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The LHCb experiment has taken a leap in precision physics at the Large Hadron Collider (LHC). In a new paper submitted to Physical Review Letters, the LHCb collaboration reports the first dedicated measurement of the Z boson mass at the LHC, using data from high-energy collisions between protons recorded in 2016 during the collider’s second run.
The Z boson is a massive, electrically neutral particle that mediates the weak nuclear force – one of nature’s fundamental forces. With a mass of around 91 billion electronvolts (GeV), it ranks among the heaviest known elementary particles. Discovered at CERN over 40 years ago, alongside the W boson, the Z boson played a central role in confirming the Standard Model of particle physics – a breakthrough that led to the 1984 Nobel Prize in Physics. Measuring its mass precisely remains essential for testing the Standard Model and searching for signs of new physics.
The new LHCb measurement is based on a sample of 174 000 Z bosons decaying into pairs of muons, heavier relatives of the electron. The measurement resulted in a mass of 91 184.2 million electronvolts (MeV) with an uncertainty of just 9.5 MeV – or about a hundredth of a per cent.
The result is in line with measurements from the electron–positron LEP collider, the LHC’s predecessor, and the CDF experiment at the former proton–antiproton Tevatron collider in the US. What’s more, it matches the precision of the Standard Model prediction, which has an uncertainty of 8.8 MeV (see figure below).
The LHCb measurement shows that this level of precision can be achieved at the LHC despite the complex environment of proton–proton collisions, in which many particles are produced simultaneously.
The achievement opens the door to more Z boson mass studies at the LHC and the future High-Luminosity LHC, including much-anticipated analyses from the ATLAS and CMS experiments. Importantly, the experimental uncertainties on Z boson mass measurements are largely independent across the LHC experiments, meaning that an average of the measurements will have a reduced uncertainty.
“The High-Luminosity LHC has the potential to challenge the precision of the Z boson mass measurement from LEP – something that seemed inconceivable at the beginning of the LHC programme,” says LHCb spokesperson Vincenzo Vagnoni. “This will pave the way for proposed future colliders, such as the FCC-ee, to achieve an even bigger leap in precision.”
Find out more on the LHCb website.
Comparison of the measured Z boson mass with the Standard Model prediction (green) and with measurements from LEP and the CDF experiment. (Image: LHCb/CERN)abelchio Fri, 06/06/2025 - 11:56 Byline Ana Lopes Publication Date Fri, 06/06/2025 - 11:37
Cosmic rays are high-energy particles from outer space that strike Earth’s atmosphere, generating showers of secondary particles, such as muons, that can reach the planet’s surface. In recent years, ground-based experiments have detected more cosmic muons than current theoretical models predict, a discrepancy known as the muon puzzle.
Underground experiments offer good conditions for the detection of cosmic muons, because the rock or soil above the experiments absorbs the other shower components. They could therefore help to solve the muon puzzle. One example is ALICE at the Large Hadron Collider (LHC). Designed to study the products of heavy-ion collisions, ALICE is also well suited for detecting cosmic muons thanks to its location in a cavern 52 metres underground, shielded by 28 metres of overburden rock and an additional 1 metre of iron magnet yoke.
In a recent article published in the Journal of Cosmology and Astroparticle Physics, the ALICE collaboration reports the detection of around 165 million events containing at least one cosmic muon, as well as 15 702 events with more than four cosmic muons. This large sample was collected between 2015 and 2018 during pauses in LHC Run 2, when no particle beams were circulating in the collider. The total data-taking time amounted to 62.5 days – more than double the duration of the previous cosmic-ray campaign in LHC Run 1 (2010–2013), which recorded approximately 22.6 million events with at least one muon.
By analysing how the number of events varies with increasing muon multiplicity (the number of muons per event), the ALICE collaboration observed a smooth, decreasing trend from a multiplicity of 5 to a multiplicity of 50, beyond which the numbers of events are very small and subject to large statistical uncertainties (figure below).
Muon multiplicity distribution of events with more than four muons, as measured by ALICE over a period of 62.5 days. (Image: ALICE)The ALICE researchers compared this decreasing muon multiplicity distribution with simulations based on three models of secondary-particle production and assuming two extreme compositions of primary cosmic rays – hydrogen nuclei (protons), representing the lightest possible composition, and iron nuclei, representing a very heavy composition.
These comparisons showed that the measured distribution corresponds to primary cosmic rays with energies ranging from 4 to 60 PeV, where 1 PeV is 1015 electronvolts. In this energy range, the composition of the primary cosmic rays is expected to be a mixture of nuclear species, from protons to iron. One of the three models reproduces the observed distribution, but only when assuming that the primary cosmic rays are composed of iron. By contrast, the other two models underpredict the event count even when assuming an iron composition. While these results suggest that heavy elements dominate the composition of the primary cosmic rays, they fail to account for the expected mixed composition and the increasing fraction of heavy elements as multiplicity, and thus primary cosmic-ray energy, increases.
Focusing on rare events with more than 100 muons, the researchers found that these high-multiplicity events are well described by two of the models when assuming an iron composition. These findings are compatible with an average energy of about 100 PeV for the primary cosmic rays that likely produced these events.
The new ALICE results confirm the discrepancy between ground-based data and models that constitutes the muon puzzle. Improving the models by incorporating these results from the LHC may help to solve the puzzle.
abelchio Mon, 06/02/2025 - 13:25 Byline ALICE collaboration Publication Date Mon, 06/02/2025 - 13:22Should a Higgs factory be a linear or circular electron–positron collider? Intriguingly, the key strategic technology may be the same in either case. Klystrons consume the majority of power at electron–positron colliders of all mature designs. The trouble is that they are typically only 60% efficient. In our cover feature, Igor Syratchev and Nuria Catalan Lasheras describe CERN’s pursuit of 90% efficiency – a feat that would have major implications for the environmental sustainability and cost effectiveness of any Higgs factory.
Klystrons date back to the development of radar in the run-up to World War II. Remarkably, astroparticle physicists are now trying to use radar to detect not enemy aircraft but ultra-high-energy neutrinos. The technology is different, but the underlying principle is the same.
This is just one facet of the ambitious global strategy described by Lu Lu of the University of Wisconsin–Madison in her feature on future neutrino observatories. Only one ultra-high-energy neutrino has been observed so far. Lu describes plans for a worldwide network of observatories offering full-sky coverage. The reward will be cosmic messengers that slice through dust and magnetic fields to point to the most extreme environments in the universe – and energies that are inconceivable in terrestrial colliders.
Elsewhere in these pages: CERN’s Verena Kain explains why AI-based automation is the future of accelerator operation; Beate Heinemann describes her strategy for DESY’s future; LHC researchers debate an unexpected pseudoscalar excess at the top–antitop threshold; DESI suggests dark energy may be evolving; an analysis of community inputs to the European Strategy for Particle Physics; how to get a job in machine learning; and much more.
Read the digital edition of this new issue on the CERN Courier website.
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Be the first to know when the new digital issue of CERN Courier is available, email cern.courier@cern.ch to request to receive a new-issue alert.
anschaef Thu, 05/22/2025 - 09:45 Publication Date Thu, 05/22/2025 - 09:42Thanks to the involvement of many early-career researchers (ECRs) over the past few months, the organisers of the ECR white paper for the European Strategy for Particle Physics update (ESPPU) were able to coordinate a collective effort that resulted in the ECR input for the Strategy update – formulated as 55 actionable recommendations to improve our future and the future of the field.
These recommendations cover such key topics as career development, well-being, recognition, science communication and the future of both collider and non-collider projects. In parallel, the organisers released a 100-page ECR white paper (arXiv:2503.19862) detailing the arguments behind those recommendations, grounded in a community-wide survey of over 800 ECRs across Europe.
Don’t miss the hybrid event on Tuesday, 27 May (1.00–4.00 p.m. CEST) to explore and discuss these recommendations and to reflect together on how to move forward as a community. The event is open to all, regardless of career stage or affiliation. We warmly welcome your participation and input.
Registration and more information at: https://indico.cern.ch/event/1540736/
The ESPPU ECR white paper organisers
(esppu-ecr-organisers@cern.ch)
The Higgs boson, discovered at the Large Hadron Collider (LHC) in 2012, plays a central role in the Standard Model of particle physics, endowing elementary particles such as quarks with mass through its interactions. The Higgs boson’s interaction with the heaviest “third-generation” quarks – top and bottom quarks – has been observed and found to be in line with the Standard Model. But probing its interactions with lighter “second-generation” quarks, such as the charm quark, and the lightest “first-generation” quarks – the up and down quarks that make up the building blocks of atomic nuclei – remains a formidable challenge, leaving unanswered the question of whether or not the Higgs boson is responsible for generating the masses of the quarks that make up ordinary matter.
Researchers study the Higgs boson's interactions by looking at how the particle decays into – or is produced with – other particles in high-energy proton–proton collisions at the LHC. At a seminar held at CERN last week, the CMS experiment collaboration reported the results of the first search for a Higgs boson decaying into a pair of charm quarks in collision events where the Higgs boson is produced alongside two top quarks. Exploiting cutting-edge AI techniques, this novel search has been used to set the most stringent limits to date on the interaction between the Higgs boson and the charm quark.
The production of a Higgs boson in association with a top-quark pair, with the Higgs boson decaying into pairs of quarks, is not only a rare process at the LHC but one that is particularly challenging to distinguish from similar-looking background collision events. That’s because quarks immediately produce collimated sprays (or “jets”) of hadrons that travel only a small distance before decaying, making it especially difficult to identify jets originating from charm quarks that are created in the decay of a Higgs boson from jets originating from other types of quark. Traditional identification methods, referred to as “tagging”, struggle to efficiently recognise charm jets, necessitating the development of more advanced discrimination techniques.
“This search required a paradigm shift in analysis techniques,” explains Sebastian Wuchterl, a research fellow at CERN. “Because charm quarks are harder to tag than bottom quarks, we relied on cutting-edge machine-learning techniques to separate the signal from backgrounds.”
The CMS researchers tackled two major hurdles using machine-learning models. The first was the identification of charm jets, which was performed by employing a type of algorithm called a graph neural network. The second was to distinguish Higgs boson signals from background processes, which was addressed with a transformer network – the type of machine learning that is behind ChatGPT but trained to classify events instead of generating dialogues. The charm-tagging algorithm was trained on hundreds of millions of simulated jets to allow it to recognise charm jets with higher accuracy.
Using data collected from 2016 to 2018, combined with the results from previous searches for the decay of the Higgs boson into charm quarks via other processes, the CMS team set the most stringent limits yet on the interaction between the Higgs boson and the charm quark, reporting an improvement of around 35% compared to previous constraints. This places significant bounds on potential deviations from the Standard Model prediction.
“Our findings mark a major step,” says Jan van der Linden, a postdoctoral researcher at Ghent University. “With more data from upcoming LHC runs and improved analysis techniques, we may gain direct insight into the Higgs boson’s interaction with charm quarks at the LHC—a task that was thought impossible a few years ago.”
As the LHC continues to collect data, refinements in charm tagging and Higgs boson event classification could eventually allow CMS, and its companion experiment ATLAS, to confirm the Higgs boson’s decay into charm quarks. This would be a major step towards a complete understanding of the Higgs boson’s role in the generation of mass for all quarks and provide a crucial test of the 50-year-old Standard Model.
jharma Mon, 05/19/2025 - 09:34 Publication Date Mon, 05/19/2025 - 09:31In a paper published in Physical Review Journals, the ALICE collaboration reports measurements that quantify the transmutation of lead into gold in CERN’s Large Hadron Collider (LHC).
Transforming the base metal lead into the precious metal gold was a dream of medieval alchemists. This long-standing quest, known as chrysopoeia, may have been motivated by the observation that dull grey, relatively abundant lead is of a similar density to gold, which has long been coveted for its beautiful colour and rarity. It was only much later that it became clear that lead and gold are distinct chemical elements and that chemical methods are powerless to transmute one into the other.
With the dawn of nuclear physics in the 20th century, it was discovered that heavy elements could transform into others, either naturally, by radioactive decay, or in the laboratory, under a bombardment of neutrons or protons. Though gold has been artificially produced in this way before, the ALICE collaboration has now measured the transmutation of lead into gold by a new mechanism involving near-miss collisions between lead nuclei at the LHC.
Extremely high-energy collisions between lead nuclei at the LHC can create quark–gluon plasma, a hot and dense state of matter that is thought to have filled the universe around a millionth of a second after the Big Bang, giving rise to the matter we now know. However, in the far more frequent interactions where the nuclei just miss each other without “touching”, the intense electromagnetic fields surrounding them can induce photon–photon and photon–nucleus interactions that open further avenues of exploration.
The electromagnetic field emanating from a lead nucleus is particularly strong because the nucleus contains 82 protons, each carrying one elementary charge. Moreover, the very high speed at which lead nuclei travel in the LHC (corresponding to 99.999993% of the speed of light) causes the electromagnetic field lines to be squashed into a thin pancake, transverse to the direction of motion, producing a short-lived pulse of photons. Often, this triggers a process called electromagnetic dissociation, whereby a photon interacting with a nucleus can excite oscillations of its internal structure, resulting in the ejection of small numbers of neutrons and protons. To create gold (a nucleus containing 79 protons), three protons must be removed from a lead nucleus in the LHC beams.
“It is impressive to see that our detectors can handle head-on collisions producing thousands of particles, while also being sensitive to collisions where only a few particles are produced at a time, enabling the study of rare electromagnetic ‘nuclear transmutation’ processes,” says Marco Van Leeuwen, ALICE spokesperson.
The ALICE team used the detector’s zero degree calorimeters (ZDC) to count the number of photon–nucleus interactions that resulted in the emission of zero, one, two and three protons accompanied by at least one neutron, which are associated with the production of lead, thallium, mercury and gold, respectively. While less frequent than the creation of thallium or mercury, the results show that the LHC currently produces gold at a maximum rate of about 89 000 nuclei per second from lead–lead collisions at the ALICE collision point. Gold nuclei emerge from the collision with very high energy and hit the LHC beam pipe or collimators at various points downstream, where they immediately fragment into single protons, neutrons and other particles. The gold exists for just a tiny fraction of a second.
The ALICE analysis shows that, during Run 2 of the LHC (2015–2018), about 86 billion gold nuclei were created at the four major experiments. In terms of mass, this corresponds to just 29 picograms (2.9 ×10-11 g). Since the luminosity in the LHC is continually increasing thanks to regular upgrades to the machines, Run 3 has produced almost double the amount of gold that Run 2 did, but the total still amounts to trillions of times less than would be required to make a piece of jewellery. While the dream of medieval alchemists has technically come true, their hopes of riches have once again been dashed.
“Thanks to the unique capabilities of the ALICE ZDCs, the present analysis is the first to systematically detect and analyse the signature of gold production at the LHC experimentally,” says Uliana Dmitrieva of the ALICE collaboration.
“The results also test and improve theoretical models of electromagnetic dissociation which, beyond their intrinsic physics interest, are used to understand and predict beam losses that are a major limit on the performance of the LHC and future colliders,” adds John Jowett, also of the ALICE collaboration.
Additional image:
Illustration of an ultra-peripheral collision where the two lead (208Pb) ion beams at the LHC pass by close to each other without colliding. In the electromagnetic dissociation process, a photon interacting with a nucleus can excite oscillations of its internal structure and result in the ejection of small numbers of neutrons (two) and protons (three), leaving the gold (203Au) nucleus behind (Image: CERN)
angerard Wed, 04/30/2025 - 16:50 Publication Date Thu, 05/08/2025 - 09:00
Hans Gutbrod, a pioneer of heavy-ion physics from its beginnings in the 1970s, passed away on 3 March 2025. He was one of the founding members of the ALICE collaboration at CERN and leader of the FAIR core team at GSI, Darmstadt.
Hans studied at the Technical University of Karlsruhe and at the University of Heidelberg, where he received his doctorate in 1970 under the supervision of Wolfgang Gentner and Rudolf Bock. He then worked in the field of heavy-ion physics at low energies in Heidelberg, in Rochester, New York, at Brookhaven National Laboratory and in Berkeley, California. In 1974, he was invited by Reinhard Stock to work with him at the newly commissioned Bevalac at the Lawrence Berkeley National Laboratory (then LBL, now LBNL). During that time, they pondered whether nuclei would be dense enough to create a compressed fireball in relativistic nuclear collisions or would pass through one another, creating shock waves. From 1978 to 1981, he collaborated with Arthur Poskanzer and Hans Georg Ritter to construct and build, at the Bevalac, the GSI–LBL “Plastic Ball” 4π detector, the first electronic detector in relativistic nuclear physics with 4π coverage and particle identification. They discovered the collective behaviour of nuclear matter, called “flow”, which is still considered one of the most significant observations ever made in relativistic heavy-ion physics. Together with Reinhard Stock, Hans was awarded the Robert-Wichard-Pohl-Preis by the German Physical Society in 1988.
In 1982, GSI, CERN and LBL signed a memorandum of understanding to bring heavy ions to CERN, first at the PS and then at the SPS. Hans moved to CERN and brought with him components of the Plastic Ball detector, which became an integral part of the first-generation SPS experiment WA80. The experiment later evolved into WA93 and WA98. His fixed-target experiments at the SPS made significant contributions to heavy-ion physics, including the first measurement of elliptic flow and numerous photon-related results, such as direct photon and neutral pion production, as well as neutral-to-charged particle ratios for studying disoriented chiral condensates.
When discussions about a heavy-ion programme at the LHC began in the early 1990s, Hans was immediately and enthusiastically involved, contributing to both physics ideas and detector concepts. His boundless energy and collaborative spirit brought together many partners from his SPS experiments, especially his colleagues from India and Russia. As one of ALICE’s founding figures, he played a crucial role in key design choices, including the L3 magnet reuse, a dedicated muon arm (a development project he led for many years), the ALICE Photon Spectrometer and large-area calorimetry – some of which could only be realised much later due to practical constraints. Hans served as ALICE’s first deputy spokesperson for a decade, from the submission of the letter of intent in 1993 until 2003.
In 1995, Hans became the director of the newly founded Subatech laboratory in Nantes, where he facilitated collaboration with international experiment groups such as STAR, PHENIX and ALICE. He founded a research group focused on nuclear applications and was an initiator of the design of the ARRONAX cyclotron, a powerful tool for research and the production of radioisotopes.
In 2001, Hans returned to GSI to work on the FAIR project, an advanced research facility on antiprotons and heavy ions. His inspiring personality and imagination were invaluable in establishing the project and he made significant contributions to the design as the leader of the FAIR Joint Core Team. He actively promoted broad cooperation and his close ties with Indian institutes were instrumental in encouraging their participation in FAIR. Hans also played a significant role in preparing the heavy-ion programme at the NICA facility at JINR in Dubna, Russia, where he served on the Programme Advisory Committee and the Detector Advisory Committee for more than 10 years.
He held an honorary professorship in the physics department of Goethe University Frankfurt am Main and was awarded an honorary doctorate by the University of Lund, Sweden. He had also been a Fellow of the American Physical Society since 1992.
Hans’s contributions to experimental physics were visionary and far-reaching. His scientific legacy lives on through the experiments that he helped shape and the many colleagues and collaborators whom he mentored and inspired throughout his career. He will be remembered as an outstanding scientist, a cherished colleague and a remarkable person. His enthusiasm and openness to new ideas will always remain in our hearts.
We extend our deepest condolences to his family, close friends and collaborators. We shall miss him.
His friends and colleagues in the ALICE collaboration
ndinmore Tue, 04/15/2025 - 09:56 Publication Date Tue, 04/15/2025 - 12:53The 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 their spin aligned with the direction of their momentum, something that is only possible for particles that have mass. The existence of longitudinally polarised W and Z bosons (WL and ZL) 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 WL-WL interaction 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.”
Read more in the supporting ATLAS note and physics briefing.
angerard Thu, 04/10/2025 - 13:23 Byline ATLAS collaboration Publication Date Thu, 04/10/2025 - 13:21The CMS collaboration at CERN has observed an unexpected feature in data produced by the Large Hadron Collider (LHC), which could point to the existence of the smallest composite particle yet observed. The result, reported at the Rencontres de Moriond conference in the Italian Alps this week, suggests that top quarks – the heaviest and shortest lived of all the elementary particles – can momentarily pair up with their antimatter counterparts to produce an object called toponium. Other explanations cannot be ruled out, however, as the existence of toponium was thought too difficult to verify at the LHC, and the result will need to be further scrutinised by CMS’s sister experiment, ATLAS.
High-energy collisions between protons at the LHC routinely produce top quark–antiquark pairs (tt-bar). Measuring the probability, or cross section, of tt-bar production 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 50-year-old theory. Many of the open questions in particle physics, such as the nature of dark matter, motivate the search for new particles that may be too heavy to have been produced in experiments so far.
CMS researchers were analysing a large sample of tt-bar production data collected in 2016–2018 to search for new types of Higgs bosons when they spotted something unusual. Additional Higgs-like particles are predicted in many extensions of the Standard Model. If they exist, such particles are expected to interact most strongly with the singularly massive top quark, which weighs in at 184 times the mass of the proton. And if they are massive enough to decay into a top quark–antiquark pair, this should dominate the way they decay inside detectors, with the two massive quarks splintering into “jets” of particles.
Observing more top–antitop pairs than expected is therefore often considered to be a smoking gun for the presence of additional Higgs-like bosons. The CMS data showed just such a surplus. Intriguingly, however, the collaboration observed the excess top-quark pairs at the minimum energy required to produce a pair of top quarks. This led the team to consider an alternative hypothesis long considered difficult to detect: a short-lived union of a top quark and a top antiquark, or toponium.
While tt-bar pairs do not form stable bound states, calculations in quantum chromodynamics – which describes how the strong nuclear force binds quarks into hadrons – predict bound-state enhancements at the tt-bar production threshold. Though other explanations – including an elementary boson such as appears in models with additional Higgs bosons – cannot be ruled out, the cross section that CMS obtains for a simplified toponium-production hypothesis is 8.8 picobarns with an uncertainty of about 15%. This passes the “five sigma” level of certainty required to claim an observation in particle physics, and makes it extremely unlikely that the excess is just a statistical fluctuation.
If the result is confirmed, toponium would be the final example of quarkonium – a term for unstable quark–antiquark states formed from pairings of the heavier charm, bottom and perhaps top quarks. Charmonium (charm–anticharm) was discovered simultaneously at Stanford National Accelerator Laboratory in California and Brookhaven National Laboratory in New York in the November Revolution in particle physics of 1974. Bottomonium (bottom–antibottom) was discovered at Fermi National Accelerator Laboratory in Illinois in 1977. Charmonium and bottomonium are approximately 0.6 and 0.4 femtometres in size respectively, where one femtometre is a millionth of a nanometre. Bottomonium is thought to be the smallest hadron yet discovered. Given its larger mass, toponium is expected to be far smaller – qualifying it as the smallest known hadron.
For a long time, it was thought that toponium bound states were unlikely to be detected in hadron–hadron collisions. The top quark decays into a bottom quark and a W boson in the time it takes light to travel just 0.1 femtometre – a fraction of the size of the particle itself. Toponium would therefore be unique among quarkonia in that its decay would be triggered by the spontaneous disintegration of one of its constituent quarks rather than by the mutual annihilation of its matter and antimatter components.
CMS and ATLAS are now working closely to study the effect, which remains an open scientific question.
For further details, consult the full report in CERN Courier magazine or visit the CMS website.
ndinmore Thu, 04/03/2025 - 14:38 Publication Date Thu, 04/03/2025 - 14:31In late 2023, Wojciech Brylinski was analysing data from the NA61/SHINE collaboration at CERN for his thesis when he noticed an unexpected anomaly – a strikingly large imbalance between charged and neutral kaons in argon–scandium collisions. He found that, instead of being produced in roughly equal numbers, charged kaons were produced 18.4% more often than neutral kaons. This suggested that the so-called “isospin symmetry” between up and down quarks might be broken by more than expected due to the differences in their electric charges and masses – a discrepancy that existing theoretical models would struggle to explain. Known sources of isospin asymmetry only predict deviations of a few percent.
“When Wojciech got started, we thought it would be a trivial verification of the symmetry,” says Marek Gaździcki, who was spokesperson of NA61/SHINE at the time of the discovery. “We expected the symmetry to be closely obeyed – although we had previously measured these types of discrepancies at the NA49 experiment, they had large uncertainties and were not significant.”
Isospin symmetry is one facet of flavour symmetry, whereby the strong interaction treats all quark flavours identically. This means that all types of quarks should behave the same under the strong interaction, except for kinematic differences arising from their different masses. Isospin is not a symmetry of the electromagnetic interaction as up and down quarks have different electric charges. According to isospin symmetry, strong interactions in heavy-ion collisions should generate nearly equal amounts of charged kaons (comprising either an up quark and a strange antiquark or an up antiquark and a strange quark) and neutral kaons (comprising either a down quark and a strange antiquark or a down antiquark and a strange quark), given the similar masses of the up and down quarks. NA61/SHINE’s data contradicts the hypothesis of equal yields with a 4.7σ significance.
“I see two ways to interpret the results,” says Francesco Giacosa, a theoretical physicist working with NA61/SHINE. “First, we might be substantially underestimating the role of electromagnetic interactions in creating quark–antiquark pairs. Second, these results could mean that strong interactions do not obey flavour symmetry. If this is true, it would contradict physicists’ current understanding of quantum chromodynamics (QCD), that is, how quarks and gluons (carriers of the strong interaction) combine.”
While the experiment routinely measures particle yields in nuclear collisions, finding a discrepancy in isospin symmetry was not something the researchers were actively looking for. NA61/SHINE’s primary focus is studying properties of the production of hadrons in the production of hadrons when beams from CERN’s Super Proton Synchrotron collide with a variety of fixed nuclear targets. This data is also shared with neutrino and cosmic ray experiments, such as T2K, to help them to refine their models.
The collaboration is now planning additional studies on this new result, using different projectiles, targets and collision energies to determine whether this effect is unique to certain heavy-ion collisions or is a more general feature of high-energy interactions. It has also put out a call to theoretical physicists to help explain what might have caused such an unexpectedly large asymmetry.
“We tried to fit the data into the current, existing models, but it didn’t work at all — it was just not possible,” says Giacosa. “We need more experimental data and more theoretical predictions to fill our gap in knowledge of the strong interaction. So the real question is: what’s next?”
Read more:
ndinmore Thu, 03/27/2025 - 17:26 Byline Alex Epshtein Publication Date Fri, 03/28/2025 - 09:23
Quantum entanglement is a fascinating phenomenon where two particles’ states are tied to each other, no matter how far apart the particles are. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments involving entangled photons. These experiments confirmed the predictions for the manifestation of entanglement that had been made by the late CERN theorist John Bell. This phenomenon has so far been observed in a wide variety of systems, such as in top quarks at CERN’s Large Hadron Collider (LHC) in 2024. Entanglement has also found several important societal applications, such as quantum cryptography and quantum computing. Now, it also explains the famous herd mentality of sheep.
A flock of sheep (ovis aries) has roamed the CERN site during the spring and summer months for over 40 years. Along with the CERN shepherd, they help to maintain the vast expanses of grassland around the LHC and are part of the Organization’s long-standing efforts to protect the site’s biodiversity. In addition, their flocking behaviour has been of great interest to CERN's physicists. It is well known that sheep behave like particles: their stochastic behaviour has been studied by zoologists and physicists alike, who noticed that a flock’s ability to quickly change phase is similar to that of atoms in a solid and a liquid. Known as the Lamb Shift, this can cause them to get themselves into bizarre situations, such as walking in a circle for days on end.
Now, new research has shed light on the reason for these extraordinary abilities. Scientists at CERN have found evidence of quantum entanglement in sheep. Using sophisticated modelling techniques and specialised trackers, the findings show that the brains of individual sheep in a flock are quantum-entangled in such a way that the sheep can move and vocalise simultaneously, no matter how far apart they are. The evidence has several ramifications for ovine research and has set the baa for a new branch of quantum physics.
“The fact that we were having our lunch next to the flock was a shear coincidence,” says Mary Little, leader of the HERD collaboration, describing how the project came about. “When we saw and herd their behaviour, we wanted to investigate the movement of the flock using the technology at our disposal at the Laboratory.”
Observing the sheep’s ability to simultaneously move and vocalise together caused one main question to aries: since the sheep behave like subatomic particles, could quantum effects be the reason for their behaviour?
“Obviously, we couldn’t put them all in a box and see if they were dead or alive,” said Beau Peep, a researcher on the project. “However, by assuming that the sheep were spherical, we were able to model their behaviour in almost the exact same way as we model subatomic particles.”
Using sophisticated trackers, akin to those in the LHC experiments, the physicists were able to locate the precise particles in the sheep’s brains that might be the cause of this entanglement. Dubbed “moutons” and represented by the Greek letter lambda, l, these particles are leptons and are close relatives of the muon, but fluffier.
The statistical significance of the findings is 4 sigma, which is enough to show evidence of the phenomenon. However, it does not quite pass the baa to be classed as an observation.
“More research is needed to fully confirm that this was indeed an observation of ovine entanglement or a statistical fluctuation,” says Ewen Woolly, spokesperson for the HERD collaboration. “This may be difficult, as we have found that the research makes physicists become inexplicably drowsy.”
“While entanglement is now the leading theory for this phenomenon, we have to take everything into account,” adds Dolly Shepherd, a CERN theorist. “Who knows, maybe further variables are hidden beneath their fleeces. Wolves, for example.”
Theoretical physicist John Ellis, pioneer of the penguin diagram, with its updated sheep version. Scientists at CERN find evidence of quantum entanglement in sheep in 2025, the year declared by the United Nations as the International Year of Quantum Science and Technology. (Image: CERN) ndinmore Thu, 03/27/2025 - 16:27 Byline Naomi Dinmore Publication Date Tue, 04/01/2025 - 08:19Yesterday, at the annual Rencontres de Moriond conference taking place in La Thuile, Italy, the LHCb collaboration at CERN reported a new milestone in our understanding of the subtle yet profound differences between matter and antimatter. In its analysis of large quantities of data produced by the Large Hadron Collider (LHC), the international team found overwhelming evidence that particles known as baryons, such as the protons and neutrons that make up atomic nuclei, are subject to a mirror-like asymmetry in nature’s fundamental laws that causes matter and antimatter to behave differently. The discovery provides new ways to address why the elementary particles that make up matter fall into the neat patterns described by the Standard Model of particle physics, and to explore why matter apparently prevailed over antimatter after the Big Bang.
First observed in the 1960s among a class of particles called mesons, which are made up of a quark–antiquark pair, the violation of “charge-parity (CP)” symmetry has been the subject of intense study at both fixed-target and collider experiments. While it was expected that the other main class of known particles – baryons, which are made up of three quarks – would also be subject to this phenomenon, experiments such as LHCb had only seen hints of CP violation in baryons until now.
“The reason why it took longer to observe CP violation in baryons than in mesons is down to the size of the effect and the available data,” explains LHCb spokesperson Vincenzo Vagnoni. “We needed a machine like the LHC capable of producing a large enough number of beauty baryons and their antimatter counterparts, and we needed an experiment at that machine capable of pinpointing their decay products. It took over 80 000 baryon decays for us to see matter–antimatter asymmetry with this class of particles for the first time.”
Particles are known to have identical mass and opposite charges with respect to their antimatter partners. However, when particles transform or decay into other particles, for example as occurs when an atomic nucleus undergoes radioactive decay, CP violation causes a crack in this mirror-like symmetry. The effect can manifest itself in a difference between the rates at which particles and their antimatter counterparts decay into lighter particles, which physicists can log using highly sophisticated detectors and data analysis techniques.
The LHCb collaboration observed CP violation in a heavier, short-lived cousin of protons and neutrons called the beauty-lambda baryon Λb, which is composed of an up quark, a down quark and a beauty quark. First, they sifted through data collected by the LHCb detector during the first and second runs of the LHC (which lasted from 2009 to 2013 and from 2015 to 2018, respectively) in search of the decay of the Λb particle into a proton, a kaon and a pair of oppositely charged pions, as well as the corresponding decay of its antimatter counterpart, the anti-Λb. They then counted the numbers of the observed decays of each and took the difference between the two.
The analysis showed that the difference between the numbers of Λb and anti-Λb decays, divided by the sum of the two, differs by 2.45% from zero with an uncertainty of about 0.47%. Statistically speaking, the result differs from zero by 5.2 standard deviations, which is above the threshold required to claim an observation of the existence of CP violation in this baryon decay.
While it has long been expected that CP violation exists among baryons, the complex predictions of the Standard Model of particle physics are not yet precise enough to enable a thorough comparison between theory and the LHCb measurement.
Perplexingly, the amount of CP violation predicted by the Standard Model is many orders of magnitude too small to account for the matter–antimatter asymmetry observed in the Universe. This suggests the existence of new sources of CP violation beyond those predicted by the Standard Model, the search for which is an important part of the LHC physics programme and will continue at future colliders that may succeed it.
“The more systems in which we observe CP violations and the more precise the measurements are, the more opportunities we have to test the Standard Model and to look for physics beyond it,” says Vagnoni. “The first ever observation of CP violation in a baryon decay paves the way for further theoretical and experimental investigations of the nature of CP violation, potentially offering new constraints for physics beyond the Standard Model.”
“I congratulate the LHCb collaboration on this exciting result. It again underlines the scientific potential of the LHC and its experiments, offering a new tool with which to explore the matter–antimatter asymmetry in the Universe,” says CERN Director for Research and Computing, Joachim Mnich.
angerard Tue, 03/25/2025 - 10:50 Publication Date Tue, 03/25/2025 - 17:00
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