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Marking a major step in shaping the future of particle physics, the Physics Briefing Book for the 2026 update of the European Strategy for Particle Physics (ESPP) was released on 2 October. The document synthesises all the current input for the community-driven ESPP process and provides the foundation, alongside the final national input and an assessment of large accelerator projects by a dedicated working group, for the European Strategy Group (ESG) to formulate its recommendations in December.
The ESPP 2026 update, which was launched by the CERN Council in March 2024, called upon the particle physics community to develop a visionary and concrete plan that greatly advances knowledge in fundamental physics through the realisation of the next flagship project at CERN. A total of 266 written submissions, ranging from individual to national perspectives, were received. These formed the basis of rich discussions at an Open Symposium held in Venice from 23 to 27 June 2025, which brought together more than 600 physicists from almost 40 countries.
More than 600 physicists attended the Open Symposium in Venice from 23 to 27 June to debate the future of European particle physics. (Image: INFN)The Briefing Book, compiled by experts in the Physics Preparatory Group (PPG), distils these discussions and all the community input received so far into a single document that has been handed to the ESG. The document does not prescribe a single path forward but evaluates the scientific potential of different facilities and experiments. Following the recommendations of the ESPP 2020 update, it prioritises the need for an electron–positron collider dedicated to precision Higgs boson studies and, in the longer term, an energy-frontier collider.
Arranged in 12 chapters, the Briefing Book summarises the outstanding questions across the various physics areas, together with a discussion of the potential of the different proposed colliders and other experiments to address them. The differences in the physics potential between the various collider options, along with the technical readiness, risks, timescales and costs, will be reviewed to enable the ESG to produce its final recommendations. Crucially, the CERN Council requested that the community indicate not only the scientifically most attractive option, but also alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive.
“We wish to deeply thank the co-conveners, scientific secretaries and all members of the PPG working groups for their hard work and dedication in summarising the main messages from the many strategy input submissions and the discussions at the Open Symposium in this book,” says Karl Jakobs, Strategy Secretary, University of Freiburg. “As we have seen from the input so far, the ESSP 2026 update has revealed a vibrant scientific landscape across high-energy physics and a community united in its desire for a future flagship collider at CERN.”
The next step towards updating the ESPP is the submission of the final national input, with a deadline of 14 November. The ESG project-assessment working group will release its findings on 17 October such that they can be taken into account. The final drafting session of the Strategy update will then take place from 1 to 5 December at Monte Verità Ascona, Switzerland, where the community recommendations will be finalised. These will be presented to the CERN Council in March 2026 and discussed at a dedicated meeting of the CERN Council in May 2026 in Budapest.
Further information:
European Strategy update: the community speaks
https://cerncourier.com/european-strategy-update-the-community-speaks/
Europe’s collider strategy takes shape
https://cerncourier.com/a/europes-collider-strategy-takes-shape/
The LHCb collaboration has released the results of its latest analysis of the rare decay of the beauty meson B0 into a K* meson and a pair of muons (B0→K*μ+μ–). Based on data from the first and second runs of the LHC, the new analysis confirms a tension with predictions from the Standard Model that was observed in previous analyses. However, such predictions are highly complex and are a topic of debate within the theoretical physics community.
The B0→K*μ+μ– decay offers a promising, indirect way to search for new phenomena, as it is sensitive to contributions from undiscovered particles that may have masses beyond the reach of direct searches at the LHC. By comparing precise measurements of the decay’s properties with Standard Model predictions, possible signs of new fundamental particles or interactions could be revealed.
The best sensitivity to new particles comes from the study of the angular distributions of the decay products, namely a kaon and a pion from the K* decay and the two muons. The LHCb team had previously measured these angular observables first using proton–proton collision datasets from LHC Run 1 and then a larger dataset that also included LHC Run 2 data taken in 2016.
In all of these earlier analyses, one of the observables, called P5’, showed a significant deviation from the Standard Model. This result had a statistical significance below the ‘five-sigma' gold standard required to claim a discovery, but it was significant enough to warrant attention in future studies.
The latest LHCb analysis represents the most sophisticated study of the B0→K*μ+μ– decay properties to date, using proton–proton collision data collected by LHCb in 2011, 2012 and 2016–2018. This analysis confirms that the P5’ observable is in significant tension with theoretical predictions. The results are in good agreement with the previous LHCb measurements and also with a recent CMS measurement.
“The new measurements show the same pattern of tensions with the Standard Model that we have seen before,” says Mark Smith, who presented the details of the analysis at a recent LHC seminar. “Further clarifying the experimental picture with the LHC Run 3 dataset and improving theoretical calculations should help determine the origin of the observed patterns,” says Leon Carus, who presented the results at the latest LHC Committee Open Session.
Read more on the LHCb website.
abelchio Tue, 09/23/2025 - 15:50 Byline LHCb collaboration Publication Date Wed, 09/24/2025 - 11:00
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.
Further material
Animation showing side-by-side comparison of lead-lead and oxygen-oxygen collision
Animations showing the quark–gluon plasma formed in collisions between heavy ions
rodrigug Thu, 09/18/2025 - 11:23 Publication Date Thu, 09/18/2025 - 14:30The quark model. The Ω-. The cosmic microwave background. Charm. The Brout–Englert–Higgs mechanism. CP violation. Colour. 1964 was a remarkable year for invention and discovery . The story of quarkonia began one year earlier, in 1963. As the ATLAS collaboration joins CMS in reporting an excess near the top–antitop production threshold, John Ellis asks whether quarkonia’s final chapter is now being written.
In 2025, the community has the opportunity to shape strategic investments for decades to come. CERN Council president Costas Fountas and European strategy secretary Karl Jakobs report a growing consensus on the future of the field.
The cover is a classic photograph of the OPAL detector at LEP – just one of the historic experiments whose software and data are being given a new lease of life, decades after data-taking ended. As the LHC surpasses one exabyte of stored data, Cristinel Diaconu and Ulrich Schwickerath call for new collaborations to join a global effort in data preservation, to allow future generations to unearth the hidden treasures.
Elsewhere in this issue of CERN Courier: should dark energy evolve?; scalable technology for precision neutrino physics with small detectors; tips from IBM’s head of science and technology on how to get a job in industry; an update on the ATOMKI anomaly; Andreas Hoecker’s highlights from EPS–HEP; and much more.
Read the digital edition of this new issue on the CERN Courier website.
________
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.
ehatters Tue, 09/09/2025 - 17:33 Publication Date Tue, 09/09/2025 - 17:24The atomic nucleus was discovered over a century ago, yet many questions remain about the force that keeps its constituent protons and neutrons together and the way in which these particles pack themselves together within it.
In the classic nuclear shell model, protons and neutrons arrange themselves in shells of increasing energy, and completely filled outer shells of protons or neutrons result in particularly stable “magic” nuclei. But the model only works for nuclei with the right mix of protons and neutrons. Get the wrong mix and the model breaks down.
Identifying the regions on the chart of nuclei where this breakdown occurs is keeping nuclear physicists busy worldwide. The goal? To develop a model that applies to all nuclei and leads to a deeper understanding of their internal structure.
In a paper just published in Physical Review C, Louis Lalanne and his colleagues report data from CERN’s ISOLDE facility that allowed them to determine the western border of one such region – the “island of inversion” associated with the neutron number 40.
The 40-neutron island of inversion is one of only a few small islands of unusual nuclei in a sea of mostly “normal” nuclei at the neutron-rich edge of the nuclear chart. In these insular regions, the usual order of nuclear shell filling breaks down and neutrons occupy shells other than those where we expect to find them. This uncommon shell filling gives these nuclei unusual shapes and properties compared to their neighbours.
To explore the 40-neutron island of inversion, Lalanne and his co-workers used ISOLDE, a unique facility for the production and study of nuclei that have too many or too few neutrons to be stable. Specifically, they created and investigated the little-studied chromium-61 nucleus, which has 24 protons and 37 neutrons and was thought to be located right at the western shore of the 40-neutron island of inversion.
Using measurements taken with the facility’s collinear resonance ionisation spectroscopy (CRIS) apparatus, which allows neutron-rich nuclei to be studied with high precision, the researchers determined two properties of chromium-61 known as spin and magnetic dipole moment.
Paired with theoretical calculations, these measurements showed that chromium-61 has a shell-filling configuration that lies between the one expected for nuclei located outside the 40-neutron island of inversion and that expected for nuclei that lie within it – thus determining the western border of the 40-neutron island of inversion.
"The ultimate goal is to understand how nuclear structure emerges and evolves across the nuclear landscape,“ says Louis Lalanne. "Islands of inversion are important because they represent regions of rapid evolution that challenge our understanding. This result is helping us to build a clearer picture of the mechanism driving this evolution."
abelchio Tue, 08/05/2025 - 14:37 Byline Ana Lopes Publication Date Tue, 09/02/2025 - 17:00In a breakthrough for antimatter research, the BASE collaboration at CERN has kept an antiproton – the antimatter counterpart of a proton – oscillating smoothly between two different quantum states for almost a minute while trapped. The achievement, reported in a paper published today in the journal Nature, marks the first demonstration of an antimatter quantum bit, or qubit, and paves the way for substantially improved comparisons between the behaviour of matter and antimatter.
Particles such as the antiproton, which has the same mass but opposite electrical charge to a proton, behave like miniature bar magnets that can “point” in one of two directions depending on their underlying quantum mechanical spin.
Measuring the way these so-called magnetic moments flip, using a technique called coherent quantum transition spectroscopy, is a powerful tool in quantum sensing and information processing. It also enables high-precision tests of the fundamental laws of nature, including charge-parity-time symmetry. This symmetry rules that matter and antimatter behave identically, which is at odds with the observation that matter vastly outweighs antimatter in the Universe.
Particles have quantum characteristics that defy our common sense, such as the characteristic of interfering with themselves, as demonstrated in the double slit experiment. Interactions with the surrounding environment can quickly suppress these interference effects through a process known as quantum decoherence. Preserving coherence is essential for controlling and tracking the evolution of quantum systems, like the transitions between the spin states of a single antiproton.
Although coherent quantum transitions have been observed before in large collections of particles and in trapped ions, they have never been seen for a single free nuclear magnetic moment – despite the latter featuring prominently in physics textbooks. The BASE collaboration has now achieved this at CERN’s antimatter factory.
In some respects, the feat can be likened to pushing a child on a playground swing. With the right push, the swing arcs back and forth in a perfect rhythm. Now imagine that the swing is a single trapped antiproton oscillating between its spin “up” and “down” states in a smooth, controlled rhythm. The BASE collaboration has achieved this using a sophisticated system of electromagnetic traps to give an antiproton the right “push” at the right time. And since this swing has quantum properties, the antimatter spin-qubit can even point in different directions at the same time when unobserved.
The BASE experiment studies antiprotons produced at CERN’s antimatter factory by storing them in electromagnetic Penning traps and feeding them one by one into a second multi-trap system to, among other things, measure and change their spin states. Using this set-up, the BASE collaboration has previously been able to show that the magnitudes of the magnetic moments of the proton and antiproton are identical within a just few parts-per-billion. Any slight difference in their magnitudes would break charge-parity-time symmetry and point to new physics beyond the Standard Model of particle physics.
However, this previous result was based on an incoherent spectroscopy technique in which the quantum transitions were disturbed by magnetic field fluctuations and measurement interference. In a substantial upgrade of the experiment, these decoherence mechanisms were suppressed and eliminated, culminating in the first coherent spectroscopy of an antiproton spin. The BASE team has now accomplished this for a period – called spin coherence time – of 50 seconds.
“This represents the first antimatter qubit and opens up the prospect of applying the entire set of coherent spectroscopy methods to single matter and antimatter systems in precision experiments,” explains BASE spokesperson Stefan Ulmer. “Most importantly, it will help BASE to perform antiproton moment measurements in future experiments with 10- to 100-fold improved precision.”
While qubits are the basic building blocks of quantum computers, where they allow information to be stored not just in one of two states but via a potentially limitless superposition of those states, the antimatter qubit demonstrated by BASE is unlikely to have immediate applications outside fundamental physics.
An even bigger leap in the precision of antiproton measurements is expected using BASE-STEP, which was designed to allow trapped antiparticles to be transported by road to magnetic environments that are “calmer” than the antimatter factory. “Once it is fully operational, our new offline precision Penning trap system, which will be supplied with antiprotons transported by BASE-STEP, could allow us to achieve spin coherence times maybe even ten times longer than in current experiments, which will be a game-changer for baryonic antimatter research,” says lead author of the paper Barbara Latacz.
angerard Mon, 07/21/2025 - 16:50 Publication Date Wed, 07/23/2025 - 17:00Studies of the properties of the Higgs boson featured prominently in the programme of the major annual physics conference, the 2025 European Physical Society Conference on High Energy Physics (EPS-HEP), held this week in Marseille, France. Among the results, the ATLAS collaboration presented two analyses narrowing in on two exceptionally rare Higgs-boson decays.
The first process under study was the Higgs-boson decay into a pair of muons (H→μμ). Despite its scarceness – occurring in just 1 out of every 5000 Higgs decays – this process provides the best opportunity to study the Higgs interaction with second-generation fermions and shed light on the origin of mass across different generations. Up to now, the interactions of the Higgs boson with matter particles have only been observed for particles from the third, heaviest, generation: the tau lepton and the top and bottom quarks.
The second process investigated was the Higgs-boson decay into a Z boson and a photon (H→Zγ), where the Z boson subsequently decays into electron or muon pairs. This rare decay is especially intriguing, as it proceeds via an intermediate “loop” of virtual particles. If new, unknown particles contribute to this loop, the process could offer hints of physics beyond the Standard Model.
Identifying these rare decays is quite the challenge. For H→μμ, researchers looked for a small excess of events clustering near a muon-pair mass of 125 GeV (the mass of the Higgs boson). This signal can be easily hidden behind the thousands of muon pairs produced through other processes (“background”). The H→Zγ decay with the Z decaying into electrons or muons is even harder to isolate, due to the Z boson only decaying this way only about 6% of the time and photons being easily mimicked by particle jets.
Event display of a candidate Higgs boson decaying to a photon and a Z boson, with the Z subsequently decaying to an electron-positron pair. (Image: ATLAS/CERN)To boost the sensitivity of their searches, ATLAS physicists combined the first three years of LHC Run 3 data with the full LHC Run 2 data. They also developed a sophisticated method to better model background processes, categorised recorded events by the specific Higgs-production modes and made further improvements to their event-selection techniques.
In previous searches for H→μμ using the full Run 2 data set, the ATLAS collaboration saw its first hint of this process at the level of 2 standard deviations, while the CMS collaboration reached a significance of 3 standard deviations with 2.5 standard deviations expected. Now, with the combined Run 2 and Run 3 data sets, the ATLAS collaboration has found evidence for H→μμ with an expected significance of 2.5 standard deviations and an observed significance of 3.4 standard deviations. This means that the chance that the result is a statistical fluctuation is less than 1 in 3000!
As for the H→Zγ process, a previous ATLAS and CMS combined analysis used Run 2 data to find evidence of this decay mode. It reported an excess over the background-only hypothesis of 3.4 standard deviations with 1.6 standard deviations expected. The latest ATLAS result, combining Run 2 and Run 3 data, reported an excess of 2.5 standard deviations. The expected sensitivity of this analysis is 1.9 standard deviations, providing the most stringent expected sensitivity to date for measuring the decay probability (“branching fraction”) of H→Zγ.
These achievements were made possible by the large, excellent data set provided by the LHC, the outstanding efficiency and performance of the ATLAS experiment and the use of novel analysis techniques. With more data on the horizon, the journey of exploration continues!
Read more on the ATLAS website.
ptraczyk Fri, 07/11/2025 - 14:56 Byline ATLAS collaboration Publication Date Fri, 07/11/2025 - 14:51Just 100 years ago, on 9 July 1925, Werner Heisenberg wrote a letter to his friend, colleague and fiercest critic, Wolfgang Pauli. A few weeks earlier, Heisenberg had returned from the North Sea outpost of Helgoland, where he had laid the foundations of modern quantum mechanics and changed our understanding of the atomic world. The letter, preserved in the Wolfgang Pauli Archive at CERN, reveals Heisenberg’s efforts to liberate physics from the semi-classical picture of atoms as planetary systems, with electrons in orbit around the nucleus.
“All of my pitiful efforts are directed at completely killing off the concept of orbits – which, after all, cannot be observed—and replacing it with something more suitable,” he explains in his letter to Pauli.
By sweeping away the old interpretation, Heisenberg could focus on building a more coherent model, based purely on what the experiments were observing. Attached to the letter was the draft of Heisenberg’s famous Umdeutung paper, which was received for publication a few weeks later, and which is often considered as the birth certificate of modern quantum theory. In the following months, Max Born, Pascual Jordan and Wolfgang Pauli himself helped turn Heisenberg’s work into matrix mechanics, the first mature formulation of quantum theory.
Today, those early reflections underlie the most precise framework in the history of science: the Standard Model of particle physics. Experiments at CERN keep pushing it to extreme regimes, and time and again, it proves astonishingly accurate.
To celebrate 100 years of quantum mechanics, the CERN Courier looks back at the impact of this theory and examines how it keeps delivering new puzzles, experimental ideas and technologies. For instance, quantum sensors may soon extend their reach from low- to high-energy applications, while quantum simulations could help overcome the limits of classical computing in describing extreme environments and complex systems.
Theoretical and philosophical considerations, too, are far from exhausted. Despite its empirical power, there is still no consensus about quantum theory’s true meaning. What guides the emergence of our classical world? Is the wavefunction a real entity, a representation of the observer’s knowledge or an artifact we should abandon altogether? Should we think of measurement apparatus and observers as quantum objects?
Heisenberg himself was cautious yet hopeful, writing to Pauli: “Perhaps people who can do more, will be able to make sense of it.” A century on, physicists are still working to fulfil that dream. Whatever the last word may be, one thing is certain: the conversation sparked on Helgoland is far from over.
One hundred years ago, Werner Heisenberg retreated to the island of Helgoland, where he built the foundations of the first full formulation of quantum mechanics. Finding its matrices repugnant, Erwin Schrödinger developed a mathematically equivalent formulation with a wavefunction and a wave equation. Either way, physics would never be the same again: in quantum mechanics, measurement affects what can be known, and predictions can only ever be probabilistic.
A century has not sufficed to fully understand or exploit the theory – and high-energy physicists today find themselves at an interesting juncture. Detector designs are beginning to push quantum limits. Quantum computing is in its “noisy intermediate-scale” era, poised to apply its remarkable parallelism to simulations beyond the reach of classical supercomputers. And with efforts to move beyond the Standard Model at least temporarily frustrated, increasing numbers of theorists are returning to grapple with the foundational assumptions of quantum mechanics. In this special edition, Carlo Rovelli and David Wallace explore a jungle of ideas to resolve questions that have defied consensus since the earliest days of the theory.
Elsewhere in these pages: Fermilab’s last word on muon g-2; DESY brings practical plasma-wakefield acceleration a step closer; ATLAS and ALICE make the first differential measurements of the radial flow of quark-gluon plasma; a farewell to Mary K. Gaillard; and much more.
Read the digital edition of this new issue on the CERN Courier website.
________
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 Tue, 07/08/2025 - 13:02 Publication Date Wed, 07/09/2025 - 08:01An 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.
“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.
Links
ATLAS Collaboration 2025 (ATLAS-CONF-2025-008): “Observation of a cross-section enhancement near the t¯t production threshold in √s=13 TeV pp collisions with the ATLAS detector”
CMS Collaboration 2024 (TOP-24-007): "Observation of a pseudoscalar excess at the top quark pair production threshold" (Accepted for publication in Reports on Progress in Physics)
jharma Mon, 07/07/2025 - 18:01 Publication Date Mon, 07/07/2025 - 17:58
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