Press Statements

The first high-energy collisions between light nuclei at the Large Hadron Collider confirm the unusual “bowling-pin” shape of neon nuclei and offer up a new tool to study the extreme state of matter produced in the aftermath of the Big Bang.

Artistic rendering of quark–gluon plasma (Image: CERN)

This summer, the Large Hadron Collider (LHC) took a breath of fresh air. Normally filled with beams of protons, the 27-km ring was reconfigured to enable its first oxygen–oxygen and neon–neon collisions. First results from the new data, recorded over a period of six days by the ALICE, ATLAS, CMS and LHCb experiments, were presented during the Initial Stages conference held in Taipei, Taiwan, on 7–12 September.

Smashing atomic nuclei into one another allows physicists to study the quark–gluon plasma (QGP), an extreme state of matter that mimics the conditions of the Universe during its first microseconds, before atoms formed. Until now, exploration of this hot and dense state of free particles at the LHC relied on collisions between heavy ions (like lead or xenon), which maximise the size of the plasma droplet created.

Collisions between lighter ions, such as oxygen, open a new window on the QGP to better understand its characteristics and evolution. Not only are they smaller than lead or xenon, allowing a better investigation of the minimum size of nuclei needed to create the QGP, but they are less regular in shape. A neon nucleus, for example, is predicted to be elongated like a bowling pin – a picture that has now been brought into sharper focus thanks to the new LHC results.

The experiments focused on measurements of subtle patterns in the angles and directions of the particles flying outward as the QGP droplet expands and cools, which are caused by small distortions in the original collision zone. Remarkably, these “flow” patterns can be described using the same fluid-dynamics calculations that are used to model everyday fluids, allowing researchers to probe both the properties of the QGP and the geometry of the colliding nuclei. Accurate model predictions enable a more precise exploration of flow in oxygen–oxygen and neon–neon collisions than in proton–proton and proton–lead collisions.

ALICE, which specialises in the study of the QGP, as well as the general-purpose experiments ATLAS and CMS, have measured sizeable elliptic and triangular flow in oxygen–oxygen and neon–neon collisions, and found that these depend strongly on whether the collisions are glancing or head-on. The level of agreement between theory and data is comparable to that obtained for collisions of heavier xenon and lead ions, despite the much smaller system size. This provides strong evidence that flow in oxygen–oxygen and neon–neon collisions is driven by nuclear geometry, supporting the bowling-pin structure of the neon nucleus and demonstrating that hydrodynamic flow emerges robustly across collision systems at the LHC.

Complementary results presented last week by the LHCb collaboration confirm the bowling-pin shape of the neon nucleus. The results are based on lead–argon and lead–neon collisions in a fixed-target configuration, using data recorded in 2024 with its SMOG apparatus. The LHCb collaboration has also started to analyse the oxygen–oxygen and neon–neon collision data.

“Taken together, these results bring fresh perspectives on nuclear structure and how matter emerged after the Big Bang,” says CERN Director for Research and Computing Joachim Mnich.

This media update was originally published on the CERN Press website (English).

Learn more

Bowling balls vs. bowling pins? ATLAS studies the unique shape of neon ions, ATLAS Physics Briefing, 8 July 2025
Hunting for jet quenching in collisions between oxygen nuclei, ATLAS Physics Briefing, 8 July 2025

Elusive romance of top-quark pairs observed at the LHC

Katarina Anthony — Tue, 08 Jul 2025 09:18:24 +0000

Elusive romance of top-quark pairs observed at the LHC

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The CMS and ATLAS experiments at CERN’s Large Hadron Collider have observed an unforeseen feature in the behaviour of top quarks that suggests that these heaviest of all elementary particles form a fleeting union.

Geneva, 8 July. An unforeseen feature in proton-proton collisions previously observed by the CMS experiment at CERN’s Large Hadron Collider (LHC) has now been confirmed by its sister experiment ATLAS. The result, reported yesterday at the European Physical Society’s High-Energy Physics conference in Marseille, suggests that top quarks – the heaviest and shortest-lived of all the elementary particles – can momentarily pair up with their antimatter counterparts to produce a “quasi-bound-state” called toponium. Further input based on complex theoretical calculations of the strong nuclear force -- called quantum chromodynamics (QCD) -- will enable physicists to understand the true nature of this elusive dance.

High-energy collisions between protons at the LHC routinely produce top quark–antiquark pairs. Measuring the probability, or cross section, of this process is both an important test of the Standard Model of particle physics and a powerful way to search for the existence of new particles that are not described by the theory.

Last year, CMS researchers were analysing a large sample of top quark–antiquark production data collected from 2016 to 2018 to search for new types of Higgs bosons when they observed something unusual. The team saw a surplus of top quark–antiquark pairs, which is often considered as a smoking gun for the presence of new particles. Intriguingly, the excess appeared at the very minimum energy required to produce such a pair of top quarks. This led the team to consider an alternative hypothesis of something that had long been considered too difficult to detect at the LHC: a short-lived union of a top quark and a top antiquark.

The top quark is typically a loner. While other quarks can get together to form bound states called hadrons, the top quark’s extremely short lifetime means that it typically decays almost instantly – disappearing before it can form a bound state. But quantum mechanics makes it possible for the top quark-antiquark pair to occasionally survive long enough that, if produced almost at rest with respect to each other, they can exchange gluons (messengers of the strong force) that bind them into the toponium state.

Basing itself on a simplified toponium production hypothesis, CMS measured the cross section for the top quark–antiquark excess to be 8.8 picobarns (pb) with an uncertainty of about 1.3 pb. This passed the “five sigma” level of certainty required to claim a discovery in particle physics and made it extremely unlikely that the excess over the background-only prediction is just a statistical fluctuation.

While there is no doubt that an unforeseen phenomenon is present in the LHC data, the challenge is to be certain of its underlying cause.

“The observation of a non-relativistic QCD effect that was thought to be too difficult to detect is a great triumph for the LHC experiment programme,” said CMS spokesperson Gautier Hamel de Monchenault. “We keenly anticipate further rich interactions with our theory colleagues so that we may learn more about this fascinating corner of the Standard Model.”

In examining the full LHC Run-2 dataset collected from 2015 to 2018, the ATLAS collaboration has now seen the same effect. The ATLAS data rejects models that ignore the formation of a quasi-bound-state with a significance of 7.7 sigma and determines the production cross section of the top quark-antiquark excess to be 9.0± 1.3 pb, in close agreement with CMS.

While there is no doubt that an unforeseen phenomenon is present in the LHC data, the challenge is to be certain of its underlying cause. An alternative or additional possibility to the formation of toponium could be, for example, the existence of a new particle with a mass close to twice that of the top quark which is produced in collisions between gluons and decays to a top quark-antiquark pair. The conclusive interpretation of this new phenomenon will rely on accurate modelling of how quarks and gluons behave in the complex environment of high-energy proton-proton collisions, involving state-of-the art QCD calculations.

“For a long time, it was considered experimentally unfeasible to measure this subtle effect at the LHC, since events close to the production threshold make up only a small fraction of the top-pairs produced and are difficult to spot in the data,” said ATLAS spokesperson Stéphane Willocq. “However, thanks to the wealth of proton-proton data recorded during Run 2 of the LHC and thanks to advances in analysis techniques, this long-held assumption is now being overturned.”

If the toponium hypothesis is confirmed, its discovery would add a new twist to the story of quarkonia–quarkonium is a term for unstable states formed from pairings of heavy quarks and antiquarks of the same flavour. Charmonium (charm–anticharm) was discovered in 1974, sparking the “November Revolution” in particle physics, and bottomonium (bottom–antibottom) was discovered three years later, both at laboratories in the United States.

“These impressive results from ATLAS and CMS prove that there is still much to learn about the Standard Model of Particle Physics at high energies,” said CERN Director of Research and Computing, Joachim Mnich. “They show that high-precision measurements, many of which were never thought possible at a hadron collider, can reveal remarkably subtle phenomena that deepen our understanding of nature.”

With the ongoing Run 3 of the LHC due to deliver significantly more data, the ATLAS and CMS collaborations are set to deepen the exploration of the strong force via top quark-antiquark interactions in the non-relativistic regime.

This press release was originally published on the CERN Press website (English, French).

About the banner image: Artist’s impression of the short-lived union of a top quark and a top antiquark formed by the exchange of gluons. (Image: D. Dominguez/CERN)

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Observation of a cross-section enhancement near the tt production threshold in 13 TeV proton-proton collisions with the ATLAS detector (ATLAS-CONF-2025-008)
CMS Collaboration: Observation of a pseudoscalar excess at the top quark pair production threshold
Bound to be discovered? ATLAS explores top-quark interactions near threshold, ATLAS Physics Briefing, 7 July 2025

ATLAS gets under the hood of the Higgs mechanism

Katarina Anthony — Thu, 10 Apr 2025 12:09:14 +0000

ATLAS gets under the hood of the Higgs mechanism

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The detection of longitudinally polarised W boson production at the Large Hadron Collider is an important step towards understanding how the primordial electroweak symmetry broke, giving rise to the masses of elementary particles

CERN

vector bosons

Display of a candidate event for the production of two W+ bosons via vector-boson scattering, followed by their decay into two muons and two muon neutrinos. The muons are represented by the red lines in the inner detector and the muon spectrometer, and the two jets by the yellow cones. The direction of the missing transverse energy associated with the two neutrinos is indicated by the dashed grey line. (Image: ATLAS Collaboration/CERN)

The discovery of the Higgs boson by the ATLAS and CMS collaborations at CERN in 2012 opened a new window on the innermost workings of the Universe. It revealed the existence of a mysterious, ancient field with which elementary particles interact to acquire their all-important masses. This process is governed by a delicate mechanism called electroweak symmetry breaking, which was first proposed in 1964 but remains among the least understood phenomena of the Standard Model of particle physics. To probe this critical mechanism in the evolution of the Universe, physicists require a very large dataset of high-energy particle collisions.

Last week, at the Rencontres de Moriond conference, the ATLAS Collaboration brought physicists a step closer to understanding the nature of the electroweak symmetry-breaking mechanism. Using the full proton-proton collision dataset from LHC Run 2, which was collected at an energy of 13 TeV from 2015 to 2018, the team presented the first evidence of a key process involving the W boson – one of the mediators of the weak force.

In the Standard Model of particle physics, the electromagnetic and the weak interactions are two sides of the same coin, unified as the electroweak interaction. It is thought that the electroweak interaction prevailed in the immediate aftermath of the Big Bang, when the Universe was extremely hot. But the symmetry between the two interactions somehow got broken, since the carriers of the weak interaction, the W and Z bosons, are observed to be massive, whereas the photon, which mediates the electromagnetic interaction, is massless. The breaking of this symmetry is realised in the Standard Model through the Brout-Englert-Higgs (BEH) mechanism. The discovery of the Higgs boson provided the first experimental confirmation of this mechanism. The next step is to measure the properties of the new particle, in particular how strongly it interacts with other elementary particles. These measurements are currently under way, with the aim of confirming that the masses of elementary matter particles are also the result of their interaction with the BEH field.

But the BEH mechanism also makes other predictions. Two processes in particular need to be measured to confirm that the mechanism is indeed as the Standard Model predicts: the interaction between longitudinally polarised W or Z bosons and the interaction of the Higgs boson with itself. While studies of Higgs self-interaction are expected to be possible at the earliest with the High-Luminosity LHC, which is due to begin operation in 2030, and will require a future collider to be pinned down in detail, first studies of the scattering of longitudinally polarised gauge bosons should be possible earlier.

For particles, polarisation refers to the way in which their spin is oriented in space. Longitudinally polarised particles have spin perpendicular to the direction of their momentum, something that is only possible for particles that have mass. The existence of longitudinally polarised W and Z bosons (W_L and Z_L) is a direct consequence of the BEH mechanism, and the way in which these states interact with each other is therefore a very sensitive test of how the electroweak symmetry is broken. Studying this interaction should allow physicists to tell whether the symmetry breaking is realised via the minimal BEH mechanism or whether some new physics beyond the Standard Model is involved. The new ATLAS result provides a first glimpse of this elusive process.

The W_L-W_Linteraction can be probed experimentally in proton-proton collisions by studying a process called vector-boson scattering (VBS). The VBS process can be visualised as a quark in each of the incoming protons emitting a W boson and those two W bosons interacting with each other, producing a pair of W or Z bosons. VBS can be identified by looking for collisions containing the decay products of the two bosons together with the two quarks that participated in the interaction forming two jets of particles going in opposite directions.

The new ATLAS analysis targets collisions in which the two W bosons decay into an electron or a muon and their respective neutrinos. In order to suppress backgrounds, mostly from processes involving top-quark pair production, both leptons are required to be of the same electrical charge. The experimental signature is thus a pair of same-charge leptons (electron-electron, muon-muon or electron-muon), two particle “jets” with opposite directions produced by the decays of the quarks, and missing energy coming from the undetectable neutrinos.

Once candidates for the VBS process are selected, the polarisation of the W bosons has to be determined. This is very challenging and can be done only via a thorough analysis of correlations between the directions of the reconstructed electrons and muons and the properties of other particles produced in the interaction. Dedicated neural networks have been trained to distinguish between transverse and longitudinal polarisation and made it possible to extract the final result: evidence with the statistical significance of 3.3 sigma that at least one of the two W bosons was longitudinally polarised.

“This measurement is a milestone in the studies of the core physics value via polarised boson interactions in vector-boson scattering processes,” says Yusheng Wu, the ATLAS Standard Model group convener. “It marks a path towards the eventual study of longitudinally polarised boson scattering using LHC Run-3 and HL-LHC data.”

This media update was originally published on the CERN Press website (English, French).

Learn more

ATLAS probes the Higgs mechanism in the scattering of W boson, ATLAS Physics Briefing, 4 April 2025

ATLAS Collaboration awarded Breakthrough Prize in Fundamental Physics

Katarina Anthony — Mon, 07 Apr 2025 06:30:18 +0000

ATLAS Collaboration awarded Breakthrough Prize in Fundamental Physics

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ATLAS Collaboration

ATLAS

The ATLAS Collaboration at CERN has been awarded the Breakthrough Prize in Fundamental Physics for its pioneering studies of the high-energy collisions from the Large Hadron Collider (LHC). The prize, shared with the ALICE, CMS and LHCb Collaborations, recognises the extraordinary contributions of 13,508 international researchers who have pushed the boundaries of particle physics to unprecedented heights. The prize celebrates the collaborative effort to establish the Brout-Englert-Higgs mechanism of spontaneous electroweak symmetry breaking, test the Standard Model with remarkable precision, uncover rare and exotic particle interactions, and search for new physics phenomena that could reshape our understanding of the universe. Through the CERN & Society Foundation, the prize will fund PhD scholarships to support the next generation of ATLAS scientists as they build on this legacy of discovery.

“The Breakthrough Prize is a testament to the dedication and ingenuity of the ATLAS Collaboration and our colleagues across the LHC experiments,” says Stephane Willocq, ATLAS Spokesperson (University of Massachusetts Amherst). “In particular, Run 2 was a transformative period for particle physics. The results we achieved with its dataset have deepened our understanding of the Higgs boson, the Standard Model and the fundamental symmetries of Nature.”

Probing new frontiers with Run 2

Let’s turn back the clock to 2012, when the ATLAS and CMS Collaborations announced the discovery of a brand-new particle: the Higgs boson. This groundbreaking achievement earned ATLAS, CMS, and LHC scientists, including former ATLAS spokespeople Peter Jenni and Fabiola Gianotti, the 2013 Special Breakthrough Prize in Fundamental Physics. The discovery of this final missing piece of the Standard Model was a triumph decades in the making, yet it marked only the beginning of the ATLAS Collaboration’s rich and ongoing scientific journey.

The second run of the LHC (2015–2018) would be a monumental leap forward for high-energy physics. For the first time, protons would collide at a centre-of-mass energy of 13 TeV — 60% higher than that during Run 1 and, at the time, the highest energy ever achieved in a particle collider. It would be a true test of the accelerator’s capabilities, and would call on precise operational expertise to keep it running at peak performance. Meanwhile, the ATLAS experiment had also undergone several upgrades during the LHC’s first long shutdown, including new event-selection capabilities and the installation of the Insertable B-Layer (IBL). The IBL, a pixel layer equipped with radiation-hard sensors positioned just 3.3 cm from the beam, significantly enhanced the resolution of the ATLAS inner tracking detector.

The stage was set and teams were poised for action — but what would Nature have in store? Over the following decade, thanks to the LHC’s stellar performance and ATLAS’ record data-taking and data-quality efficiency, physicists would venture into completely uncharted territories in particle physics.

The prize, shared with the ALICE, CMS and LHCb Collaborations, recognises the extraordinary contributions of 13,508 international researchers who have pushed the boundaries of particle physics to unprecedented heights.

Mapping the Higgs boson

The Breakthrough Prize specifically recognised ATLAS for its precise measurements of the Higgs boson’s properties and its exploration of the Brout-Englert-Higgs (BEH) mechanism. “Each particle’s relationship with the Higgs boson is unique, providing critical tests of the Standard Model,” says Andreas Hoecker, former ATLAS Spokesperson (CERN). “Our goal with Run 2 was to understand as many of these as we could, creating a detailed map of the properties and interactions of the Higgs boson.”

The Collaboration made tremendous strides, observing and measuring all of the primary production and decay mechanisms of the Higgs boson, enabling ATLAS to derive the predicted non-universal, mass-dependent interaction strengths with other particles. These results emerged as early as 2018, with ATLAS detecting the Higgs boson decaying into b-quarks — a process accounting for more than half of all Higgs decays — and observing its rare associated production with top-quark pairs. This latter measurement provided direct insight into the top-quark Yukawa coupling, a key aspect of the Higgs mechanism. More recently, ATLAS measured the Higgs-boson mass with 0.09% precision and set direct constraints on its charm-quark Yukawa coupling, showcasing how a deeper understanding of the dataset and improved algorithms have led to significant advancements in Higgs physics.

“These achievements also underscore the transformative power of Run 2 in science,” says Guillaume Unal, ATLAS Deputy Spokesperson (CERN). “With just a few short years of data-taking, we were able to confirm the BEH mechanism of electroweak symmetry breaking and deepen our understanding of how particles acquire mass.”

Entering the precision era

The 13 TeV era saw a surge in the breadth and depth of ATLAS’ physics results. To date, the Collaboration has released 408 papers using the full Run-2 dataset, along with a similar number of results based on partial datasets. ATLAS observed numerous rare processes for the first time, with each iteration of analysis delivering more refined and precise results. This progress is driven not only by the sheer volume of data collected but also by improved reconstruction algorithms and the adoption of advanced analysis techniques, most prominently the widespread adoption of machine learning algorithms.

Among the many Standard Model results, ATLAS greatly expanded the understanding of the strong and electroweak forces. The Collaboration observed the simultaneous production of three W bosons and the production of W-boson pairs through the interaction of two photons — both processes that test the very foundations of the Standard Model. ATLAS also made the first observation of four-top-quark production, a process 4,000 times rarer than Higgs boson production, probing the boundaries of quantum chromodynamics (QCD) and the top quark’s role in the Standard Model.

“Run 2 also demonstrated the remarkable precision capabilities of the ATLAS experiment,” says Stephane. “From measuring the strong force coupling strength with 0.8% precision to the first observation of quantum entanglement at high energy, we pushed the boundaries of what was thought possible at the LHC.”

“The Breakthrough Prize is a testament to the dedication and ingenuity of the ATLAS Collaboration and our colleagues across the LHC experiments,” says Stephane Willocq, ATLAS Spokesperson.

While the Standard Model has so far withstood ATLAS’ rigorous testing, the absence of new particles at the TeV scale remains a mystery. Using 13 TeV data, ATLAS conducted extensive searches for new phenomena, including supersymmetry, heavy resonances and dark matter, setting stringent limits on these scenarios. Innovative techniques, such as jet substructure analysis and machine learning algorithms, enhanced the sensitivity of these searches, allowing researchers to explore uncharted territories in the high-energy frontier.

“While we’ve yet to find compelling evidence of new physics phenomena, these searches provide crucial direction for the field,” says Anna Sfyrla, ATLAS Deputy Spokesperson (University of Geneva). “They challenge us to rethink what lies beyond the Standard Model, explore new avenues and develop ever more inventive approaches. This mentality will be crucial as we tackle increasingly difficult searches for elusive new physics phenomena.”

Of course, Run 2 was about more than just colliding protons. ATLAS recorded its very first xenon-xenon collisions, and the highest-energy lead-lead collisions. These “heaviest” of collisions allowed researchers to observe one of the “lightest” processes in Nature, as physicists reported the first direct observation of light-by-light scattering in ultra-peripheral collisions. This has since developed into a growing field of study, offering opportunities to advance the physics of electroweak interactions and the search for new phenomena.

The next breakthrough

“The successes of Run 2 showcase the ingenuity of the ATLAS Collaboration — not only in collecting data with a detector of outstanding precision, but also in our relentless drive to improve our understanding of it,” says Andreas. “Many of the analyses described here were years in the making, requiring the development of pioneering techniques and advanced reconstruction methods. The fresh approaches evident in the research highlight an unwavering commitment to pushing the boundaries of scientific exploration.”

While the ATLAS Collaboration celebrates the recognition of the Breakthrough Prize, its focus remains firmly on the future. LHC Run 3 is currently underway and preparations for the High-Luminosity LHC are advancing rapidly. “Although Run 2 still has much to reveal, and analyses on the dataset are ongoing, the LHC’s journey is already taking ATLAS into uncharted waters,” concludes Stephane. “The HL-LHC is set to deliver 10 times more data in its lifetime, up to 2041. We are now preparing the ATLAS detectors of the future — designed to harness these unprecedented data and further push our understanding of the universe’s fundamental building blocks.”

About the image banner: Members of the ATLAS Collaboration outside the ATLAS Control Room. (Image: M. Struik/CERN)

Learn more

A transformative leap in physics: ATLAS results from LHC Run 2, ATLAS Feature, 7 April 2025
The LHC experiment collaborations at CERN receive Breakthrough Prize, CERN Press, 7 April 2025
Breakthrough Prize Announces 2025 Laureates, News, 5 April 2025

The LHC experiment collaborations at CERN receive Breakthrough Prize

Katarina Anthony — Mon, 07 Apr 2025 06:20:30 +0000

The LHC experiment collaborations at CERN receive Breakthrough Prize

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The Breakthrough Prize in Fundamental Physics was awarded to the ALICE, ATLAS, CMS and LHCb collaborations during a ceremony held in Los Angeles on 5 April

CERN

ATLAS collaboration

This weekend, the ALICE, ATLAS, CMS and LHCb collaborations at the Large Hadron Collider at CERN were honoured with the Breakthrough Prize in Fundamental Physics by the Breakthrough Prize Foundation. The prize is awarded to the four collaborations, which unite thousands of researchers from more than 70 countries, and concerns the papers authored based on LHC Run-2 data up to July 2024. It was received by the spokespersons who led the collaborations during that time.

The prize was awarded to the collaborations for their “detailed measurements of Higgs boson properties confirming the symmetry-breaking mechanism of mass generation, the discovery of new strongly interacting particles, the study of rare processes and matter-antimatter asymmetry, and the exploration of nature at the shortest distances and most extreme conditions at CERN’s Large Hadron Collider”.

“I am extremely proud to see the extraordinary accomplishments of the LHC collaborations honoured with this prestigious Prize,” said Fabiola Gianotti, Director-General of CERN. “It is a beautiful recognition of the collective efforts, dedication, competence and hard work of thousands of people from all over the world who contribute daily to pushing the boundaries of human knowledge.”

Following consultation with the experiments’ management teams, the Breakthrough Prize Foundation will donate the $3 million Prize to the CERN & Society Foundation. The Prize money will be used to offer grants for doctoral students from the collaborations’ member institutes to spend research time at CERN, giving them experience in working at the forefront of science and new expertise to bring back to their home countries and regions.

ATLAS and CMS are general-purpose experiments, which pursue the full programme of exploration offered by the LHC’s high-energy and high-intensity proton and ion beams. They jointly announced the discovery of the Higgs boson in 2012 and continue to investigate its properties.

"This prize recognises the collective vision and monumental effort of thousands of ATLAS collaborators worldwide," says ATLAS spokesperson Stephane Willocq. "Their talent and dedication, and the support of our public funding agencies, enabled the scientific breakthroughs that are being celebrated today. These results have transformed our understanding of the Universe at the most fundamental level.”

"CMS is deeply honoured to receive this prestigious prize,” said CMS spokesperson Gautier Hamel de Monchenault. “Through continuous innovation in exploiting the data from the Large Hadron Collider over the past fifteen years, the CMS collaboration is conducting a thorough characterisation of the Higgs boson, exploring the electroweak scale and beyond and probing the hot, dense state of nuclear matter that prevailed in the early Universe.”

ALICE studies quark-gluon plasma, a state of extremely hot and dense matter that existed in the first microseconds after the Big Bang, while LHCb explores minute differences between matter and antimatter, violation of fundamental symmetries and the complex spectra of composite particles (“hadrons”) made of heavy and light quarks, among other things.

“The ALICE collaboration is honoured to receive the Breakthrough Prize for the investigation of the properties of the hottest and densest matter available in a laboratory, quark-gluon plasma,” says ALICE spokesperson Marco Van Leeuwen. “The new grants funded through this prize will contribute to training the next generation of ALICE scientists.”

"The award of the 2025 Breakthrough Prize is a great honour for the LHCb collaboration. It underlines the importance of the many measurements made by the LHCb experiment in flavour physics and spectroscopy through the exploration of subtle differences between matter and antimatter and the discovery of several new heavy quark hadrons,” says LHCb spokesperson Vincenzo Vagnoni.

By performing these extraordinarily precise and delicate tests, the LHC experiments have pushed the boundaries of knowledge of fundamental physics to unprecedented limits. They will continue to do so with the upcoming upgrade of the Large Hadron Collider, the High-Luminosity LHC, which aims to ramp up the performance of the LHC, starting in 2030, in order to increase the potential for discoveries.

This press release was originally published on the CERN Press website (English).

Learn more

ATLAS Collaboration awarded Breakthrough Prize in Fundamental Physics, ATLAS Press Statement, 7 April 2025
A transformative leap in physics: ATLAS results from LHC Run 2, ATLAS Feature, 7 April 2025

LHC experiments at CERN observe quantum entanglement at the highest energy yet

Katarina Anthony — Wed, 18 Sep 2024 11:14:16 +0000

LHC experiments at CERN observe quantum entanglement at the highest energy yet

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The results open up a new perspective on the complex world of quantum physics

CERN

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physics results

Quantum entanglement is a fascinating feature of quantum physics – the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, which has no analogue in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell and pioneered quantum information science.

Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published today in Nature, the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet. First reported by ATLAS in September 2023 and since confirmed by two observations made by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.

"While particle physics is deeply rooted in quantum mechanics, the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable,” says ATLAS spokesperson Andreas Hoecker. “It paves the way for new investigations into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow."

The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study entanglement.

The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum traits to its decay particles. Physicists observe and use these decay products to infer the top quark’s spin orientation.

"While particle physics is deeply rooted in quantum mechanics, the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable,” says ATLAS spokesperson Andreas Hoecker.

To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018. In particular, they looked for pairs in which the two quarks are simultaneously produced with low particle momentum relative to each other. This is where the spins of the two quarks are expected to be strongly entangled.

The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted. By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations.

In its second study, the CMS collaboration also looked for pairs of top quarks in which the two quarks are simultaneously produced with high momentum relative to each other. In this domain, for a large fraction of top quark pairs, the relative positions and times of the two top quark decays are predicted to be such that classical exchange of information by particles traveling at no more than the speed of light is excluded, and CMS observed spin entanglement between top quarks also in this case.

“With measurements of entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible, we can test the Standard Model of particle physics in new ways and look for signs of new physics that may lie beyond it.” says CMS spokesperson Patricia McBride.

This press release was originally published on the CERN Press website (English, French).

About the banner image: Artistic visualisation of top-quark entanglement. The line between the particles emphasises the non-separability of the top-quark pair, which is produced by LHC collisions and recorded by ATLAS. (Image: Daniel Dominguez/CERN)

Learn more

Observation of quantum entanglement with top quarks at the ATLAS detector, Nature, 18 September 2024
ATLAS achieves highest-energy detection of quantum entanglement, ATLAS Physics Briefing, 23 September 2023

ATLAS measures strength of the strong force with record precision

Edite Prado Felgueiras — Mon, 25 Sep 2023 08:22:58 +0000

ATLAS measures strength of the strong force with record precision

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The result showcases the power of the LHC to push the precision frontier and improve our understanding of nature

Binding together quarks into protons, neutrons and atomic nuclei is a force so strong, it’s in the name. The strong force, which is carried by gluon particles, is the strongest of all fundamental forces of nature – the others being electromagnetism, the weak force and gravity. Yet, it’s the least precisely measured of these four forces. In a paper just submitted to Nature Physics, the ATLAS collaboration describes how it has used the Z boson, the electrically neutral carrier of the weak force, to determine the strength of the strong force with an unprecedented uncertainty of below 1%.

The strength of the strong force is described by a fundamental parameter in the Standard Model of particle physics called the strong coupling constant. While knowledge of the strong coupling constant has improved with measurements and theoretical developments made over the years, the uncertainty on its value remains orders of magnitude larger than that of the coupling constants for the other fundamental forces. A more precise measurement of the strong coupling constant is required to improve the precision of theoretical calculations of particle processes that involve the strong force. It is also needed to address important unanswered questions about nature. Could all of the fundamental forces be of equal strength at very high energy, indicating a potential common origin? Could new, unknown interactions be modifying the strong force in certain processes or at certain energies?

With a relative uncertainty of only 0.8%, the result is the most precise determination of the strength of the strong force made by a single experiment to date.

In its new study of the strong coupling constant, the ATLAS collaboration investigated Z bosons produced in proton–proton collisions at CERN's Large Hadron Collider (LHC) at a collision energy of 8 TeV. Z bosons are typically produced when two quarks in the colliding protons annihilate. In this weak-interaction process, the strong force comes into play through the radiation of gluons off the annihilating quarks. This radiation gives the Z boson a “kick” transverse to the collision axis (transverse momentum). The magnitude of this kick depends on the strong coupling constant. A precise measurement of the distribution of Z-boson transverse momenta and a comparison with equally precise theoretical calculations of this distribution allows the strong coupling constant to be determined.

In the new analysis, the ATLAS team focused on cleanly selected Z-boson decays to two leptons (electrons or muons) and measured the Z-boson transverse momentum via its decay products. A comparison of these measurements with theoretical predictions enabled the researchers to precisely determine the strong coupling constant at the Z-boson mass scale to be 0.1183 ± 0.0009. With a relative uncertainty of only 0.8%, the result is the most precise determination of the strength of the strong force made by a single experiment to date. It agrees with the current world average of experimental determinations and state-of-the-art calculations known as lattice quantum chromodynamics (see figure below).

The new ATLAS value of the strong coupling constant compared with other measurements. (Image: ATLAS Collaboration/CERN)

This record precision was accomplished thanks to both experimental and theoretical advances. On the experimental side, the ATLAS physicists achieved a detailed understanding of the detection efficiency and momentum calibration of the two electrons or muons originating from the Z-boson decay, which resulted in momentum precisions ranging from 0.1% to 1%. On the theoretical side, the ATLAS researchers used, among other ingredients, cutting-edge calculations of the Z-boson production process that consider up to four “loops” in quantum chromodynamics. These loops represent the complexity of the calculation in terms of contributing processes. Adding more loops increases the precision.

“The strength of the strong nuclear force is a key parameter of the Standard Model, yet it is only known with percent-level precision. For comparison, the electromagnetic force, which is 15 times weaker than the strong force at the energy probed by the LHC, is known with a precision better than one part in a billion,” says CERN physicist Stefano Camarda, a member of the analysis team. “That we have now measured the strong force coupling strength at the 0.8% precision level is a spectacular achievement. It showcases the power of the LHC and the ATLAS experiment to push the precision frontier and enhance our understanding of nature.”

This press release was originally published on the CERN Press website (english).

Learn more

A precise determination of the strong-coupling constant from the recoil of Z bosons with the ATLAS experiment at 8 TeV (arXiv:2309.12986)
ATLAS measures the strength of the strong force, ATLAS Physics Briefing, 23 March 2023

ATLAS sets record precision on Higgs boson’s mass

Edite Prado Felgueiras — Fri, 21 Jul 2023 14:54:08 +0000

ATLAS sets record precision on Higgs boson’s mass

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Press Statement

New result from the ATLAS experiment at CERN reaches the unprecedented precision of 0.09%

Candidate Higgs boson decays into two photons in the ATLAS experiment. (Image: ATLAS Collaboration/CERN)

In the 11 years since its discovery at the Large Hadron Collider (LHC), the Higgs boson has become a central avenue for shedding light on the fundamental structure of the Universe. Precise measurements of the properties of this special particle are among the most powerful tools physicists have to test the Standard Model, currently the theory that best describes the world of particles and their interactions. At the Lepton Photon Conference this week, the ATLAS collaboration reported how it has measured the mass of the Higgs boson more precisely than ever before.

The mass of the Higgs boson is not predicted by the Standard Model and must therefore be determined by experimental measurement. Its value governs the strengths of the interactions of the Higgs boson with the other elementary particles as well as with itself. A precise knowledge of this fundamental parameter is key to accurate theoretical calculations which, in turn, allow physicists to confront their measurements of the Higgs boson’s properties with predictions from the Standard Model. Deviations from these predictions would signal the presence of new or unaccounted-for phenomena. The Higgs boson’s mass is also a crucial parameter driving the evolution and the stability of the Universe’s vacuum.

The ATLAS and CMS collaborations have been making ever more precise measurements of the Higgs boson’s mass since the particle’s discovery. The new ATLAS measurement combines two results: a new Higgs boson mass measurement based on an analysis of the particle’s decay into two high-energy photons (the “diphoton channel”) and an earlier mass measurement based on a study of its decay into four leptons (the “four-lepton channel”).

With a precision of 0.11%, this diphoton-channel result is the most precise measurement to date of the Higgs boson’s mass from a single decay channel.

The new measurement in the diphoton channel, which combines analyses of the full ATLAS data sets from Runs 1 and 2 of the LHC, resulted in a mass of 125.22 billion electronvolts (GeV) with an uncertainty of only 0.14 GeV. With a precision of 0.11%, this diphoton-channel result is the most precise measurement to date of the Higgs boson’s mass from a single decay channel.

Summary of individual and combined Higgs-boson mass measurements. (Image: ATLAS Collaboration/CERN)

Compared to the previous ATLAS measurement in this channel, the new result benefits both from the full ATLAS Run 2 data set, which reduced the statistical uncertainty by a factor of two, and from dramatic improvements to the calibration of photon energy measurements, which decreased the systematic uncertainty by almost a factor of four to 0.09 GeV.

“The advanced and rigorous calibration techniques used in this analysis were critical for pushing the precision to such an unprecedented level,” says Stefano Manzoni, convener of the ATLAS electron-photon calibration subgroup. “Their development took several years and required a deep understanding of the ATLAS detector. They will also greatly benefit future analyses.”

When the ATLAS researchers combined this new mass measurement in the diphoton channel with the earlier mass measurement in the four-lepton channel, they obtained a Higgs boson mass of 125.11 GeV with an uncertainty of 0.11 GeV. With a precision of 0.09%, this is the most precise measurement yet of this fundamental parameter.

“This very precise measurement is the result of the relentless investment of the ATLAS collaboration in improving the understanding of our data,” says ATLAS spokesperson Andreas Hoecker. "Powerful reconstruction algorithms paired with precise calibrations are the determining ingredients of precision measurements. The new measurement of the Higgs boson’s mass adds to the increasingly detailed mapping of this critical new sector of particle physics."

This press release was originally published on the CERN Press website (english).

Learn more

Measurement of the Higgs boson mass with H→γγ decays in 140 fb⁻¹ of 13 TeV proton-proton collisions with the ATLAS detector (ATLAS-CONF-2023-036)
Combined measurement of the Higgs boson mass from the H→γγ and H→ZZ∗→4ℓ decay channels with the ATLAS detector using 7, 8 and 13 TeV proton-proton collision data (ATLAS-CONF-2023-037)
ATLAS measures Higgs boson mass with unprecedented precision, ATLAS Physics Briefing, 21 July 2023
Resources: Event Display image collection

LHC experiments see first evidence of a rare Higgs boson decay

Edite Prado Felgueiras — Fri, 26 May 2023 09:13:26 +0000

LHC experiments see first evidence of a rare Higgs boson decay

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Press Statement

The ATLAS and CMS collaborations have joined forces to establish the first evidence of the rare decay of the Higgs boson into a Z boson and a photon

The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 marked a significant milestone in particle physics. Since then, the ATLAS and CMS collaborations have been diligently investigating the properties of this unique particle and searching to establish the different ways in which it is produced and decays into other particles.

At the Large Hadron Collider Physics (LHCP) conference this week, ATLAS and CMS report how they teamed up to find the first evidence of the rare process in which the Higgs boson decays into a Z boson, the electrically neutral carrier of the weak force, and a photon, the carrier of the electromagnetic force. This Higgs boson decay could provide indirect evidence of the existence of particles beyond those predicted by the Standard Model of particle physics.

The decay of the Higgs boson into a Z boson and a photon is similar to that of a decay into two photons. In these processes, the Higgs boson does not decay directly into these pairs of particles. Instead, the decays proceed via an intermediate "loop" of “virtual” particles that pop in and out of existence and cannot be directly detected. These virtual particles could include new, as yet undiscovered particles that interact with the Higgs boson.

The Standard Model predicts that, if the Higgs boson has a mass of around 125 billion electronvolts, approximately 0.15% of Higgs bosons will decay into a Z boson and a photon. But some theories that extend the Standard Model predict a different decay rate. Measuring the decay rate therefore provides valuable insights into both physics beyond the Standard Model and the nature of the Higgs boson.

"Each particle has a special relationship with the Higgs boson, making the search for rare Higgs decays a high priority." – ATLAS physics coordinator Pamela Ferrari

Previously, using data from proton–proton collisions at the LHC, ATLAS and CMS independently conducted extensive searches for the decay of the Higgs boson into a Z boson and a photon. Both searches used similar strategies, identifying the Z boson through its decays into pairs of electrons or muons – heavier versions of electrons. These Z boson decays occur in about 6.6% of the cases.

In these searches, collision events associated with this Higgs boson decay (the signal) would be identified as a narrow peak, over a smooth background of events, in the distribution of the combined mass of the decay products. To enhance the sensitivity to the decay, ATLAS and CMS exploited the most frequent modes in which the Higgs boson is produced and categorised events based on the characteristics of these production processes. They also used advanced machine-learning techniques to further distinguish between signal and background events.

In a new study, ATLAS and CMS have now joined forces to maximise the outcome of their search. By combining the data sets collected by both experiments during the second run of the LHC, which took place between 2015 and 2018, the collaborations have significantly increased the statistical precision and reach of their searches.

This collaborative effort resulted in the first evidence of the Higgs boson decay into a Z boson and a photon. The result has a statistical significance of 3.4 standard deviations, which is below the conventional requirement of 5 standard deviations to claim an observation. The measured signal rate is 1.9 standard deviations above the Standard Model prediction.

“Each particle has a special relationship with the Higgs boson, making the search for rare Higgs decays a high priority,” says ATLAS physics coordinator Pamela Ferrari. "Through a meticulous combination of the individual results of ATLAS and CMS, we have made a step forward towards unravelling yet another riddle of the Higgs boson."

“The existence of new particles could have very significant effects on rare Higgs decay modes,” says CMS physics coordinator Florencia Canelli. “This study is a powerful test of the Standard Model. With the ongoing third run of the LHC and the future High-Luminosity LHC, we will be able to improve the precision of this test and probe ever rarer Higgs decays.”

This press release was originally published on the CERN Press website (english).

Learn more

Evidence for the Higgs boson decay to a Z boson and a photon at the LHC (ATLAS-CONF-2023-025)
LHC experiments see first evidence for rare Higgs boson decay into two different bosons, ATLAS Physics Briefing, 26 May 2023
Resources: Event Display image collection

Improved ATLAS result weighs in on the W boson

Steven Goldfarb — Wed, 22 Mar 2023 13:16:25 +0000

Improved ATLAS result weighs in on the W boson

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Press Statement

An improved ATLAS measurement of the W boson mass is in line with the Standard Model of particle physics

The W boson, a fundamental particle that carries the charged weak force, is the subject of a new precision measurement of its mass by the ATLAS experiment at CERN.

The measured value of the W-boson mass is compared to other published results. The vertical bands show the Standard Model prediction, and the horizontal bands and lines show the statistical and total uncertainties of the published results. (Image: ATLAS Collaboration/CERN)

The preliminary result, reported in a new conference note presented today at the Rencontres de Moriond conference, is based on a reanalysis of a sample of 14 million W boson candidates produced in proton–proton collisions at the Large Hadron Collider (LHC), CERN’s flagship particle accelerator.

The new ATLAS measurement concurs with, and is more precise than, all previous W mass measurements except one – the latest measurement from the CDF experiment at the Tevatron, a former accelerator at Fermilab.

Together with its electrically neutral counterpart, the Z boson, the electrically charged W boson mediates the weak force, a fundamental force that is responsible for a form of radioactivity and initiates the nuclear fusion reaction that powers the Sun.

The particle’s discovery at CERN 40 years ago helped to confirm the theory of the electroweak interaction that unifies the electromagnetic and weak forces. This theory is now a cornerstone of the Standard Model of particle physics. CERN researchers who enabled the discovery were awarded the 1984 Nobel Prize in physics.

Since then, experiments at particle colliders at CERN and elsewhere have measured the W boson mass ever more precisely. In the Standard Model, the W boson mass is closely related to the strength of the electroweak interactions and the masses of the heaviest fundamental particles, including the Z boson, the top quark and the Higgs boson. In this theory, the particle is constrained to weigh 80354 million electronvolts (MeV), within an uncertainty of 7 MeV.

"The W mass measurement is among the most challenging precision measurements performed at hadron colliders." – ATLAS spokesperson Andreas Hoecker

Any deviation of the measured mass from the Standard Model prediction would be an indicator of new physics phenomena, such as new particles or interactions. To be sensitive to such deviations, mass measurements need to be extremely precise.

In 2017, ATLAS released its first measurement of the W boson mass, which was determined using a sample of W bosons recorded by ATLAS in 2011, when the LHC was running at a collision energy of 7 TeV. The W boson mass came out at 80370 MeV, with an uncertainty of 19 MeV.

At the time, this result represented the most precise W boson mass value ever obtained by a single experiment, and was in good agreement with the Standard Model prediction and all previous experimental results, including those from experiments at the Large Electron–Positron Collider (LEP), the LHC’s predecessor at CERN.

Last year, the CDF collaboration at Fermilab announced an even more precise measurement, based on an analysis of its full dataset collected at the Tevatron. The result, 80434 MeV with an uncertainty of 9 MeV, differed significantly from the Standard Model prediction and from the other experimental results, calling for more measurements to try to identify the cause of the difference.

In its new study, ATLAS reanalysed its 2011 sample of W bosons, improving the precision of its previous measurement. The new W boson mass, 80360 MeV with an uncertainty of 16 MeV, is 10 MeV lower than the previous ATLAS result and 16% more precise. The result is in agreement with the Standard Model.

To attain this result, ATLAS used an advanced data-fitting technique to determine the mass, as well as more recent, improved versions of what are known as the parton distribution functions of the proton. These functions describe the sharing of the proton’s momentum amongst its constituent quarks and gluons. In addition, ATLAS verified the theoretical description of the W boson production process using dedicated LHC proton–proton runs.

“Due to an undetected neutrino in the particle’s decay, the W mass measurement is among the most challenging precision measurements performed at hadron colliders. It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modelling uncertainties,” says ATLAS spokesperson Andreas Hoecker. “This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions.”

Further measurements of the W boson mass are expected from ATLAS and CMS and from LHCb, which has also recently weighed the boson.

This press release was originally published on the CERN Press website (English, French).

Learn more

Improved W boson Mass Measurement using 7 TeV Proton-Proton Collisions with the ATLAS Detector (ATLAS-CONF-2023-004, see figures)
New ATLAS result weighs in on the W boson, ATLAS Physics Briefing, 23 March 2023
Resources: Event Display image collection, Animated Event Display (video), Interactive Event Display

ATLAS Experiment records “first physics” at new high-energy frontier

Katarina Anthony — Tue, 05 Jul 2022 06:53:29 +0000

ATLAS Experiment records “first physics” at new high-energy frontier

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Press Statement

ATLAS Collaboration

Run 3

“We have proton collisions in the ATLAS experiment.” At 16.47 CEST, the Large Hadron Collider (LHC) officially kicked off its third period of operation (Run 3). The LHC is colliding proton beams at a world-record-breaking energy of 13.6 tera electron volts (TeV). The higher beam energy and intensity of Run 3 will allow the ATLAS experiment to push the very limits of its physics research.

Today’s declaration of “stable beams” marks the end of the LHC’s re-commissioning period, which began in April 2022. Within a month from today, the LHC will have reached its full Run-3 operating intensity. ATLAS has now switched on all systems to begin recording data to be used for physics analysis. The LHC will run around the clock for nearly 4 years, with only some technical stops during the winter months, delivering an unprecedented wealth of proton and heavy-ion collisions.

“The ATLAS experiment stands ready to record this new harvest of data,” says Andreas Hoecker, ATLAS Spokesperson. “The new run promises to more than triple the currently accumulated dataset at a new energy frontier. We have prepared a broad scientific programme, taking advantage of new upgrades to our experiment."

The return of LHC beams comes after more than three years of upgrade and maintenance work, with new detector systems and electronic infrastructure installed 100 metres underground in the ATLAS cavern. “We are excited to see the ATLAS control room busy again as we prepare for Run-3 data taking,” says Jörg Stelzer, ATLAS Run Coordinator. “We have spent the past several months re-commissioning the experiment, and re-establishing the expertise and procedures that led to the outstanding data-taking efficiency of Run 2. This will be our model for Run 3.”

The higher beam energy and intensity of LHC Run 3 will allow the ATLAS experiment to push the very limits of its physics research.

“Run 3 will see our sensitivity to new physics processes increase, as we explore new types of collision events that were previously out of reach,” says Pamela Ferrari, ATLAS Deputy Physics Coordinator. “During the shutdown, we paid particular attention to improving our online event filtering system (or ‘trigger’). By refining our selection upstream with new detector systems, we should be able to identify some of the most difficult-to-spot signatures which could be left by new particles.”

Among the event signatures that would benefit from the new triggers is dark matter production. “The search for dark matter is a very important part of our physics research programme,” says Andreas. “We know from numerous observations of gravitational effects that dark matter exists in the universe. If it is made of particles, as most physicists expect, these may interact with protons or the Higgs boson and be produced in LHC collisions.” Dark matter is invisible and can only be spotted when produced with other, visible particles. The dark matter then appears as localised missing energy in a collision event, which physicists can measure.

Andreas Hoecker, ATLAS spokesperson, beams as he connects to the live broadcast, while members of the ATLAS collaboration cheer in the background. (Image: A. Chrul/CERN)

Applause in the ATLAS Control Room as first 13.6 TeV collisions are recorded. (Image: D. Price/ATLAS Collaboration)

ATLAS Data Preparation team members discuss preparations for the start of data-taking. (Image: D. Price/ATLAS Collaboration)

"ATLAS Run-3 Crew" waiting for the start of collisions. (Image: R. Gonzalez Suarez/ATLAS Collaboration)

Eagerly waiting for first collisions. (Image: R. Gonzalez Suarez/ATLAS Collaboration)

ATLAS Spokesperson Andreas Hoecker on stand-by for interview on CERN live-stream. (Image: R. Gonzalez Suarez/ATLAS Collaboration)

Of course, a centrepiece of the ATLAS physics programme remains the Higgs boson. Discovered by the ATLAS and CMS experiments just ten years ago in 2012, the Higgs boson plays a unique role in the universe. While it was discovered in Run 1 essentially through its interactions with force-carrier bosons, the larger and higher-energy Run-2 data sample allowed ATLAS to measure Higgs-boson interactions to matter particles (fermions) of the heaviest, third generation, and to observe all of its major production modes. In Run 3, these measurements will be further improved and the Higgs boson interactions with second-generation particles, such as muons, and its interaction with itself will become a focus.

"The Higgs sector is a rich environment for study, as precise measurements of its properties can yield spectacular results,” adds Pamela. “Constraining and eventually measuring its self-coupling will inform us about the energy potential of the Higgs field, which drives the phase transition in the early universe that transformed massless elementary particles into massive ones.”

These are but a few of the scientific highlights expected from Run 3. “As we enter this new era of exploration, it is important to champion the competence, dedication and hard work of the several hundreds of ATLAS members who implemented the upgrades and maintenance of our experiment during the long shutdown,” says Andreas. "Their work planted the seeds for the upcoming data harvest."

The many improvements to the sophisticated trigger and data acquisition system permit a much wider range of collision events to be explored, compared to Run 2, while maintaining the same particle acceptance rate. "In addition to the new hardware, ATLAS upgraded large parts of its software, simulation and computing environment to boost performance, save resources and enable it for use with heterogeneous computing systems," concludes Andreas.

The launch of the LHC Run 3 was streamed live on CERN’s social media channels, with live commentary in five languages (English, French, German, Italian, Spanish). Watch the stream below.

About the event display: Run 427394 Evt 3038977 Event display of a collision event recorded in ATLAS on 5 July 2022, when stable beams of protons at the energy of 6.8 TeV per beam were delivered to ATLAS for the first time by the LHC. The bottom-right figure shows a 3D view of the ATLAS detector. Starting from the point where the two beams of protons from the LHC collide, the figure shows the tracks of charged particles as they are reconstructed in the inner detector (orange tracks), the energy deposits in the electromagnetic (green boxes) and hadronic (yellow boxes) calorimeters, as well as the reconstructed particle jets (yellow cones). The barrel magnet and the muon chambers in the barrel region of the detector (blue boxes) are shown as a semi-transparent cut-out. At the centre, the picture shows the beam pipe, where the protons accelerated by the LHC enter the detector. The top-left view shows a projection of the same event on the transverse plane, showing the tracks of charged particles reconstructed in the inner detector (orange tracks) and the energy deposits in the electromagnetic (green boxes) and hadronic (yellow boxes) calorimeters, as well as the reconstructed particle jets (yellow cones). (Image: ATLAS Collaboration/CERN)

Learn more

LHC Run 3: physics at record energy starts tomorrow, CERN Press Statement, 4 July 2022
10 years of discovery with the Higgs boson, ATLAS Press Statement, 4 July 2022
About ATLAS upgrades during Long Shutdown 2
ATLAS Run 3 Resources

LHC Run 3: physics at record energy starts tomorrow

Steven Goldfarb — Mon, 04 Jul 2022 19:00:00 +0000

LHC Run 3: physics at record energy starts tomorrow

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The Large Hadron Collider is ready to once again start delivering proton collisions to experiments, this time at an unprecedented energy of 13.6 TeV, marking the start of the accelerator’s third run of data taking for physics.

Operators in the LHC control centre prepare for the start of LHC Run 3 physics. (Image: CERN)

A new period of data taking begins on Tuesday, 5 July for the experiments at the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), after more than three years of upgrade and maintenance work. Beams have already been circulating in CERN’s accelerator complex since April, with the LHC machine and its injectors being recommissioned to operate with new higher-intensity beams and increased energy. Now, the LHC operators are ready to announce “stable beams”, the condition allowing the experiments to switch on all their subsystems and begin taking the data that will be used for physics analysis. The LHC will run around the clock for close to four years at a record energy of 13.6 trillion electron volts (TeV), providing greater precision and discovery potential than ever before.

“We will be focusing the proton beams at the interaction points to less than 10 micron beam size, to increase the collision rate. Compared to Run 1, in which the Higgs was discovered with 12 inverse femtobarns, now in Run 3 we will be delivering 280 inverse femtobarns . This is a significant increase, paving the way for new discoveries,” says director for accelerators and technology Mike Lamont.

The four big LHC experiments have performed major upgrades to their data readout and selection systems, with new detector systems and computing infrastructure. The changes will allow them to collect significantly larger data samples, with data of higher quality than in previous runs. The ATLAS and CMS detectors expect to record more collisions during Run 3 than in the two previous runs combined. The LHCb experiment underwent a complete revamp and looks to increase its data taking rate by a factor of ten, while ALICE is aiming at a staggering fifty-fold increase in the number of recorded collisions.

With the increased data samples and higher collision energy, Run 3 will further expand the already very diverse LHC physics programme. Scientists at the experiments will probe the nature of the Higgs boson with unprecedented precision and in new channels. They may observe previously inaccessible processes, and will be able to improve the measurement precision of numerous known processes addressing fundamental questions, such as the origin of the matter–antimatter asymmetry in the universe. Scientists will study the properties of matter under extreme temperature and density, and will also be searching for candidates for dark matter and for other new phenomena, either through direct searches or – indirectly – through precise measurements of properties of known particles.

“We’re looking forward to measurements of the Higgs boson decay to second-generation particles such as muons. This would be an entirely new result in the Higgs boson saga, confirming for the first time that second-generation particles also get mass through the Higgs mechanism,” says CERN theorist Michelangelo Mangano.

“We will measure the strengths of the Higgs boson interactions with matter and force particles to unprecedented precision, and we will further our searches for Higgs boson decays to dark matter particles as well as searches for additional Higgs bosons,” says Andreas Hoecker, spokesperson of the ATLAS collaboration. “It is not at all clear whether the Higgs mechanism realised in nature is the minimal one featuring only a single Higgs particle.”

A closely watched topic will be the studies of a class of rare processes in which an unexpected difference (lepton flavour asymmetry) between electrons and their cousin particles, muons, was studied by the LHCb experiment in the data from previous LHC runs. “Data acquired during Run 3 with our brand new detector will allow us to improve the precision by a factor of two and to confirm or exclude possible deviations from lepton flavour universality,” says Chris Parkes, spokesperson of the LHCb collaboration. Theories explaining the anomalies observed by LHCb typically also predict new effects in different processes. These will be the target of specific studies performed by ATLAS and CMS. “This complementary approach is essential; if we’re able to confirm new effects in this way it will be a major discovery in particle physics,” says Luca Malgeri, spokesperson of the CMS collaboration.

The heavy-ion collision programme will allow the investigation of quark–gluon plasma (QGP) – a state of matter that existed in the first 10 microseconds after the Big Bang – with unprecedented accuracy. “We expect to be moving from a phase where we observed many interesting properties of the quark–gluon plasma to a phase in which we precisely quantify those properties and connect them to the dynamics of its constituents,” says Luciano Musa, spokesperson of the ALICE collaboration. In addition to the main lead–lead runs, a short period with oxygen collisions will be included for the first time, with the goal of exploring the emergence of QGP-like effects in small colliding systems.

The smallest experiments at the LHC – TOTEM, LHCf, MoEDAL, with its entirely new subdetector MAPP, and the recently installed FASER and SND@LHC – are also poised to explore phenomena within and beyond the Standard Model, from magnetic monopoles to neutrinos and cosmic rays.

A new physics season is starting, with a broad and promising scientific programme in store. The launch of LHC Run 3 will be streamed live on CERN’s social media channels and high-quality Eurovision satellite link starting at 4.00 p.m. (CEST) on 5 July. Live commentary from the CERN Control Centre, available in five languages (English, French, German, Italian and Spanish), will walk the viewers through the operation stages that take proton beams from injection into the LHC to collisions for physics at the four interaction points where the experiments are located. A live Q&A session with experts from the accelerators and experiments will conclude the live stream.

This media update was originally published on the CERN Press website (English, French).

To follow the live stream on EBU satellite, you will need to create an account. The event will be accessible here.

Pictures of the day will be added here.

Run 3 background information can be found here.

Press contact: press@cern.ch

10 years of discovery with the Higgs boson

Katarina Anthony — Mon, 04 Jul 2022 09:46:42 +0000

10 years of discovery with the Higgs boson

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Press Statement

ATLAS Collaboration

Higgs boson

4 July 2022. The ATLAS Collaboration at CERN has released its most comprehensive overview of the Higgs boson. The new paper, published in the journal Nature, comes exactly ten years after ATLAS announced the discovery of the Higgs boson. In celebration of this anniversary, a special all-day symposium on the Higgs boson is currently underway at CERN.

"The ATLAS Collaboration has made tremendous leaps in the understanding of the Higgs boson over the past ten years," says Guillaume Unal, ATLAS Physics Coordinator. "Today's paper summarises these insights, presenting a detailed map of the Higgs boson’s interactions with other particles and its properties.”

The results are based on the full dataset collected by the ATLAS experiment during Run 2 (2015–2018) of the Large Hadron Collider (LHC). The wealth of data allowed researchers to study the Higgs boson in unprecedented detail. Alongside these highlights (see timeline below), brand-new studies of the Higgs boson self interaction are being presented at the CERN symposium. Studies of this property can shed light on the form of the energy potential of the Higgs field, which governs the Brout-Englert-Higgs mechanism that gave mass to elementary particles a fraction of a second after the Big Bang.

“The successful decade of Higgs boson exploration would not have been possible without the exceptional performance of the LHC and the ATLAS detector, as well as the data reconstruction and calibration chains following the acquisition of the data, optimised during countless hours by skilled and dedicated ATLAS members,” says Andreas Hoecker, ATLAS Spokesperson. “That performance together with powerful novel analysis approaches and improved theoretical modelling have made our studies surpass expectations in every aspect.”

The singular characteristics of the Higgs boson, as the only elementary scalar particle, have made it the subject of a very rich field of research. “The Higgs sector is directly connected with very profound questions related to the evolution of the early Universe and the stability of the vacuum, as well as to the striking mass pattern of matter particles,” says Andreas. “Each particle’s relationship with the Higgs boson is special, and provides a unique test of the Standard Model. Over the past ten years, we have observed and measured all of the main production and decay mechanisms of the Higgs boson as summarised in today’s paper.”

The ATLAS Collaboration has made tremendous leaps in the understanding of the Higgs boson over the past ten years, painting a detailed picture of its properties and interactions.

The Higgs boson is also being deployed as a tool in the search for new, unknown phenomena. ATLAS’ new paper provides an up-to-the-minute look at these searches, including possible decays of the Higgs boson to invisible particles that could make up the dark matter observed in the universe. “Any new particle with mass might interact with the Higgs boson," says Guillaume. “We could observe these particles directly in dedicated searches or indirectly by precisely measuring the kinematic and symmetry properties of the Higgs boson. New particles occurring in quantum loops might alter these properties from those predicted by the Standard Model.”

While today’s Nature paper draws a detailed map of the Higgs boson, it is still far from complete. The upcoming Run 3 of the LHC, scheduled to begin tomorrow, will triple the amount of proton-collision data for ATLAS to explore – providing a new opportunity for physicists to piece together the nature of the Higgs boson.

Members of the public are invited to tune in to today’s CERN symposium on the Higgs boson to learn more (watch the webcast here). Seminar talks explore the decades of detector and accelerator innovation that were required to search for the Higgs boson, as well as the process and people behind the discovery. The very latest Higgs studies are also described, with special focus given to the exciting future prospects of LHC Run 3.

About the event display: One of the first Higgs boson candidate events, recorded 10 June 2012 by the ATLAS Experiment. The event is consistent with a H→ZZ*→4μ decay. (Image: ATLAS Collaboration/CERN)

About the font: To celebrate the Higgs-boson discovery anniversary, we annually switch the ATLAS public website font to Comic Sans. This is an homage to use of Comic Sans in ATLAS' Higgs announcement slides, presented on 4 July 2012.

Learn more

Nature paper: A detailed map of Higgs boson interactions by the ATLAS experiment ten years after the discovery
The Higgs boson, ten years after its discovery, CERN Press Statement, 4 July 2022
Happy birthday, Higgs boson! What we do and don’t know about the particle, Nature article, 4 July 2022
ATLAS and CMS release results of most comprehensive studies yet of Higgs boson’s properties, CERN Media Update, 4 July 2022
ATLAS explores the self-interaction of the Higgs boson, ATLAS Physics Briefing, 4 July 2022
Watch the CERN Higgs10 Symposium
About the Higgs boson: a landmark discovery
About ATLAS Long Shutdown 2 Upgrades
About LHC Run 3

Rare phenomenon observed by ATLAS features the LHC as a high-energy photon collider

Steven Goldfarb — Wed, 05 Aug 2020 10:21:00 +0000

Rare phenomenon observed by ATLAS features the LHC as a high-energy photon collider

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An ATLAS event display consistent with the production of a pair of W bosons from two photons, where the W bosons decay into a muon and an electron (visible in the detector) and neutrinos (not detected).

During the International Conference on High-Energy Physics (ICHEP 2020), the ATLAS collaboration presented the first observation of photon collisions producing pairs of W bosons, elementary particles that carry the weak force, one of the four fundamental forces. The result demonstrates a new way of using the LHC, namely as a high-energy photon collider directly probing electroweak interactions. It confirms one of the main predictions of electroweak theory – that force carriers can interact with themselves – and provides new ways to probe it.

According to the laws of classical electrodynamics, two intersecting light beams would not deflect, absorb or disrupt one another. However, effects of quantum electrodynamics (QED), the theory that explains how light and matter interact, allow interactions among photons.

Indeed, it is not the first time that photons interacting at high energies have been studied at the LHC. For instance, light-by-light “scattering”, where a pair of photons interact by producing another pair of photons, is one of the oldest predictions of QED. The first direct evidence of light-by-light scattering was reported by ATLAS in 2017, exploiting the strong electromagnetic fields surrounding lead ions in high-energy lead–lead collisions. In 2019 and 2020, ATLAS further studied this process by measuring its properties.

The new result reported at this conference is sensitive to another rare phenomenon in which two photons interact to produce two W bosons of opposite electric charge via (among others) the interaction of four force carriers^{^[1]}. Quasi-real photons from the proton beams scatter off one another to produce a pair of W bosons. A first study of this phenomenon was previously reported by ATLAS and CMS in 2016, from data recorded during LHC Run 1, but a larger dataset was required to unambiguously observe it.

The ATLAS Collaboration reports the observation of photon collisions producing weak-force carriers and provides further insights into their interactions.

The observation was obtained with a highly significant statistical evidence of 8.4 standard deviations, corresponding to a negligible chance of being due to a statistical fluctuation. ATLAS physicists used a considerably larger dataset taken during Run 2, the four-year data collection in the LHC that ended in 2018, and developed a customised analysis method.

Owing to the nature of the interaction process, the only particle tracks visible in the central detector are the decay products of the two W bosons, an electron and a muon with opposite electric charge. W-boson pairs can also be directly produced from interactions between quarks and gluons in the colliding protons considerably more often than from photon–photon interactions, but these are accompanied by additional tracks from strong interaction processes. This means that the ATLAS physicists had to carefully disentangle collision tracks to observe this rare phenomenon.

“This observation opens up a new facet of experimental exploration at the LHC using photons in the initial state,” said Karl Jakobs, Spokesperson of the ATLAS Collaboration. “It is unique as it only involves couplings among electroweak force carriers in the strong-interaction dominated environment of the LHC. With larger future datasets it can be used to probe in a clean way the electroweak gauge structure and possible contributions of new physics.”

Indeed, the new result confirms one of the main predictions of electroweak theory, namely that, besides interacting with ordinary particles of matter, the force carriers, also known as gauge bosons – the W bosons, the Z boson and the photon – are also interacting with each other. Photon collisions will provide a new way to test the Standard Model and to probe for new physics, which is necessary for a better understanding of our Universe.

This media update was originally published on the CERN Press website (English, French).

[1] The four force-carrier interaction is one of the predictions of the electroweak theory that explains how force-carrier particles, also known as gauge bosons, interact not only with matter particles, but also with one another.

Learn more

Observation of photon-induced W⁺ W^–production in proton–proton collisions at 13 TeV using the ATLAS detector (ATLAS-CONF-2020-038)
ATLAS observes W-boson pair production from light colliding with light, Physics Briefing, 5 August 2020

CERN experiments announce first indications of a rare Higgs boson process

Steven Goldfarb — Mon, 03 Aug 2020 15:50:00 +0000

CERN experiments announce first indications of a rare Higgs boson process

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A Run 2 ATLAS event containing two muons (red) with mass compatible with that of the Higgs boson, and two forward jets (yellow cones). (Image: ATLAS Collaboration/CERN)

At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

Physicists at CERN have been studying the Higgs boson since its discovery in 2012 in order to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analysing how it decays into the various fundamental particles and the rate of disintegration.

CMS achieved evidence of this decay with 3 sigma, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s two-sigma result means the chances are one in 40. The combination of both results would increase the significance well above 3 sigma and provides strong evidence for the Higgs boson decay to two muons.

"These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector, as well as the use of novel analysis techniques,” says Karl Jakobs, ATLAS spokesperson.

“CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.

The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.

“This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics programme. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.

What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimised to categorise physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.

The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.

This press release is also available on the CERN Press website (English, French).

ATLAS Experiment releases 13 TeV Open Data for Science Education

Steven Goldfarb — Mon, 10 Feb 2020 15:25:00 +0000

ATLAS Experiment releases 13 TeV Open Data for Science Education

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Press Statement

Highest-energy particle-collision data ever released through open access.

ATLAS Collaboration

open data

13 TeV

Animation of ATLAS detector using ROOTJS. (Image: ATLAS Collaboration/CERN)

Geneva, 10 February. The ATLAS Collaboration at CERN has just released the first open dataset from the Large Hadron Collider’s (LHC) highest-energy run at 13 teraelectronvolts (TeV). The new release is specially developed for science education, underlining the Collaboration’s long-standing commitment to students and teachers using open-access ATLAS data and related tools.

"The ATLAS Collaboration is proud to make public these data for advanced learning," says Karl Jakobs, ATLAS Spokesperson. “Our high-energy collision open data, recorded during the second run of the LHC, provide insight into the real world of particle-physics analysis. Students, scholars and interested publics will be able to reproduce ATLAS physics results in a fully realistic manner, understanding for themselves the fascinating study of Nature at its deepest level.”

The ATLAS Collaboration makes public 10 inverse femtobarns (fb^–1) of the 13 TeV data. For context, this corresponds to about 1 quadrillion proton-proton collisions (that’s 1 followed by 15 zeros), or 500 thousand produced Higgs bosons. It is also approximately the same amount of data that the ATLAS Collaboration used to discover the Higgs boson in 2012. Explore ATLAS Open Data using the software and tools available here, or on the CERN Open Data Portal.

Alongside impressive new open datasets, the ATLAS Collaboration has also released new simulated datasets, web-based and offline analysis software, as well as extensive documentation and tutorials. “These are the tools of a particle physicist’s trade, allowing us to go from data-taking to physics measurements and eventually discovery,” says Arturo Sánchez Pineda, co-leader of the ATLAS Open Data team (University of Udine, ICTP and INFN, Italy). “Simulated datasets allow physicists to compare theory with real data. They are based on theoretical models of the expected physics processes taking place in the collisions, together with a detailed description of the ATLAS detector. By providing such resources, we hope to empower students, professors and dedicated self-learners worldwide to learn and teach experimental particle physics, as well as the computer science behind the field.”

“Do-it-yourself discoveries”

One of the most exciting features of the new ATLAS Open Data release is its ability to put learners in the role of the ‘discoverer’. “For the first time, students will be able to ‘re-discover’ the Higgs boson (in three different decay channels) and can even search through the data for physics beyond the Standard Model, such as Dark Matter,” explains Kate Shaw, co-leader of the ATLAS Open Data team (University of Sussex, UK). “These new avenues for study will greatly enhance understanding of the experimental side of data analysis - a particular advantage for budding researchers.”

The ATLAS Open Data team worked closely with students and teachers during the development, carefully curating the release to ensure that students get the best educational experience straight out of the box. “We wanted to build upon our experience with the 8 TeV open data release by providing more complex physics analyses for study and shortening the required setup process,” says Leonid Serkin of the ATLAS Open Data team (University of Udine, ICTP and INFN, Italy). “This time, students can access the datasets, write analysis code and begin producing results within minutes with the help of cloud computing and tools such as the CERN ROOT analysis framework. The millions of independent 13 TeV collision events can thus be analysed from almost any commercial computer.”

Enriching global physics education

Every year, hundreds of students on every continent explore ATLAS Open Data as part of their curriculum. Their scope can range from one-day high-school projects to in-depth analyses for their Master’s thesis.*

For example, at the University of Montréal, Canada, Jean-François Arguin uses ATLAS Open Data to reproduce a more realistic research environment for his second-year undergraduate students. “It makes a welcome change from learning from books, which is how physics is traditionally taught,” he explains. “Using ATLAS Open Data, the students spend three weeks recreating the major particle discoveries of the late twentieth century: the Z boson, W boson and top quark. The process develops their skills as researchers, which are not always correlated to the ones they develop through traditional courses.”

Preparing students for research work is also an important part of Lund University’s (Sweden) programme. There, Else Lytken and Caterina Doglioni divide their undergraduate students into mini-analysis groups, each focusing on one ATLAS Open Data analysis. “An important part of this course is the reconstruction and identification of particles, as well as the understanding of analysis strategies,” says Lytken. “ATLAS Open Data plays a key role in introducing students to these concepts while letting them ‘feel like a physicist’.”

Now, students and teachers turn their eyes to the possibilities of 13 TeV. “The students can’t wait to get their hands on a Higgs boson,” adds Doglioni. “It’s an incredibly exciting prospect: to follow the steps of that famed discovery, using real data from the experiment that found it.”

While the world takes its first look at 13 TeV open data, the ATLAS Collaboration is already preparing its next release of educational content. Look forward to more tools, tutorials and analyses to be published this year, as the ATLAS Open Data project continues to grow.

* ATLAS Open Data has been incorporated into the curriculums of multiple universities, including: Maastricht University (Belgium); University of Montréal (Canada); Industrial University of Santander (Colombia); University of Athens (Greece); TU Dresden and TU Dortmund (Germany); University of Valencia (Spain); KTH Royal Institute of Technology and Lund University (Sweden); University of Oslo (Norway); LIP (Portugal), CERN (Switzerland), University of Manchester, University of Sussex, University of Birmingham and Queen Mary University of London (UK); University of Michigan, Ohio State University and California State University (USA); University of the Andes, Central University of Venezuela and Simón Bolívar University (Venezuela).

Learn more

Explore ATLAS Open Data using the software and tools available here, or on the CERN Open Data Portal.
ATLAS PUB Note: Review of the 13 TeV ATLAS Open Data release (ATL-OREACH-PUB-2020-001)
Boosting high-energy physics education around the world with ATLAS Open Data, ATLAS Blog, July 2018
Explore LHC data on new ATLAS educational platform, ATLAS News, July 2016

ATLAS observes elusive Higgs boson decay to a pair of bottom quarks

Steven Goldfarb — Tue, 28 Aug 2018 09:17:00 +0000

ATLAS observes elusive Higgs boson decay to a pair of bottom quarks

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A candidate event display for the production of a Higgs boson decaying to two b-quarks (blue cones), in association with a W boson decaying to a muon (red) and a neutrino. The neutrino leaves the detector unseen, and is reconstructed through the missing transverse energy (dashed line). (Image: ATLAS Collaboration/CERN)

Geneva, 28 August 2018. The ATLAS Collaboration at CERN’s Large Hadron Collider (LHC) has – at long last – observed the Higgs boson decaying into a pair of bottom (b) quarks. This elusive interaction is predicted to make up almost 60% of the Higgs boson decays and is thus primarily responsible for the Higgs natural width. Yet it took over six years after the 2012 discovery of the Higgs boson to accomplish this observation.

“ATLAS is proud to announce the observation of this important and challenging Higgs boson decay," says Karl Jakobs, ATLAS Spokesperson. “While the result is certainly a confirmation of the Standard Model, it is equally a triumph for our analysis teams. During the early preparations of the LHC, there were doubts on whether this observation could be achieved. Our success is thanks to the excellent performance of the LHC and the ATLAS detector, and the application of highly sophisticated analysis techniques to our large dataset.”

The ATLAS Collaboration first presented a preliminary result of this observation on 9 July at the 2018 International Conference on High-Energy Physics (ICHEP) in Seoul. Today, in a seminar together with the CMS Collaboration, ATLAS presented results which have been submitted for publication to Physics Letters B. They are based on combined Run 2 and Run 1 data, and utilise machine learning technology and new analysis techniques to reach a significance of 5.4 standard deviations.^[1]

This elusive interaction is predicted to make up almost 60% of the Higgs boson decays and is thus primarily responsible for the Higgs natural width.

Distribution of b-quark pair mass from all search channels combined after subtraction of all backgrounds except for WZ and ZZ production. The data (points with error bars) are compared to the expectations from the production of WZ and ZZ (in grey) and of WH and ZH (in red). (Image: ATLAS Collaboration/CERN)

This is among the most demanding analyses carried out by ATLAS so far. “LHC collisions produce b-quark pairs in great abundance, making it hard to spot those originating from Higgs boson decays,” says Kerstin Tackmann, ATLAS Higgs working group convenor. “The analysis teams therefore focused on signatures, in particular the production of a Higgs boson in association with a vector boson, which increased substantially the purity of the signal.” This technique proved highly successful.

Today’s announcement is a new confirmation of the so-called “Yukawa couplings”. Similar to the Higgs mechanism, these couplings to the Higgs field provide mass to charged fermions (quarks and leptons), which are the building blocks of matter. Combined analyses of the Run 1 and Run 2 datasets have resulted in the first measurements of these couplings, as seen in the recent ATLAS observation of Higgs boson production in association with a top-quark pair and the observation of the Higgs boson decaying into pairs of tau leptons.

The new ATLAS result also establishes, for the first time, the production of a Higgs boson in association with a vector boson above 5 standard deviations. ATLAS has now observed all four main production modes of the Higgs boson, two of which this year.

These observations mark a new milestone in the study of the Higgs boson, as ATLAS transitions from observations to precise measurements of its properties. “We now have the opportunity to study the Higgs boson in unprecedented detail and will be able to further challenge the Standard Model,” concludes Karl Jakobs.

[1] Physicists consider five standard deviations (or “sigma”) the significance threshold past which they claim a discovery. There is only a one in a 3.5 million chance that such a signal originates from a statistical fluctuation of the background.

Learn more:

Observation of H→bb decays and VH production with the ATLAS detector (arXiv: 1808.08238, submitted to Phys. Lett. B.)
CERN Press Release: Long-sought decay of Higgs boson observed
LHC Seminar: Observation of the H→bb decay at ATLAS and CMS (Watch the webcast live.)
ATLAS Physics Briefing: Higgs boson observed decaying to b quarks – at last!
CMS Collaboration: Observation of Higgs boson decay to bottom quarks (arXiv: 1808.08242)
See also the full lists of ATLAS Physics Papers and ATLAS Conference Notes.

ATLAS observes direct interaction of Higgs boson with top quark

Steven Goldfarb — Mon, 04 Jun 2018 14:00:00 +0000

ATLAS observes direct interaction of Higgs boson with top quark

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Visualization of an event from the tt̄H(γγ) analysis. The event contains two photon candidates displayed as green towers in the electromagnetic calorimeter, and six particle (b-jets) are shown as yellow (blue) cones. (Image: ATLAS Collaboration/CERN)

Visualization of an event from the tt̄H(γγ) analysis. The event contains two photon candidates displayed as green towers in the electromagnetic calorimeter, and six jets (b-jet) shown as yellow (blue) cones. (Image: ATLAS Collaboration/CERN)

CERN and Bologna, Italy, 4 June 2018. The ATLAS Collaboration at CERN has announced the observation of Higgs bosons produced together with a top-quark pair. Observing this extremely rare process is a significant milestone for the field of High-Energy Physics. It allows physicists to test critical parameters of the Higgs mechanism in the Standard Model of particle physics.

The result exploits the full dataset delivered to ATLAS by the Large Hadron Collider (LHC) and establishes the signal with a statistical significance of 6.3 standard deviations. It concurs with a recent observation by the CMS Collaboration with a significance of 5.2 standard deviations using a smaller dataset. “This measurement constitutes a landmark achievement in the exploration of the Higgs mechanism and the interaction of Standard Model particles with the Higgs boson,” says Karl Jakobs, ATLAS Spokesperson. “It provides direct evidence that the heaviest known particle, the top quark, interacts with the predicted large strength with the Higgs boson.” The signal for this production has slowly built up over years of accumulating data, with first evidence reported by ATLAS last December.

As only 1% of all Higgs bosons are produced in association with top quarks, its observation was extremely challenging. ATLAS physicists examined five years of collision data to arrive at this result. “This was one of the most demanding searches ever carried out by the ATLAS Collaboration, requiring a concerted effort from several analysis teams,” says Fabio Cerutti, ATLAS Higgs working group convenor. “As it is such a rare process, we had to look across many different Higgs boson decay channels. Some of these were limited by experimental uncertainties, while others by the quantity of the data we collected. It was only by combining all of these different analyses that we could achieve this observation.”

The result exploits the full dataset delivered to ATLAS by the Large Hadron Collider and establishes the signal with a statistical significance of 6.3 standard deviations.

Time-lapse animation showing the increasing ttH signal in the diphoton mass spectrum as more data are included in the measurement. (Image: ATLAS Collaboration/CERN)

The new result may also provide insight into one of the most puzzling aspects of the Standard Model: the wide range of masses among fermions, the class of particles that constitute matter and include quarks and leptons. Understanding the nature of the top quark mass would go a long way to solving this open question. “The measurement gives a strong indication that the Higgs boson has a key role in the large value of the top quark mass,” says Cerutti. “While this is certainly a key feature of the Standard Model, this is the first time it has been verified experimentally with overwhelming significance.”

The success of this result hints at the new analysis possibilities of the ATLAS experiment. Thanks to the wealth of data being produced by the LHC and the excellent performance of the ATLAS experiment, physicists will be able to study the Higgs boson in further rare and experimentally challenging interactions for the first time. Such studies will continue to challenge the limits of the Standard Model and may open new avenues of discovery.

“The full power of the entire dataset collected during Run 2, including those taken until the end of this year, will be used in the future to improve the precision of this result and to look for smaller deviations with reduced experimental uncertainties,” concludes Jakobs. “A combination of all Higgs boson measurements at the end of Run 2 will be important and exciting.”

Find out more

The Higgs boson reveals its affinity for the top quark, CERN Press Release
Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector (arXiv: 1806.00425, see figures)
New ATLAS result establishes production of Higgs boson in association with top quarks, ATLAS Physics Briefing
Status and highlights from the ATLAS experiment, LHCP18 presentation by Kunihiro Nagano
CMS Collaboration: Observation of ttH production (Phys. Rev. Lett. 120, 231801, arXiv:1804.02610)
Beyond discovery: ATLAS explores the Higgs boson, ATLAS News, 11 April 2018

First high-precision measurement of the mass of the W boson at the LHC

Steven Goldfarb — Mon, 12 Feb 2018 15:00:00 +0000

First high-precision measurement of the mass of the W boson at the LHC

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Geneva, 12 February 2018. In a paper published today in the European Physical Journal C, the ATLAS Collaboration reports the first high-precision measurement at the Large Hadron Collider (LHC) of the mass of the W boson. This is one of two elementary particles that mediate the weak interaction – one of the forces that govern the behaviour of matter in our universe. The reported result gives a value of 80370±19 MeV for the W mass, which is consistent with the expectation from the Standard Model of Particle Physics, the theory that describes known particles and their interactions.

The measurement is based on around 14 million W bosons recorded in a single year (2011), when the LHC was running at the energy of 7 TeV. It matches previous measurements obtained at LEP, the ancestor of the LHC at CERN, and at the Tevatron, a former accelerator at Fermilab in the United States, whose data made it possible to continuously refine this measurement over the last 20 years.

The W boson is one of the heaviest known particles in the universe. Its discovery in 1983 crowned the success of CERN’s Super Proton Synchrotron, leading to the Nobel Prize in physics in 1984. Although the properties of the W boson have been studied for more than 30 years, measuring its mass to high precision remains a major challenge.

Any deviation of the measured W boson mass from predictions could reveal new phenomena.

Display of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions recorded by ATLAS with LHC stable beams at a collision energy of 7 TeV. (Image: ATLAS Collaboration/CERN)

“Achieving such a precise measurement despite the demanding conditions present in a hadron collider such as the LHC is a great challenge,” said the physics coordinator of the ATLAS Collaboration, Tancredi Carli. “Reaching similar precision, as previously obtained at other colliders, with only one year of Run 1 data is remarkable. It is an extremely promising indication of our ability to improve our knowledge of the Standard Model and look for signs of new physics through highly accurate measurements.”

The Standard Model is very powerful in predicting the behaviour and certain characteristics of the elementary particles and makes it possible to deduce certain parameters from other well-known quantities. The masses of the W boson, the top quark and the Higgs boson for example, are linked by quantum physics relations. It is therefore very important to improve the precision of the W boson mass measurements to better understand the Higgs boson, refine the Standard Model and test its overall consistency.

Remarkably, the mass of the W boson can be predicted today with a precision exceeding that of direct measurements. This is why it is a key ingredient in the search for new physics, as any deviation of the measured mass from the prediction could reveal new phenomena conflicting with the Standard Model.

The measurement relies on a thorough calibration of the detector and of the theoretical modelling of the W boson production. These were achieved through the study of Z boson events and several other ancillary measurements. The complexity of the analysis meant it took almost five years for the ATLAS team to achieve this new result. Further analysis with the huge sample of now-available LHC data, will allow even greater accuracy in the near future.

This update is also available on the CERN Press website (English, French).

Links

Read the paper in European Physical Journal C: Measurement of the W-boson mass in proton-proton collisions at 7 TeV with the ATLAS detector
Measuring the W boson mass, ATLAS Physics Briefing, December 2016
See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.
Printer-friendly version of this Statement [pdf]

ATLAS sees first direct evidence of light-by-light scattering at high energy

Steven Goldfarb — Mon, 14 Aug 2017 17:00:00 +0000

ATLAS sees first direct evidence of light-by-light scattering at high energy

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A light-by-light scattering candidate event measured in the ATLAS detector. (Image: ATLAS Collaboration/CERN)

Geneva, 14 August 2017. Physicists from the ATLAS experiment at CERN have found the first direct evidence of high energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics, confirms one of the oldest predictions of quantum electrodynamics (QED).

"This is a milestone result: the first direct evidence of light interacting with itself at high energy,” says Dan Tovey (University of Sheffield), ATLAS Physics Coordinator. “This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism."

Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider’s second run began in 2015. As the accelerator collided lead ions at unprecedented collision rates, obtaining evidence for light-by-light scattering became a real possibility. “This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2,” says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group convenor.

Heavy-ion collisions provide a uniquely clean environment to study light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as ‘ultra-peripheral collisions’.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

“Finding evidence of this rare signature required the development of a sensitive new ‘trigger’ for the ATLAS detector,” says Steinberg. “The resulting signature – two photons in an otherwise empty detector – is almost the diametric opposite of the tremendously complicated events typically expected from lead nuclei collisions. The new trigger’s success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it.”

ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of the result and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.

This statement is also available on the CERN Press website (English, French).