), the CMS Outer Tracker and Inner Tracker, the CMS High-Granularity Calorimeter and the CMS MIP Timing Detector for the High‑Luminosity LHC upgrade project.

“For decades, Hamamatsu Photonics’ exceptional sensor technology has been a cornerstone of CMS, enabling groundbreaking physics results documented in more than 1500 scientific publications. This unique partnership and shared mission will continue to drive the experiment’s success in the demanding High-Luminosity LHC era,” says CMS Spokesperson Anadi Canepa

“Over many years, ATLAS and Hamamatsu Photonics have built a partnership founded on mutual trust, technical excellence and a steadfast commitment to scientific discovery. As we enter the High-Luminosity LHC era, we are proud to continue working with a partner that has made such an important contribution to the success of ATLAS,” adds ATLAS Spokesperson Stéphane Willocq.

Hamamatsu Photonics’ expertise, responsiveness and flexibility in accommodating evolving requirements have been instrumental in transforming the ambitious detector concept for the ATLAS Inner Tracker into reality.

Based on technical excellence, steadfast commitment and collegial R&D, and having weathered demanding technical requirements and project schedules, this partnership has borne fruit over several decades, becoming essential to the success of the ATLAS and CMS scientific endeavour. Achievements include the development of high-quality p-type sensors for strip and pixel detectors on six-inch production lines, the first large-scale eight-inch production for pad sensors, novel LGADs (low gain avalanche diodes), numerous SiPMs (silicon photomultipliers), and significant manufacturing support throughout the bump‑bonding process.

Across both ATLAS and CMS, about 1000 m² of silicon wafers were procured from Hamamatsu Photonics, reflecting the scale and importance of the contribution to the effort to prepare the detectors for the high-luminosity era.

The Hamamatsu delegation spells out ATLAS while visiting the experimental cavern.

Visiting the ATLAS Inner Tracker (ITk) assembly clean room at CERN.

Examining ITk Pixel sensors in the clean room.

With a prototype of the ITk Pixel outer ring.

A peak through the barrel of the new tracker.

Success! Reviewing the results of an ITk Pixel sensor test.

At the ITk Pixel clean room in CERN's Building 186.

A few tokens of appreciation from the ATLAS Collaboration to celebrate the long-standing partnership.

“The ATLAS ITk is the most ambitious detector upgrade ever undertaken by our Collaboration. Throughout its development, Hamamatsu Photonics K.K. has made outstanding contributions, from the development of high-quality sensors for the ITk strip detector to the joint development of p-type sensors for the ITk pixel detector and substantial manufacturing support throughout the bump‑bonding process. Their expertise, responsiveness and flexibility to accommodate our evolving requirements have been instrumental in turning an ambitious detector concept into a reality,” says Benedetto Gorini, ATLAS Upgrade Coordinator.

“Recently, we ordered an unprecedented amount (over 800 m²) of silicon sensors across five different base technologies. All the sensors were developed and prototyped together with HPK, and all the deliveries were timely and of remarkable quality. Without HPK's unique expertise and commitment, the ambitious CMS upgrade would not be possible. This is not a standalone occurrence; I have been working with HPK since 1997 on numerous projects. R&D, prototyping, process optimisation and reliable mass production,” says Frank Hartmann, CMS Upgrade Coordinator.

The ATLAS and CMS Collaborations are proud to honour these achievements with the 2026 Industrial Award and look forward to the continued collaboration with Hamamatsu Photonics into the future.

Challenging symmetries with the heaviest particles

Katarina Anthony — Mon, 13 Jul 2026 08:28:17 +0000

Challenging symmetries with the heaviest particles

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Katarina Anthony Mon, 13/07/2026 - 10:28

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The ATLAS Collaboration is studying the two heaviest known elementary particles – the Higgs boson and the top quark – looking for signs of fundamental symmetry-breaking. Using a single optimised strategy, researchers simultaneously measured rare Higgs-top production processes and mapped how they interact.

“We are made of matter.” This simple statement hides one of the deepest questions in particle physics: why do the laws of nature allow matter to dominate over antimatter?

One place to look for answers is CP symmetry, which compares the behaviour of particles with that of their mirror-image antiparticles. Although CP violation is a necessary condition to explain the matter-dominated Universe, the Standard Model’s small effects – so far limited to W boson interactions with quarks – are not sufficient.

Figure 1: Weighted diphoton invariant-mass (mγγ) distributions in the 32 analysis categories. The top panel shows the tH signal; the bottom panel shows the ttH signal. The peak near the Higgs-boson mass (125 GeV) is used to extract the ttH and tH contributions. The total background (orange dashed line) includes the continuum background (grey dotted line), arising from non-Higgs diphoton events with a smoothly falling mγγ distribution, together with Higgs bosons produced through other mechanisms that also decay to two photons, creating a smaller peak near the Higgs-boson mass. (Image: ATLAS Collaboration/CERN)

The Higgs boson may provide new clues. While its behaviour is largely consistent with Standard Model predictions, the precision of current measurements still leaves open the possibility that additional CP-violating effects are hiding in its interactions with other particles. As the heaviest known elementary particle, the top quark has the strongest coupling with the Higgs boson, making the top-Higgs interaction especially sensitive to such effects. Even slight deviations from the Standard Model could change how often Higgs bosons are produced with top quarks or leave subtle traces in collision event shapes.

In a new analysis, the ATLAS Collaboration examined 164 fb⁻¹ of 13.6 TeV proton-proton collisions (collected between 2022 and 2024) to study Higgs-boson production in association with either a top-quark pair (ttH) or a single top quark (tH). Researchers focused on events where the Higgs boson decays into two photons, due to its clean experimental signature and excellent mass resolution. As shown in Figure 1, the Higgs-boson signal appears as a narrow peak in the diphoton mass distribution, distinct from background events.

A major challenge of this analysis arises from the similarity and complexity of the signatures involved in the studied production modes. In particular, the tH process is difficult to isolate. The Standard Model predicts it to be especially rare because the mechanisms that produce tH do not simply add up. Just as overlapping waves can cancel each other out, these quantum mechanisms can interfere destructively. In the tH process, this destructive interference arises between diagrams in which the Higgs boson couples to the top quark and those in which it couples to the W boson; their cancellation suppresses the tH production rate. Any CP-violating component of the top-Higgs interaction would change the rates and kinematic patterns of ttH and tH events. These changes provide a handle for studying the CP structure, but they also further complicate the separation of closely related ttH and tH signal patterns from one another and from background events.

Even slight deviations from the Standard Model could change how often Higgs bosons are produced with top quarks or leave subtle traces in collision event shapes

To address these challenges, ATLAS researchers used graph neural networks that examine reconstructed particles and their relationships. This approach allowed the team to avoid explicit top-quark reconstruction, which can be ambiguous and introduce its own uncertainties, and instead directly sort events into regions enriched with ttH or tH signals. Events with topologies that are more CP-odd or CP-even were also identified.

Figure 2: Allowed values of the CP-even and CP-odd components of the Higgs-top interaction, based on Run 2 (13 TeV), partial Run 3 (13.6 TeV) data and their combination. Values inside the contours are allowed at the indicated confidence levels. The combined result is consistent with the Standard Model and excludes large CP-odd contributions. (Image: ATLAS Collaboration/CERN)

The results show no significant deviation from the Standard-Model prediction. Physicists measured the ttH signal strength to be 1.13 +0.33/-0.28, with a corresponding production cross section times decay branching ratio of 1.46 +0.40/-0.35 fb. This is the first ttH cross-section measurement at 13.6 TeV. Given how rare the tH process is, the amount of data used was not enough to establish its production cross section. Therefore, researchers set an upper limit of 6.2 times the Standard-Model prediction (4.4 expected) at 95% confidence level. This represents the most stringent single-measurement limit on tH production to date.

Researchers then tested whether the top–Higgs interaction could contain a mixture of different CP components. The degree of CP mixing is parameterised by the mixing angle α, where 0° corresponds to a purely CP-even interaction and 90° to a purely CP-odd one. Mixing angles larger than 53° were excluded at the 95% confidence level. To strengthen their interpretation, the team combined these results with a previous study of 13 TeV data (collected in 2015-2018) to achieve the most stringent ATLAS constraint to date on the CP structure of the top–Higgs interaction (see Figure 2). They excluded mixing angles larger than 38° and rejected the possibility of a purely CP-odd top–Higgs interaction at 5.8 standard deviations.

These results significantly improve our understanding of the top–Higgs interaction, including its strength, its rare single-top production mode and its CP structure. While no deviation from the Standard Model has yet been observed, this new analysis strategy gives ATLAS researchers stronger tools with which to search for signs of new physics in this interaction.

About the banner image: Candidate tH event display for banner use. The event was recorded by ATLAS at 13.6 TeV in 2022 and contains two photons from the Higgs boson decay, a muon, one b-tagged jet from the top quark decay and a forward jet. (Image; ATLAS Collaboration/CERN)

Learn more

Probing the Higgs-top Yukawa interaction in the ttH and tH processes using H→γγ with the ATLAS detector (arXiv:2606.04855, see figures)
Search for the associated production of a Higgs boson and a single top quark in the H→ττ decay mode and combined measurement of tH production using pp collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2026-002)
Search for the production of a Higgs boson in association with a single top quark in pp collisions at 13 TeV with the ATLAS detector (JHEP 10 (2025) 093, arXiv:2508.14695, see figures)
𝐶𝑃 Properties of Higgs Boson Interactions with Top Quarks in the 𝑡𝑡𝐻 and 𝑡𝐻 Processes Using 𝐻 -> 𝛾𝛾 with the ATLAS Detector (Phys. Rev. Lett. 125 (2020) 061802, arXiv:2004.04545, see figures)
ATLAS maps the top quark–Higgs boson interaction with multileptons, Physics Briefing, February 2026
Searching for new sources of matter–antimatter symmetry breaking in Higgs boson interaction with top quarks, Physics Briefing, April 2020

ATLAS enters the high-luminosity era

Katarina Anthony — Mon, 29 Jun 2026 10:30:41 +0000

ATLAS enters the high-luminosity era

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Press Statement

ATLAS Collaboration

HL-LHC

At 05:52 a.m. on 27 June 2026, the final protons of the Large Hadron Collider's third data-taking period circulated through the accelerator, bringing Run 3 to a close and marking the end of a remarkable chapter in particle physics. But for the thousands of scientists, engineers and students of the ATLAS Collaboration, this is not an ending so much as a transition.

The next several years will be devoted to transforming both the LHC and the ATLAS experiment for the High-Luminosity LHC (HL-LHC or HiLumi LHC), an ambitious upgrade programme that will culminate in the return of particle beams in 2030. The HL-LHC upgrade will dramatically increase the collider's intensity, or “luminosity”. Around 200 proton-proton collisions are expected every time the beams cross, creating an extraordinarily dense environment and delivering immense statistical power to explore the structure of matter and its fundamental interactions.

"The HL-LHC will shape particle physics for decades to come, and preparing for it is among the most ambitious scientific undertakings our collaboration has ever engaged in," says ATLAS Spokesperson Stéphane Willocq. "To record data under these extreme conditions, the ATLAS experiment's core systems have been fundamentally reinvented. This will allow us to continue pushing the frontiers of knowledge, exploring the limits of our current theories and looking for answers to the questions they leave open.”

"The HL-LHC will shape particle physics for decades to come, and preparing for it is among the most ambitious scientific undertakings our collaboration has ever engaged in." – ATLAS Spokesperson Stéphane Willocq

Despite its name – "Long Shutdown 3” – this upgrade period will be anything but quiet, demanding one of the most complex engineering campaigns in CERN's history. Across ATLAS institutes around the world, researchers have spent years designing, constructing and testing the next generation of detector technologies that will now begin making their way to CERN for installation.

The overhaul includes a brand-new, all-silicon Inner Tracker (ITk) featuring 178 square metres of active silicon detectors and 5 billion readout channels, able to reconstruct particle trajectories with micrometric precision. This is paired with a novel High-Granularity Timing Detector (HGTD) utilising Low-Gain Avalanche Detectors to separate overlapping collisions with a 30-to-50-picosecond track resolution, and a next-generation event-selection system driven by programmable hardware, capable of selecting events at a rate of 1 MHz. The selected data will then be processed by a powerful GPU-accelerated computing farm, isolating the rare and scientifically significant events within a final 10 kHz output stream for detailed analysis. Along with electronics upgrades throughout the experiment, physicists will be transforming nearly every aspect of the experiment.

"It is an ambitious and deeply complex programme that demands an all-hands-on-deck effort from our entire collaboration, and thousands of ATLAS members worldwide have been engaged in these activities," says Benedetto Gorini, ATLAS Upgrade Coordinator. "Our upgrade projects have been underway for several years, advancing steadily from design and prototyping into large-scale production and global integration. We are now entering a critical phase: finalising construction and preparing these systems for installation in the ATLAS experiment."

Alignment check in the Inner Tracker (ITk) barrel prior to the practice insertion of an ITk Strips stave. (Image: A. Barr/ATLAS Collaboration)

Low-Gain Avalanche Detector (LGAD) sensors for the ATLAS High-Granularity Timing Detector. (Image: K. Wu/ATLAS Collaboration)

ITk Pixel Module assembly at CERN. (Image: K. Anthony/ATLAS Collaboration)

ATLAS members pose next to the Pixel Support Tube at Berkeley Lab. (Image: The Regents of the University of California, Lawrence Berkeley National Laboratory)

Testing of the HGTD demonstrator at CERN. (Image: S Guindon and Z. Ge/ATLAS Collaboration) (Image: CERN) Testing of the HGTD demonstrator at CERN. (Image: S Guindon and Z. Ge/ATLAS Collaboration)

Upgraded Barrel Inner Large (BIL) sector singlet Resistive Plate Chambers (RPCs) at the CERN Muon assembly site, ready for integration. (Image: Paolo Iengo and Rosy Nikolaidou/ATLAS Collaboration)

A 2-Phase Accumulator Control Loop (2PACL) installed in the cooling and ventilation room in the ATLAS service cavern. This is part of a major upgrade to the experiment's cooling system to an environmentally friendly CO2 system. (Image: B. Verlaat/ATLAS Collaboration)

Assembly of new small Muon Drift Tube (sMDT) detectors to be added to the ATLAS Experiment’s Muon Spectrometer. Image: ATLAS Collaboration/CERN)

Installing these upgrades is far from a plug-and-play exercise. The original construction of ATLAS has often been compared to building a ship in a bottle, with detector components lowered through narrow shafts and assembled piece by piece in an underground cavern. Engineers must now carefully dismantle that ship and rebuild it with a new generation of detector technologies. The work includes removing and replacing major detector systems, rerouting and installing kilometres of cabling, implementing an upgraded carbon-dioxide cooling system and integrating extensive new computing and services infrastructure.

"Every phase of this engineering effort must be timed to perfection," says Martin Aleksa, ATLAS Technical Coordinator. "We are coordinating teams from around the world, each with their own schedules and responsibilities, while ensuring access to the detector and protecting the delicate systems that remain in place. These operations must be completed on time if we are to be ready when the beams return."

This colossal engineering effort is only part of the story. While teams transform the experiment for the HL-LHC, the global physics collaboration is shifting into high gear to analyse the largest dataset in its history – collected during Run 3 of the LHC.

Luminosity delivered by the LHC and recorded by the ATLAS experiment over the full lifetime of the accelerator. (Image: ATLAS Collaboration/CERN)

Over its lifetime, ATLAS has now recorded 505 fb⁻¹ of proton-proton collision data, of which 332 fb⁻¹ was collected during Run 3 alone. "Throughout Run 3, the ATLAS operations teams maintained high data-taking efficiency across proton-proton, heavy-ion and specialised low-energy runs," says Eric Torrence, Run Coordinator. "Their dedication, together with the excellent performance of the LHC and improvements implemented during the previous shutdown, enabled ATLAS to record its largest dataset yet. This rich pool of data means that the coming years will be among the most scientifically productive in the experiment's history."

"These data underpin a rich and diverse physics analysis programme spanning a broad range of subjects, from precision measurements of the Higgs boson to searches for rare phenomena that could point to physics beyond the Standard Model,” says Kerstin Tackmann, ATLAS Physics Coordinator. “When combined with data collected during LHC Run 1 and Run 2, we will more than double our statistical power. This will allow us to stress test the Standard Model of particle physics like never before.”

As one era of the LHC comes to a close, another is preparing to begin.

"We have our work cut out for us, but this is a challenge the ATLAS Collaboration is ready to meet," concludes Stéphane. "The strength of ATLAS lies in its people. Our collaboration brings together individuals from across the globe and from diverse fields of expertise to tackle some of the most profound questions in nature. As we set course for the HL-LHC era, we do so with confidence, knowing that our collective efforts have repeatedly delivered major scientific advances. The years ahead will be defined by both extraordinary scientific discoveries and remarkable technical achievements."

The future, quite literally, is luminous.

Learn more

CERN bids farewell to the LHC and enters Long Shutdown 3, CERN Press Release, 29 June 2026
What can you do with 380 million Higgs bosons? CERN Courier, January 2026
ATLAS prepares for High-Luminosity LHC, ATLAS News, April 2025
About the High-Luminosity LHC

Smallest droplet of the early Universe: ATLAS observes “jet quenching” in oxygen and neon collisions

Katarina Anthony — Sun, 28 Jun 2026 06:02:58 +0000

Smallest droplet of the early Universe: ATLAS observes “jet quenching” in oxygen and neon collisions

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Physics Briefing

ATLAS Collaboration

light ion

The ATLAS Collaboration reports the observation of jet quenching in oxygen–oxygen and neon–neon collisions — a phenomenon where particle jets lose energy as they travel through the quark-gluon plasma (QGP). These measurements establish oxygen–oxygen and neon–neon collisions as the smallest collision systems in which jet quenching has been observed to date.

During the summer of 2025, the ATLAS experiment recorded its first-ever oxygen–oxygen and neon-neon collisions, opening a new window onto the study of the QGP. This extreme state of matter mimics the conditions of the early Universe during the first microseconds after the Big Bang. While QGP studies have traditionally focused on collisions of large nuclei such as lead or xenon, there is growing interest in probing smaller systems, such as oxygen or neon, to understand how QGP behaviour evolves with system size.

Figure 1: A dijet system shown in: a proton–proton collision (left), where no QGP is created and the jets emerge balanced in energy; and in a nucleus–nucleus collision (right), which produces a QGP that the jets must travel through, leading to jet quenching. (Image: ATLAS Collaboration/CERN)

The QGP is typically characterised by two phenomena: hydrodynamic flow and partonic energy loss. Hydrodynamic flow occurs because the QGP behaves as a very low viscosity fluid and so the geometry of the initial collision is imprinted on the distributions of particles observed in the detector. Partonic energy loss occurs when high-energy quarks and gluons lose energy while traversing the medium. When this energy loss affects particle jets, the phenomenon is known as jet quenching.

Earlier analyses released last year showed that the QGP created in these light-ion collisions produced hydrodynamic flow and provided first measurements suggestive of jet quenching in oxygen–oxygen collisions. This week, the ATLAS Collaboration presented new results that extend this picture, reporting the first observation of jet quenching in both oxygen–oxygen and neon–neon collisions.

This is the first observation of jet quenching in oxygen–oxygen and neon–neon collisions at the LHC, and firmly establishes these as the smallest systems so far in which the phenomenon has been measured.

Figure 2: Distributions of x_J, the ratio of the lower jet transverse momentum to the higher one, in oxygen-oxygen collisions (left) and neon-neon collisions (right) compared to that of proton-proton collisions. The 0-10% points in each panel are head-on (central) collisions and the higher percentiles are for increasingly less head-on collisions. (Image: ATLAS Collaboration/CERN)

This milestone was achieved through studies of dijet momentum balance. In proton-proton collisions, pairs of particle jets (dijets) are typically produced back-to-back with nearly equal transverse momentum. In heavy-ion collisions, however, physicists have observed an increased number of imbalanced dijets arising from differences in the jets’ path length through the QGP (see Figure 1) and from fluctuations in the energy-loss process itself. Researchers quantify this imbalance using x_J, which is the ratio of the lower jet transverse momentum to the higher one. This phenomenon can then be mapped across different collision geometries, considering the degree of overlap of the colliding nuclei (their centrality). Head-on (central) collisions create the largest QGP droplets and show stronger jet quenching effects, manifesting in less balanced jets compared to the energy-loss-free proton baseline. Conversely, glancing (peripheral) collisions should create smaller droplets and show little to no jet quenching effects, with an x_J distribution similar to that seen in proton–proton collisions.

Figure 3: Distributions of the ratio of xJ, in oxygen-oxygen (top row) and neon-neon (bottom row) collisions to those in proton-proton collisions for head-on (central) collisions on the left and increasingly less head-on collisions moving to the right. Each panel shows the significance of the deviation between the light-ion collisions and proton-proton collisions. (Image: ATLAS Collaboration/CERN)

In their analysis of light-ion collisions, the ATLAS team successfully measured this effect. As shown in Figure 2, the gradual modification with increasing centrality is qualitatively compatible with the formation of QGP droplets of increasing transverse size inducing greater energy-loss effects on particle jets formed in the collisions. They then considered the modification in the x_J distributions relative to proton-proton collisions as a function of centrality and leading jet transverse momentum. Jet quenching was observed in central oxygen–oxygen and neon–neon collisions, with statistical significances exceeding the five standard deviations observation threshold (see Figure 3). Peripheral light-ion collisions were found to be most similar to proton–proton results, while the most central light-ion collisions showed x_J distributions similar to those seen in lead–lead collisions when they produce a similar size QGP droplet.

This is the first observation of jet quenching in oxygen–oxygen and neon–neon collisions at the LHC, and firmly establishes these as the smallest systems so far in which the phenomenon has been measured. It provides an important clue to help researchers pinpoint the system size at which QGP begins to affect high-energy jets. Future studies of these datasets will explore this phenomenon in greater detail, offering new insights into this exotic state of matter and informing plans for the heavy-ion programme in LHC Run 4 and beyond.

About the banner image: First oxygen-oxygen collisions recorded by the ATLAS experiment at a centre-of-mass energy per nucleon pair of 5.36 TeV. (Image: ATLAS Collaboration/CERN)

Learn more

Observation of centrality-dependent dĳet transverse momentum imbalance in O+O and Ne+Ne collisions at 5.36 TeV with the ATLAS detector (arXiv:2606.20463, see figures)
Measurements of the suppression and correlations of dijets in Pb+Pb collisions at 5.02 TeV (Phys. Rev. C 107 (2023) 054908, arXiv:2205.00682, see figures)
Measurement of jet pT correlations in Pb+Pb and pp collisions at 2.76 TeV with the ATLAS detector (Phys. Lett. B 774 (2017) 379, arXiv:1706.09363, see figures)
Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at 2.76 TeV with the ATLAS Detector at the LHC (Phys. Rev. Lett. 105 (2010) 252303, arXiv:1011.6182, see figures)
Giacalone et al., The unexpected uses of a bowling pin: exploiting 20Ne isotopes for precision characterizations of collectivity in small systems (arXiv:2402.05995)
Hunting for jet quenching in collisions between oxygen nuclei, Physics Briefing, September 2025
Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions, Physics Briefing, September 2025
ATLAS takes a breath of oxygen, ATLAS News, July 2025
ATLAS gives new insight into dijet suppression in heavy-ion collisions. Physics Briefing, April 2022
Looking inside trillion degree matter with ATLAS at the LHC, ATLAS Feature, May 2022

ATLAS hunts for a new "soft" signature of the dark sector

Katarina Anthony — Fri, 12 Jun 2026 07:40:59 +0000

ATLAS hunts for a new "soft" signature of the dark sector

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Katarina Anthony Fri, 12/06/2026 - 09:40

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The ATLAS Collaboration releases its first dedicated search for soft unclustered energy patterns (SUEPs) – an elusive possible signature of new physics phenomena. The analysis targets a unique muon-rich experimental signature to substantially extend sensitivity to hidden phenomena beyond previous results.

Figure 1: Illustration of a possible SUEP production process. Two gluons (g) produced in a proton–proton collision fuse to create a heavy mediator particle (S). Under a new strong force in the hidden sector, S produces an isotropic shower of many dark hadrons (φ). The φ particles decay into ordinary Standard-Model particles, producing the high-multiplicity signature targeted by this search. (Image: ATLAS Collaboration/CERN)

Long-standing tensions between simple theoretical models of Dark Matter and astrophysical observations have motivated physicists to consider more complex theories involving a hidden “dark sector” of particles and forces. Of particular interest is dark QCD – a new force analogous to quantum chromodynamics (QCD), which describes the strong force – that could bind dark-sector quarks into exotic forms of matter. If such a force exists, particles produced in LHC collisions would generate showers of dark-sector particles. These may in turn decay into familiar Standard-Model particles, leaving observable signatures in the ATLAS experiment (see Figure 1).

Previous searches for dark QCD focused on signatures similar to those produced through Standard-Model QCD. In high-energy collisions, energetic quarks and gluons generate branching showers of particles. Due to the way the strong interaction operates, this showering is restricted to small angles relative to the original particle's trajectory, and forms highly collimated sprays known as jets. ATLAS researchers previously searched for dark-QCD signatures based on this picture, focusing on so-called "dark jets", “semi-visible jets” and "emerging jets". But dark-sector showers may not behave this way. They may expand at wider angles and not form jets at all, thus evading those studies. Physicists would instead need to look for a softer, uniformly distributed signature (a SUEP).

This analysis targets a new signature of dark QCD – a hypothetical force analogous to quantum chromodynamics (QCD) that binds dark-sector quarks into exotic forms of matter.

Figure 2: Distribution of events as a function of the number of inner detector tracks and Sμ, which quantifies the isotropy of the muon system, with values close to one corresponding to nearly uniform angular distributions. The left panel shows collision data, while the right panel shows the expected distribution for representative SUEP signal models. (Image: ATLAS Collaboration/CERN)

For their search, the ATLAS Collaboration studied the full Run-2 dataset (140 fb^-1 of proton–proton collisions at 13 TeV), searching for signs of dark-sector particles decaying into a large number of muons. This signature is more easily distinguishable from Standard-Model processes – due to the unusually large number of muons – and is predicted by many well-motivated new-physics theories. The new analysis also takes the search for SUEPs into a new direction, as previous analyses focused primarily on hadronic final states.

Figure 3: Distribution of the number of inner detector tracks for events in the search region and with Sμ >0.6. The data are compared with the expected Standard Model background and with a representative signal model after performing a combined fit to signal and background contributions. The lower panel shows the difference between the observed data and the total prediction. (Image: ATLAS Collaboration/CERN)

Researchers selected events using specialised multi-muon triggers. They then characterised the data using specific observables sensitive to the unusual topology of SUEP events. Specifically, they looked for a large number of charged particles (high multiplicity) and a highly isotropic distribution of muons (see Figure 2). One of the main experimental challenges came from the large background noise arising predominantly from QCD multi-jet-production processes. To combat this, researchers determined their background prediction directly from the data, carefully extrapolating the behaviour of background processes from neighboring "sideband" regions with similar event topologies.

Ultimately, the ATLAS Collaboration recorded two events that strongly resemble the SUEP signatures expected from dark-sector interactions, one of which is shown in the banner event display. However, this event yield is still compatible with Standard-Model background expectations, given that the local significance of the excess is only 1.7 sigma (see Figure 3). Researchers used the data to set limits on the production of scalar mediator particles decaying into SUEPs, excluding cross sections down to 0.05 fb for a mediator mass of 750 GeV. In scenarios where this mediator is identified as the Standard-Model Higgs boson, the analysis restricts its decay probability into SUEPs to approximately 0.2%. These constraints represent a substantial improvement in sensitivity over previous results.

About the banner image: Visualization of a proton–proton collision which resembles the SUEP signature expected from dark-sector interactions. It contains six muons (five passing quality selections) and 174 tracks compatible with originating from the primary vertex. The muons are indicated by red lines. Reconstructed tracks with transverse momentum above 500 MeV are shown in yellow. (Image: ATLAS Collaboration/CERN)

Learn more

Search for soft unclustered energy patterns containing muons in the final state in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2605.20015, see figures)
Shedding light with jets from the dark side, Physics Briefing, May 2025
When jets go dark: identifying elusive "dark jets" at ATLAS, Physics Briefing, September 2023
Not a jet all the way: is dark matter hiding in plain sight? Physics Briefing, May 2023
CMS Collaboration: Search for soft unclustered energy patterns produced in association with a W or Z boson in proton-proton collisions at 13 TeV (arXiv: 2605.20015)
CMS Collaboration: Search for Soft Unclustered Energy Patterns in Proton-Proton Collisions at 13 TeV (Phys. Rev. Lett. 133, 191902)

ATLAS explores quantum entanglement using Higgs boson decays, while charting its properties

Katarina Anthony — Fri, 05 Jun 2026 12:45:04 +0000

ATLAS explores quantum entanglement using Higgs boson decays, while charting its properties

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At the ATLAS experiment at CERN, physicists are using the Higgs boson as a testbed for complex quantum phenomena at high energy. Their latest results deliver precise measurements of Higgs-boson properties and provide the first strong evidence of quantum entanglement between two massive vector bosons.

Exploring the "Golden Channel"

In an updated study, ATLAS physicists examined the rare H → ZZ* → 4ℓ decay process, where a Higgs boson decays into two Z bosons – one of which is a lower-mass state (denoted by the asterisk). These Z bosons then decay into four leptons (electron or muon pairs).

While this process accounts for just ~3% of Higgs decays, it is ideal for precision measurements. The ATLAS experiment was specifically designed to identify and measure electrons and muons with high efficiency and precision, allowing the four-lepton final state to be fully reconstructed. Further, very few background processes mimic this signature. These features earned H → ZZ* → 4ℓ the nickname of “the golden channel”, and it was one of the two key signatures that enabled the ATLAS Collaboration’s discovery of the Higgs boson in 2012.

Researchers analysed three years of Run-3 proton–proton collision data collected at a centre-of-mass energy of 13.6 TeV between 2022 and 2024. Figure 1 shows the number of H → ZZ* → 4ℓ candidate events measured in this period. The primary goal was to measure the Higgs-boson production rate (cross section), and compare the results with predictions from the Standard Model of particle physics. Several complementary measurements were performed: the fiducial cross section, the Higgs-boson production rate measured within the coverage of the ATLAS detector; differential fiducial cross sections, the Higgs-boson production rate as a function of eight variables sensitive to its production and decay kinematics; and production cross sections, the Higgs-boson production rate in each of its four main production modes – via gluon fusion (ggF), vector boson fusion (VBF), associated production with a vector boson (VH) or a top-quark pair (ttH).

Figure 1: Distribution of the four-lepton invariant mass, comparing the observed collision data (black points) with the expectations from the Standard Model. Different colours refer to different Standard-Model processes that contribute to the final states. (Image: ATLAS Collaboration/CERN)

Figure 2: The observed and expected Standard Model (SM) values of the inclusive and individual production-mode cross-sections normalised to their Standard-Model values. The branching ratio (B) for the ZZ* →4ℓ decay is assumed to be given by the Standard-Model prediction. (Image: ATLAS Collaboration/CERN)

All cross-section measurements were found to be in agreement with the Standard-Model predictions, with a result precision rivalling previous ATLAS measurements performed with 13 TeV data. The fiducial cross-section was measured to be σ_fid = 3.65 ± 0.35 fb, agreeing with the Standard Model prediction of 3.68 ± 0.17 fb. The observed production cross-sections are compared to the Standard Model prediction, as shown in Figure 2. Researchers also used these measurements to constrain possible anomalous Higgs boson interactions caused by new physics phenomena within an Effective Field Theory framework and the κ-framework.

These new results sharpen our understanding of Higgs-boson properties and provide the first strong evidence of quantum entanglement between two massive vector bosons.

Entangled bosons

Quantum entanglement – where the state of one particle cannot be described independently of another – is one of the most mind-bending features of quantum mechanics. Traditionally studied in low-energy experiments, entanglement has recently been put to the test at the LHC's high-energy scales. In 2023, the ATLAS Collaboration achieved the first-ever observation of quantum entanglement between quarks, detecting it in top-antitop pairs at their production threshold. Now, the ATLAS experiment’s exceptional sensitivity to the H → ZZ* → 4l decay channel has enabled physicists to explore this frontier in pairs of massive vector bosons.

The Higgs boson is unique for having a spin of zero. The Z boson, by contrast, is a spin-1 particle, meaning it can exist in three possible polarisation states, corresponding to spin projections of +1, 0, or −1 along a chosen direction. The polarisation of two Z bosons describes how their spin states are correlated. When a Higgs boson decays into two Z bosons, the pair must conserve the Higgs boson’s total spin of zero. By studying these polarisation correlations, physicists can perform a precise test for both electroweak interactions and quantum entanglement.

The new analysis looked at combined data from LHC Run 2 at 13 TeV together with the three years of 13.6 TeV data described above. Researchers measured two parameters, C_2,1,2,−1 and C_2,2,2,−2, which describe correlations between the spin states of the Z-boson pair. A non-zero value for either parameter would signal quantum entanglement. The ATLAS team also compared their data to two possible scenarios: the Standard Model prediction, in which the Z bosons are entangled, and an alternative non-entangled scenario, where the entanglement-sensitive parameters would be zero.

Figure 3: The observed distributions of events (full circles) overlaid on the expected (shaded) distributions of the estimator C_2,1,2,-1 (a) and C_2,2,2,-2 (b) for the entangled (SM-QE) hypothesis (blue solid line) and the separable (non-QE) hypothesis (orange dashed line) and backgrounds. (Image: ATLAS Collaboration/CERN)

The distributions of entanglement-sensitive observables agree well with the Standard Model and are inconsistent with a non-entangled hypothesis (see Figure 3). From these distributions, the mean values of the entanglement-sensitive parameters were measured to be C_2,1,2,−1 =−0.71 ± 0.45 (compared with an expected Standard-Model value of −0.97 ± 0.47) and C_2,2,2,−2 = 0.08 ± 0.44 (0.64 ± 0.43 expected), where uncertainties are dominated by statistical errors. Researchers carried out a complementary likelihood-ratio test using the C_2,2,2,−2 distribution, relying on several Standard-Model assumptions in the decays, which provided substantially higher sensitivity to quantum correlations. This test rejected the non-entangled hypothesis with an observed significance of 4.7 standard deviations (4.9 expected). Together, these results provide strong evidence for quantum entanglement between the two massive Z bosons produced in Higgs-boson decays.

This result highlights the Higgs boson’s unique role as a natural laboratory for studying quantum entanglement at high energies, opening broad opportunities in future spin studies of Z bosons as well as in other decay channels.

About the banner image: Display of a H → ZZ* → 2e2μ candidate event in 13.6 TeV proton-proton collisions. The two electrons are highlighted in green and the two muons in red. (Image: ATLAS Collaboration/CERN)

Learn more

Measurements of the Higgs boson production, fiducial and differential cross-sections in the four lepton decay channel using 164 fb^-1 of data collected at 13.6 TeV with the ATLAS detector (arXiv:2605.19016, see figures)
Measurements of Z-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment (arXiv:2603.26463, see figures)
Rencontres de Moriond presentation by Lailin Xu: Recent Single Higgs Measurements with the ATLAS Detector
QOfCP2026 presentation by Tairan Xu: Measurements of Z-boson pair entanglement in decays of Higgs bosons at the ATLAS experiment
ATLAS achieves highest-energy detection of quantum entanglement, Physics Briefing, September 2023
Aguilar-Saavedra, J. A., et al., Testing entanglement and Bell inequalities in 𝐻 →𝑍⁢𝑍 (Phys. Rev. D 107 (2023) 016012)

ATLAS tackles a new double-Higgs frontier

Katarina Anthony — Tue, 02 Jun 2026 06:14:37 +0000

ATLAS tackles a new double-Higgs frontier

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Katarina Anthony Tue, 02/06/2026 - 08:14

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The ATLAS Experiment is on the hunt for “ttHH production" – an extremely rare process where two top quarks and two Higgs bosons pop out of a single collision.

More than a decade after the discovery of the Higgs boson, the ATLAS Collaboration is pushing into a new frontier in Higgs research: first evidence of di‑Higgs production. This process, in which two Higgs bosons are produced simultaneously, is incredibly rare. Its observation is a flagship goal for the High-Luminosity LHC (HL-LHC), as it holds the key to the Higgs boson’s self‑interaction – a property that can shed light on the origin and evolution of the universe.

The ATLAS Collaboration has launched its first search for ttHH production, one of the rarest di-Higgs production modes. The analysis uses the full LHC Run 2 dataset (collected during 2015-2018) and early Run 3 data (collected 2022-2023), looking for any hints that these particles are interacting in ways current theories don’t expect.

Tracking ten objects at once

Of the many di-Higgs production modes, ttHH stands out as the most complex (see Figure 1). Top quarks and Higgs bosons are heavy and unstable, decaying almost immediately into a chaotic spray of leptons, photons and particle “jets” (see banner image). This makes it incredibly hard to reconstruct what happened in the collision.

But that’s also where this analysis can shine. Over the past several years, the ATLAS Collaboration has developed new detector calibration and event reconstruction methods to achieve precise identification of leptons, photons and jets (particularly those arising from bottom quarks). A single ttHH event can contain around ten well-reconstructed physics objects, providing many correlated clues to help researchers identify a potential signal. The downside? Background processes in these busy events are much more difficult to simulate, as combinatorial complexity and detector effects can mimic signals. Researchers developed advanced strategies to constrain these effects.

Figure 1: Feynman diagrams for non-resonant tt̄HH production at leading-order. Each diagram shows a distinct subprocess arising from different Higgs boson couplings. The diagram on the right shows the non-Standard-Model tt̄HH quartic coupling (highlighted with a yellow circle), which is described by the Higgs effective field theory. (Image: ATLAS Collaboration/CERN)

The ATLAS search focused on three ttHH decay signatures: the one-lepton channel (1L), defined by a single lepton (electron or muon) amid a massive "multijet" background; the multi-lepton channel (SSML), targeting a rare phenomenon where events have at least two leptons with the same charge; and the di-photon channel (bbγγ), defined by the two photons created by a Higgs-boson decay. The team used two different machine-learning classifiers to identify signal-like events: an attention-based neural network (transformer model) that learns relationships between many objects in an event was used in the 1L and SSML channels; and boosted decision trees, used in the bbγγ channel.

Even with multiple channels combined and the enormous LHC datasets, fewer than four ttHH events were expected in the selected signal regions, with the largest contribution expected from the 1L channel, followed by SSML and bbγγ.

This first ATLAS search for ttHH production pushes into a new regime where the signal is tiny and the backgrounds vast – demanding precision at every step.

Rare but powerful

Figure 2: The observed and expected upper limits on the signal strength from the individual 1L, SSML, and bb̄γγ channels, and from their combination. The expected limits are derived under the background-only hypothesis, i.e. assuming μ = 0. (Image: ATLAS Collaboration/CERN)

All decay channels and signal regions were combined into a statistical fit. The resulting limits on the signal strength (Figure 2), which is the number of observed events divided by the number of expected events, are fully compatible with current theory predictions for all three channels. Remarkably, despite the production rate being nearly 40 times smaller than that of the dominant di-Higgs modes, the upper limit on the signal cross-section is only about a factor of 8 weaker than the ATLAS and CMS combination. Additionally, this is the most sensitive ttHH search to date and the first one targeting 1L and SSML channels.

Why does this matter? A null result can constrain how far Nature might deviate from the Standard Model. The ATLAS team interpreted their data using a framework called Higgs Effective Field Theory (HEFT), which parameterises how undiscovered particles or interactions beyond the direct energy reach of the LHC might subtly alter their measurements. This is described by an “anomalous coupling coefficient” (c_ttHH, also shown in Figure 1). The new ATLAS result constrained this coefficient to -3.9 < c_ttHH < 3.3 at a 95% confidence level, so far consistent with the Standard-Model prediction of c_ttHH=0. This process is sensitive to a different combination of EFT parameters than the leading di-Higgs results, providing a complementary probe of possible new physics effects.

This search pushes the ATLAS Collaboration deeper into unexplored territory of the Higgs sector, advancing knowledge into complex heavy-flavour-rich regimes. As analysis continues on Run-3 data – and with data from HL-LHC on the horizon – expect sensitivity to ttHH production to improve substantially in the years ahead.

About the banner images: Candidates tt̄HH collision events in the single-lepton, multi-lepton and di-photon final states (shown from left to right). The cones represent reconstructed jets, with cyan cones indicating b-tagged jets associated with a candidate Higgs boson. (Image: ATLAS Collaboration/CERN)

Learn more

Search for Higgs boson pair production in association with top-quark pairs using 196 fb⁻¹ of proton-proton collision data at 13 and 13.6 TeV with the ATLAS detector (arXiv:2603.13113, see figures)
Combination of ATLAS and CMS searches for Higgs boson pair production at 13 TeV (arXiv:2602.23991, see figures)
Combination of searches for Higgs boson pair production in proton-proton collisions at 13 TeV with the ATLAS detector (Phys. Rev. Lett. 133 (2024) 101801, arXiv:2406.09971, see figures)
ATLAS sets record limits on Higgs self-interaction using Run 3 data, Physics Briefing, May 2025

ATLAS observes new Bc meson excited state

Katarina Anthony — Thu, 21 May 2026 10:10:25 +0000

ATLAS observes new Bc meson excited state

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The ATLAS Collaboration reports the first observation of the B_c*⁺ meson – a new composite particle state containing a charm quark and a bottom antiquark.

Protons and neutrons – the building blocks of matter – belong to a huge class of particles called hadrons. Hadrons are composite particles made of quarks that are bound together by the strong force. They are classified into two groups: baryons, which consist of three quarks (like protons and neutrons), and mesons, which are formed by a quark–antiquark pair.

Figure 1: Sketch of final state particles produced by the decay chain used in the ATLAS analysis. (Image: ATLAS Collaboration/CERN)

Despite decades of study, many aspects of the strong force remain poorly understood, particularly the way it binds quarks together inside hadrons. Mesons made of heavy quarks – such as charm or bottom quarks – can provide an important laboratory for testing theoretical descriptions of these effects. Of particular interest to physicists are B_c⁺ mesons, as they contain two types of heavy quarks: a charm quark and a bottom antiquark (b̅c).

In a new result presented at the Large Hadron Collider Physics 2026 conference, physicists from the ATLAS Collaboration report the first observation of a particle with properties consistent with the B_c*⁺ meson, the lowest excited B_c⁺ meson. Much like electrons shifting orbits inside an atom, quarks inside hadrons can exist in different energy states. Excited states are expected to be heavier than the corresponding ground state. In the B_c*⁺ meson, the charm quark and bottom antiquark should have spin orientations aligned in the same direction, whereas in the ground-state B_c⁺ meson their spins are oppositely oriented.

The new particle appears as a striking peak in the distribution, with a significance exceeding 8 standard deviations.

This new member of the meson family decays into a B_c⁺ meson and a photon (see Figure 1). One of the main challenges physicists faced when searching for the B_c*⁺ meson is that its mass is expected to exceed that of the B_c⁺ meson by only a few tens of MeV. As a result, the photon produced in the decay carries very little energy and is very difficult to measure in a detector like ATLAS.

For their search, researchers focused on B_c⁺ meson decays into three muons and an “invisible” neutrino. This decay mode is not typically studied in searches for new hadron states since neutrinos cannot be reconstructed. However, it occurs around twenty times more frequently than other fully-reconstructable decay modes – a gain that outweighs the complication of partial reconstruction.

Figure 2: The fitted distributions of the mass difference between the reconstructed mass of the B_c*⁺ and that of the B_c⁺ meson candidates. Despite the missing neutrino from the Bc+ decay, the distributions feature a significant excess corresponding to the B_c*⁺→ B_c⁺+γ decay (purple area). The distributions correspond to different regions with partially different sources of background events; the signal is seen in both. (Image: ATLAS Collaboration/CERN)

Detecting the low-energy photons produced in B_c*⁺ decays posed an additional challenge. Instead of using standard photon-identification techniques, which would be completely insensitive to these photons, physicists looked at cases of “photon conversion”. This is where the photon produces an electron-positron pair within the ATLAS tracking detector, leaving behind closely-spaced charged-particle tracks originating from a common point. These tracks can have transverse momenta as low as 100 MeV – significantly lower than those typically studied in ATLAS analyses – requiring a dedicated track-reconstruction procedure.

The new particle appears as a striking peak, with a significance exceeding 8 standard deviations, in the distribution of the invariant mass difference between the combined muon-triplet-plus-photon system and the muon triplet alone (see Figure 2). The measured mass difference between the B_c*⁺ meson and the B_c⁺ meson is 64.5 ± 1.4 (stat.) ^+1.0_−1.4 (syst.) MeV. This is within the range of the available theoretical expectations, though slightly deviating from the most recent, high-precision modern calculations. This result provides valuable new input for theoretical models describing the heavy-hadron mass spectra and will help to improve the understanding of the strong interaction.

About the banner: Graphical representation of a B_c⁺ meson, based on an image created by Daniel Dominguez/CERN.

Learn more

Observation of a B_c*⁺ meson with the ATLAS detector (arXiv:2605.16228, see figures)
LHCP 2026 presentation by Semen Turchikhin: Heavy flavor spectroscopy at ATLAS
LHCP 2026 presentation by Anna Sfyrla: ATLAS Highlights
ATLAS observes new composite particle, CERN News, 22 May 2026

A new trio: ATLAS finds first evidence of ZZy production

Katarina Anthony — Wed, 13 May 2026 08:31:37 +0000

A new trio: ATLAS finds first evidence of ZZy production

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Katarina Anthony Wed, 13/05/2026 - 10:31

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The ATLAS Collaboration reports the first evidence of the simultaneous production of a photon and two neutral Z bosons at the LHC.

The electroweak sector of the Standard Model combines the weak interaction, mediated by charged W bosons and the neutral Z boson, with electromagnetism, carried by the photon (γ). It governs a vast amount of physics processes, from particle decays to the radiative output of the Sun. In the 1990s, physicists measured the properties of the W and Z bosons with remarkable precision using data from CERN’s Large Electron–Positron (LEP) collider. Studying the simultaneous production of three of these force-carrying particles, however, would have to wait for the higher collision energies of the Large Hadron Collider (LHC).

Multi-boson processes are among the most sensitive tests of the Standard Model. Theoretical predictions for these interactions are extremely precise, so even small discrepancies could point to new physics phenomena. Triboson processes are the rarest of this kind, offering unique sensitivity to probe quartic boson couplings and complementarity with Higgs physics and searches beyond the Standard Model. The ATLAS Collaboration has explored such processes in collision events with three W bosons (WWW), a W or Z boson in combination with two photons (Wγγ, Zγγ), two W or Z bosons together with a Z boson (VVZ), two W bosons with a photon (WWγ), and a W boson, Z boson and a photon (WZγ).

In a new publication, ATLAS physicists report the simultaneous production of two Z bosons together with a photon (ZZγ) for the first time at the LHC. With a statistical significance of 4.4 standard deviations, the study extends the LHC’s repertoire of multi-boson studies and provides a complementary test to those already observed.

In a new publication, ATLAS physicists report the simultaneous production of two Z bosons together with a photon (ZZγ) for the first time at the LHC.

Figure 1: Data are compared with the signal and background expectations in the signal region. The event yields are shown in the 4e, 4μ, 2e2μ and 4ℓ channel, where the 4ℓ channel is the sum of the other three channels, as distinguished by the dashed line. (Image: ATLAS Collaboration/CERN)

The analysis uses the full LHC Run-2 dataset (collected between 2015 and 2018) of proton-proton collisions. Physicists focused on collision events where two Z bosons each decay into pairs of electrons or muons, accompanied by a photon of sizable energy. They applied a dedicated selection to exclude events in which the photon originates from the electrons or muons themselves. Each selected event therefore contains four charged particles (4e, 2e2μ, or 4μ) and one photon – an extremely rare combination. Theory predicts only seven signal events in the entire Run-2 dataset and only about one background event, arising mainly from “jets” of particles being misidentified as photons.

To avoid any bias, the analysis was carried out “blinded”, i.e. event selection and background studies were performed using simulated events as well as control samples. After these cross checks were completed, the data were unblinded and eight events met all the required selection criteria – a result spot on with the theoretical prediction.

Figure 1 shows the number of expected and observed events for the three different decay channels of the Z-boson pair (4e, 2e2μ, or 4μ) and for their combination (4ℓ). The expectation includes the theoretical prediction for the signal as well as estimated background contributions. The results agree with expectations for the three event classes separately and for the full sample.

The result has a statistical significance of 4.4 sigma, meaning the probability that the eight events are a random fluctuation from background-only processes is roughly one in ten thousand. This is strong evidence, though still below the five-sigma threshold required to claim a discovery. Studies of the LHC Run-3 dataset (2022–2026) and the future High-Luminosity LHC dataset will allow a discovery of this process and enable even more detailed tests of the electroweak sector.

About the banner image: Event display of a ZZγ candidate event, where the two Z bosons decay to two electrons and two muons. Green and orange/yellow boxes indicate energy deposits in the calorimeters, the green lines indicate the two electrons, and red lines indicate the two muons. The photon is indicated by a purple cone. (Image: ATLAS Collaboration/CERN)

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Evidence of ZZy production with the ATLAS detector (arXiv:2602.17165, see figures)
SM@LHC presentation by Anke Ackermann: Measurement of ZZγ production with the ATLAS detector
Shining light on the Weak force: ATLAS observes WWγ production, Physics Briefing, October 2025
Massive vector bosons also come in triplets, Physics Briefing, January 2025
Three’s no crowd: ATLAS measures tri-boson production, Physics Briefing, August 2023

ATLAS records vital low-intensity data during special LHC run

Katarina Anthony — Thu, 07 May 2026 07:55:30 +0000

ATLAS records vital low-intensity data during special LHC run

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Katarina Anthony Thu, 07/05/2026 - 09:55

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For three weeks in April 2026, the Large Hadron Collider (LHC) did something counterintuitive: it eased off the throttle. Instead of maximising collision rates, the accelerator entered a deliberately quieter operating mode. This shift offered the ATLAS Collaboration a rare opportunity to study fundamental particles in an exceptionally clean experimental environment.

In its usual configuration, the LHC is designed for intensity. Every time the proton beams cross, around 60 interactions take place simultaneously. While this is vital for hunting extremely rare processes, it also creates a dense and noisy environment that can obscure more subtle effects. Last month, the LHC significantly reduced the average number of interactions for a special operation period (a “low-μ run”), providing a unique, invaluable dataset for physicists to explore.

Their top priority is one of the most demanding measurements in particle physics: the mass of the W boson. This particle plays a fundamental role in the Standard Model, acting as a mediator of the weak force. Its mass is closely related to the masses of nature’s heaviest known fundamental particles, the top quark and the Higgs boson. If additional heavy particles exist, the W-boson mass might deviate from the Standard Model prediction, making a precise measurement an essential goal for physicists.

Side-by-side comparison of proton–proton collisions recorded with the LHC operating in a low-μ configuration (left, average of 3 simultaneous interactions) and at nominal intensity (right, average of 64 simultaneous interactions). Charged-particle tracks in the inner detector are shown as orange lines and energy deposits in the calorimeters as coloured (green, teal, yellow) boxes. Both events are consistent with involving the production of a W boson decaying into a muon and neutrino, with the reconstructed muon track shown as a red line and the neutrino as a white dashed line. The comparison illustrates the much cleaner environment of low-μ operation, with significantly fewer overlapping tracks and reduced detector activity. (Image: ATLAS Collaboration/CERN)

Measuring the W boson is especially challenging because its decay produces a neutrino, which escapes the experiment undetected. Physicists have to indirectly reconstruct the neutrino’s presence by balancing momentum in each event – a task made significantly harder in “noisy” conditions. With a reduced number of overlapping interactions, a low-μ dataset will give physicists a much clearer view of each W-boson decay. While ATLAS researchers have successfully measured the W-boson mass with sub-permille precision (±0.02%), this “quiet” dataset – with over 15 millions W bosons decaying to electron or muon – offers an important opportunity for them to reduce some of the largest remaining uncertainties.

Beyond this flagship measurement, the low-μ run enables several other important goals: studying subtle effects of the strong interaction; performing delicate measurements in flavour physics; and collecting key data for calibrating the detector to improve a whole range of future ATLAS measurements. While the High-Luminosity LHC upgrade will push the accelerator to higher intensity, this successful low-intensity run highlights the flexibility of the LHC and the diverse research approach taken by the ATLAS Collaboration.

Comparison of two proton–proton collision events recorded with the LHC operating in the low-μ configuration (left, average of 3 simultaneous interactions) and at nominal intensity (right, average of 64 simultaneous interactions). Both events are consistent with a Z boson decaying into a pair of muons, with the reconstructed muon tracks shown as cyan lines. (Image: ATLAS Collaboration/CERN)

About the banner image: Display of a collision event recorded by the ATLAS experiment during the low-μ run, consistent with a W-boson decaying into an electron (green line) and neutrino (white dashed line). (Image: ATLAS Collaboration/CERN)

Learn more

CERN Seminar by S. Clawson and B. Gilbert: Nuclear cross sections at the frontier: from cosmic rays to spatially resolved nuclear structure with ATLAS
Accelerator Report: Excellent performance at the LHC, CERN News, March 2026
Exploring the Weak Force with ATLAS, ATLAS News, November 2023
New ATLAS result weighs in on the W boson, ATLAS Physics Briefing, March 2023

ATLAS maps the rare four-way intersections of the weak force

Katarina Anthony — Fri, 24 Apr 2026 13:26:07 +0000

ATLAS maps the rare four-way intersections of the weak force

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Katarina Anthony Fri, 24/04/2026 - 15:26

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vector boson scattering

Despite its resounding success at describing high-energy phenomena, the Standard Model of particle physics provides no explanation for some of the fundamental questions of the Universe, such as the nature of dark matter or the origin of the matter–antimatter asymmetry. With no new fundamental particles beyond the Higgs boson yet observed at the LHC, physicists are searching for subtle deviations in the behaviour of known particles caused by potential new particles or forces.

Figure 1: Representative Feynman diagrams for vector-boson scattering with quartic-gauge boson vertices (left) and tri-boson production with quartic-gauge vertices (right). The aQGCs act on those diagrams through anomalous values or vertices forbidden in the Standard Model. (Image: ATLAS Collaboration/CERN)

The self-interactions of W and Z bosons – the carriers of the weak force – are tightly constrained in the Standard Model, making them uniquely sensitive to deviations from Standard Model predictions at high energies. By studying rare LHC processes such as vector boson scattering (VBS) – where two vector bosons (photons, W or Z bosons) scatter off one another – and tri-boson production – the simultaneous production of three vector bosons – physicists are able to directly examine these interactions. In particular, they can measure quartic gauge couplings, where four bosons interact simultaneously (see Figure 1).

The ATLAS Collaboration has released a new search for anomalous quartic gauge couplings (aQGCs) that combines eight separate analyses of VBS and tri-boson production in the full LHC Run-2 dataset (2015–2018). The search relies on the Éboli model, a framework that classifies all possible aQGCs in terms of 17 parameters. Think of these parameters as different "dials" that could be turned up if new phenomena are present. Researchers were able to harmonise the Éboli model treatment across all eight analyses, leading to consistent and comprehensive coverage of its parameters.

ATLAS reports the first combination of VBS and triboson analyses providing the world's best constraints on anomalous electroweak boson quartic self-couplings.

The combination is presented as confidence intervals on the 17 Éboli model parameters, obtained by varying one or two coefficients at a time while fixing the others to zero (see Figure 2). The combined limits are up to 96% tighter than the best previously published individual constraints, establishing them as the best constraints to date. Additional theoretical constraints, which ensure all parameter values are physically possible (e.g. ensuring no negative values), were compared to these experimental limits (see Figure 3).

This analysis represents the most complete and rigorous experimental study of aQGCs performed to date. It establishes a new benchmark for studies of vector boson self-interactions and will serve as a foundation for future global combinations. Looking ahead, ATLAS researchers will extend the search in analyses of the larger Run-3 dataset and prepare for the High-Luminosity LHC, further increasing sensitivity to possible signs of new physics beyond the Standard Model.

Figure 2: Combined constraints on anomalous quartic gauge couplings. The top panel illustrates the contribution of each analysis to the total sensitivity, showing the complementarity of the different analyses. The middle panel shows the expected and observed 68% and 95% confidence level intervals from the combined fit. The bottom panel shows, for each parameter, the maximum energy scale probed by the constraints for two illustrative values of the parameters (blue bars). In this figure, a 1.5 TeV cutoff (unitarization) is applied to the anomalous coupling contributions to ensure the results remain physically consistent with theoretical constraints. (Image: ATLAS Collaboration/CERN)

Figure 3: Expected (red) and observed (black) 68% (dashed) and 95% (solid) confidence level contours when considering two parameters simultaneously in the bottom left panel. Theoretical constraints preserving positivity are shown as blue dashed regions. The surrounding plots display individual likelihood scans considering one parameter at a time. (Image: ATLAS Collaboration/CERN)

About the banner image: Visualisation of a candidate event for the electroweak production of two Z bosons in association with two jets (ZZjj). The Z bosons decay into two muons (red tracks) and two electrons (green tracks). Energy deposits in the electromagnetic calorimeter associated with the electrons are visible as green blocks, while the red lines extending through the outer layers of the detector identify the muons. The two yellow cones illustrate the jets produced in the forward and backward regions of the detector signature of the VBS topology. (Image: ATLAS Collaboration/CERN)

Learn more

Combined effective field theory interpretation of measurements sensitive to quartic gauge boson couplings in proton-proton collisions at 13 TeV with the ATLAS detector (arXiv:2603.18630, see figures)
Mapping the genuine bosonic quartic coupling (O. J. P. Eboli, M. C. Gonzalez-Garcia, arXiv:1604.03555)
Shining light on the Weak force: ATLAS observes WWγ production, Physics Briefing, October 2025
ATLAS probes the Higgs mechanism in the scattering of W boson, Physics Briefing, April 2025
Unraveling Nature's secrets: vector boson scattering at the LHC, ATLAS Feature, September 2020