Your shopping cart is empty.
One of the biggest open questions in particle physics today is how the Higgs boson interacts with itself. This “self-coupling” could help explain the evolution of the early Universe and the mechanism that gives mass to elementary particles. To try to shed light on this fundamental interaction, the ATLAS Collaboration has recently studied one of the “golden” decay channels of a pair of Higgs bosons, where one Higgs boson decays into two photons and the other into a pair of bottom quarks.
By combining the entire LHC Run 2 dataset (2015–2018) and a partial Run 3 dataset (2022–2024), the ATLAS team has significantly enhanced the statistical power of the analysis of this decay channel. The result, just published in Physics Letters B, marks the first ATLAS measurement based on over 300 inverse femtobarns (fb⁻¹) of proton–proton collision data, where one inverse femtobarn corresponds to approximately 100 trillion collisions.
Studying this decay channel is particularly challenging due to the extremely rare nature of Higgs boson pair production – predicted to occur once in a trillion proton–proton collisions – and the significant background from Standard Model processes that mimic this decay mode. To overcome these challenges, ATLAS physicists used advanced data analysis techniques, such as machine learning, to help to isolate the decay signal from the background.
As a result of these advancements and the addition of the partial Run 3 dataset, the ATLAS researchers set more stringent limits than they did before on the signal strength (the observed signal divided by the Standard Model prediction) and two key interaction parameters. These are the magnitude of the Higgs boson’s self-coupling divided by its Standard Model prediction, limited to be between −1.6 and 6.6, and the interaction strength between two Higgs bosons and two vector bosons (W or Z bosons) divided by its Standard Model prediction, limited to be between −0.5 and 2.6.
The results underscore the ATLAS Collaboration’s growing ability to explore Higgs boson pair production in this golden decay channel. They also lay the foundation for future measurements of the Higgs boson’s self-coupling – key to understanding the evolution of the Universe after the Big Bang. With the full Run 3 dataset soon to be available and the High-Luminosity LHC on the horizon, ATLAS is well positioned to push these studies even further – sharpening our understanding of the Higgs boson and exploring potential signs of physics beyond the Standard Model.
Read more on the ATLAS website.
ehatters Wed, 04/22/2026 - 10:17 Byline ATLAS collaboration Publication Date Wed, 04/22/2026 - 10:10Tens of kilometres above Earth’s surface, high-energy particles from outer space constantly strike the atmosphere, creating showers of energetic secondary particles that rain down from the sky. Approximately one of these particles passes through your head every second, but the “cosmic rays” that produce them are still not fully understood. In a recent paper, the ATLAS Collaboration describes how its first measurement of proton–oxygen collisions at the LHC could help us learn more about them.
Cosmic rays were discovered over a century ago by physicist Victor Hess in experiments conducted aboard hot-air balloons. Today, astrophysicists use detectors on the ground to image cosmic-ray showers and computer simulations of the showers to understand that data.
However, these simulations depend on properties of the strong force – one of the fundamental forces of the Universe – which is difficult to accurately model. Current simulations disagree with one another, making it difficult for astrophysicists to interpret their measurements of cosmic rays.
In part to help improve these simulations, the LHC was configured to collide protons with oxygen ions for the first time in July 2025. This meant physicists could study ‘recreated’ cosmic-ray collisions in more detail. The beam of protons acted as a cosmic ray, while the beam of oxygen ions played the role of Earth’s atmosphere, which is composed primarily of nitrogen and oxygen.
The new paper describes how ATLAS physicists analysed these collisions by measuring the tracks left in the experiment from electrically charged particles. They measured key properties of the collision, including how often the particles were created, how many were created, and the energies and angles at which they flew out.
They then compared the measured distributions of charged particles with the numbers predicted by various simulations typically used to interpret data from cosmic-ray observatories. These simulations, which are tuned to reproduce data from previous collisions of protons with heavier nuclei, disagree with one another.
The new ATLAS measurements achieve a precision level of a few percent, significantly improving knowledge of proton–oxygen collisions. Theorists can now use this input to refine their models and help shed more light on the mysterious high-energy particles arriving from our cosmos.
Read more on the ATLAS website.
ehatters Mon, 04/20/2026 - 17:33 Byline ATLAS collaboration Publication Date Tue, 04/21/2026 - 10:29According to our current understanding of the Universe, quarks are fundamental, point-like particles: basic building blocks that are not made up of smaller particles. A recent paper from the CMS Collaboration describes how it probed quarks to the scale of 10-20 metres to test this premise.
At this scale, no evidence of constituent particles was identified, but history shows that structures once considered fundamental can reveal deeper layers: matter was found to consist of molecules, which were then found to be made of atoms, which were in turn found to consist of a dense nucleus surrounded by a cloud of electrons.
Rutherford discovered the nucleus by sending a beam of helium nuclei onto a gold-foil target. These nuclei scattered off the gold atoms of the foil at various angles, which Rutherford then measured. By studying the distribution of the scattering angles, he was able to prove that atoms contained a point-like nucleus at the centre. This was possible because the helium beam in the experimental set-up had enough energy to probe the inside of the atoms.
The nucleus was then shown to be made of protons and neutrons, which were themselves later found to consist of quarks. LHC experiments including CMS are now continuing this quest, colliding particles at extremely high energies to probe the potential inner structure of quarks.
When two beams of protons collide within CMS, they break apart into their constituent quarks. These outgoing quarks become two jets – sprays of particles – that can be measured and used to reconstruct the scattering angle between the quarks.
The distribution of the scattering angle between the two jets can be compared to the distribution that would be expected if the quark was indeed a point-like particle. The recent results from the CMS Collaboration, which were based on data from the second run of the LHC, showed no significant disagreement with the scattering distribution of a point-like quark. This means that quarks are not likely to be larger than 10-20 metres if they are composite structures.
This size estimate is derived from the constraints on the energy scale at which quark ‘compositeness’ reveals itself. For the benchmark model of the recent CMS paper, which assumed that quarks were composite, the recent results set the most stringent limit to date at 37 TeV.
Similarly to how Rutherford was able to identify the components of the atom only because his beam of particles had enough energy, studying particle collisions with higher energies could help us to identify smaller potential structures within quarks. Data from the third run of the LHC and the upcoming High-Luminosity LHC could help to reduce the uncertainties on the measurement of the scattering angle, allowing us to identify even smaller structures and continue the search for the smallest building blocks of matter.
A collision event recorded by the CMS detector with two outgoing jets. (Image: CMS) ehatters Thu, 04/16/2026 - 14:57 Byline CMS collaboration Publication Date Thu, 04/16/2026 - 14:46It was with profound shock and sadness that we learned of the sudden passing of staff member Mark Rayner, Editor of CERN Courier magazine, on 23 March.
Mark was born in Hounslow, England, on 7 October 1983 and studied physics at Worcester College, University of Oxford, from 2002 to 2006. His journey in particle physics began in 2005, when he spent three months at CERN as a Summer Student working on tests of the ATLAS transition-radiation-tracker end caps. He continued at Oxford with a PhD, participating in the Muon Ionisation Cooling Experiment (MICE) based at the Rutherford Appleton Laboratory. His thesis described the development of a novel technique for characterising the MICE muon beam and demonstrating its suitability for a muon cooling measurement, an essential step on the path towards a possible neutrino factory and muon collider. In 2011, Mark moved from accelerator physics to neutrino physics, joining the University of Geneva both as a lecturer and as a researcher working on the T2K, Hyper-Kamiokande and BabyMIND experiments, to which he contributed with data analysis and detector development.
A passionate educator and communicator, Mark trained as an apprentice physics teacher at Ecole Internationale de Genève in 2018. The following year he joined CERN as a senior fellow working on the CERN Courier magazine, disseminating the latest developments in global high-energy physics. He played a major role in the launch of the CERN Courier website and rose quickly to become the magazine’s deputy editor. When his fellowship ended, Mark moved to the World Economic Forum, where he managed the production of a portfolio of publications and tools relating to education, skills and learning and served as lead author for the Future of Jobs report 2023.
Mark returned to CERN as a staff member in 2024, and as Editor of the CERN Courier. Over a short period, his talent for language and his creativity in graphics and data journalism raised the bar for CERN’s flagship publication. He also paid particular attention to improving the visibility of gender diversity in the Courier and to developing the magazine’s online presence, enabling him to connect particle physics with new audiences. He took great pride in his work and in engaging with authors to shape their stories, and was widely recognised for his dignity and professionalism both among his colleagues and members of the international particle-physics community.
Above all, Mark cared deeply about everything he did, and especially about the well-being of others. His pursuit of excellence and his remarkable attention to detail set a standard that inspired those around him, and this is reflected in the deeply motivated team that he built and nurtured. He was highly cultured and sang in the Geneva Gospel Choir.
Mark was a man of great warmth and spirit and of quiet generosity, whose presence brought light to those fortunate enough to know him. He will be remembered with great respect and will be profoundly missed.
His friends and colleagues
ehatters Tue, 04/14/2026 - 13:08 Publication Date Tue, 04/14/2026 - 12:59The top quark, the heaviest and most short-lived elementary particle known, has long been thought to decay too quickly to form bound states. However, a new result from the CMS Collaboration, presented this week at the Rencontres de Moriond conference, strengthens last year's observation that top quarks may, in fact, briefly pair up with their antimatter counterparts. This fleeting bound state – known as toponium – would be the most massive composite particle ever observed, completing the family of quark–antiquark states bound by the strong nuclear force.
Most matter around us is made of atoms, in which electrons cling to protons through the electromagnetic force. But protons themselves are not elementary. They belong to a broad family of composite particles called hadrons, in which quarks are held together by the strong nuclear force. Among them, the simplest are pairings of a quark with its own antiquark, which provide an especially clean window on the workings of the strong force. For decades, such states have been known for every type of quark but the most elusive: the top.
First discovered more than 30 years ago at the Tevatron accelerator near Chicago, the top quark has been extensively studied ever since, with experiments at the LHC going so far as to measure quantum entanglement between top quarks and antiquarks. Even when produced alongside its antiquark, the top typically decays before any bound state can form. Yet the hundreds of millions of top quark–antiquark pairs produced at the LHC, effectively making it a top-quark factory, provide such an enormous dataset that the rarest phenomena can leave a detectable trace.
The first hints of toponium appeared in searches for heavy Higgs-boson-like particles that could decay into a top quark–antiquark pair. An unexpected excess of collision events was observed at a mass close to twice the mass of the top quark, which is more characteristic of a bound state rather than a new fundamental particle. Detailed studies by the CMS and ATLAS experiments confirmed this excess using events in which both top quarks decay into leptons (electrons or muons).
The new CMS study approaches the problem from a different angle, examining events in which one top quark decays into a bottom quark, a charged lepton and a neutrino while the other decays into quarks that produce sprays, or “jets”, of particles. “Isolating the signal in this decay channel was challenging,” says Otto Hindrichs, a researcher at the University of Rochester who developed a new AI-assisted technique to reconstruct these collision events.
“Instead of reconstructing the mass of the top quark–antiquark pair directly, we focused on the relative velocity of the top quark and antiquark,” explains Yu-Heng Yu, a graduate student involved in the analysis. “If they form a bound state, their relative velocity should be much smaller than when they are produced independently,”
These new techniques proved highly effective. They resulted in the observation of an excess with a statistical significance of more than five standard deviations – the gold standard for a discovery in high-energy physics. The result provides a new, statistically independent confirmation of toponium production.
“Toponium is heavier than the heaviest known atomic nucleus, oganesson, making it the most massive bound state ever observed,” says Regina Demina, leader of the CMS group at the University of Rochester. “Its discovery deepens our understanding of the strong nuclear force and its ability to bind the fundamental constituents of matter.”
Find out more on the CMS website.
roryalex Tue, 03/24/2026 - 11:35 Byline CMS collaboration Publication Date Wed, 03/25/2026 - 11:31According to the theory of supersymmetry, there is a mirror world of hypothetical particles that could help resolve several physics puzzles, such as the surprisingly small mass of the Higgs boson and the nature of dark matter. The ATLAS Collaboration at the Large Hadron Collider (LHC) has conducted new searches for these so-called supersymmetric (SUSY) particles using machine-learning techniques. The results of these searches, presented this week at the Moriond conference, have placed some of the strongest bounds yet on the properties of SUSY particles.
Supersymmetry proposes that each particle in the Standard Model has a “superpartner”. The higgsino is the SUSY counterpart of the Higgs boson and is the subject of many SUSY searches. But detecting the higgsino, if it exists, is far from simple. The higgsino would not appear on its own but as a mixture of other SUSY particles, creating states known as neutralinos and charginos. Theorists predict that the lightest neutralino could be stable and, therefore, a strong candidate for dark matter. The other, heavier neutralinos and charginos would decay into this stable SUSY particle. However, these decays are expected to produce very little energy and the resulting low-energy particles would be extremely difficult to detect.
By deploying machine-learning techniques, the ATLAS Collaboration has been able to significantly improve the experiment’s sensitivity to low-energy particles. ATLAS now reports the results of two new searches for signs of SUSY particles in analyses of data from the LHC’s second run, which was collected between 2015 and 2018.
One of these searches involved hunting for signs of a disappearing track left by a chargino decaying into a stable neutralino, which is invisible to the detectors, and a low-energy pion. The pion follows a highly curved trajectory that is extremely difficult to identify in a busy proton–proton collision, causing the chargino’s track to “disappear”. The ATLAS Collaboration additionally searched for signs of heavier neutralinos decaying into the lightest and only stable neutralino and two low-momentum leptons, such as electrons. The researchers deployed neural networks to search deep into the low-momentum region of pions and leptons to find signs of them being produced through the decay of SUSY particles.
No signs of these SUSY particles were observed in either of these searches. However, these results have now set some of the most stringent limits yet on the masses and lifetimes of charginos and neutralinos, superseding the longstanding limits set by the Large Electron–Positron Collider, the LHC’s predecessor.
These limits help guide future searches for SUSY particles at the LHC and the High-Luminosity LHC. The search continues for the mirror world of SUSY.
roryalex Thu, 03/19/2026 - 09:25 Byline Rory Harris Publication Date Thu, 03/19/2026 - 11:23The LHCb experiment at CERN’s Large Hadron Collider (LHC) has discovered a new particle consisting of two charm quarks and one down quark, a similar structure to the familiar proton, but with two heavy charm quarks replacing the two up quarks of the proton, thus quadrupling its mass. The discovery, presented at the ongoing Moriond conference, will help physicists better understand how the strong force binds protons, neutrons and other composite particles together.
Quarks are fundamental building blocks of matter and come in six flavours: up, down, charm, strange, top and bottom. They usually combine in groups of twos and threes to form mesons and baryons, respectively. Unlike the stable proton, however, most of these mesons and baryons, which are collectively known as hadrons, are unstable and short-lived, making them a challenge to observe. Producing them requires smashing together high-energy particles in a machine such as the Large Hadron Collider (LHC). These unstable hadrons will quickly decay, but the more stable particles that are produced as a result of this decay can be detected and the properties of the original particle can therefore be deduced.
Researchers have used this approach many times to find new hadrons, and the new particle just announced by the LHCb Collaboration brings the total number of hadrons discovered by LHC experiments up to 80.
“This is the first new particle identified after the upgrades to the LHCb detector that were completed in 2023, and only the second time a baryon with two heavy quarks has been observed, the first having being observed by LHCb almost 10 years ago,” says LHCb Spokesperson Vincenzo Vagnoni. “The result will help theorists test models of quantum chromodynamics, the theory of the strong force that binds quarks into not only conventional baryons and mesons but also more exotic hadrons such as tetraquarks and pentaquarks.”
In 2017, LHCb reported the discovery of a very similar particle, which consists of two charm quarks and one up quark. This up quark is the only difference between this particle and the new one, which has a down quark in its place. Despite the similarity, the new particle has a predicted lifetime that is up to six times shorter than its counterpart, due to complex quantum effects. This makes it even more challenging to observe.
By analysing data from proton–proton collisions recorded by the LHCb detector during the third run of the LHC, the LHCb Collaboration observed the new baryon with a statistical significance of 7 sigma, well above the threshold of 5 sigma required to claim a discovery.
“This major result is a fantastic example of how LHCb’s unique capabilities play a vital role in the success of the LHC,” says Mark Thomson, CERN Director-General. “It highlights how experimental upgrades at CERN directly lead to new discoveries, setting the stage for the transformative science we expect from the High-Luminosity LHC. These achievements are only possible thanks to the exceptional performance of CERN’s accelerator complex and the teams who make it all work and to the commitment of the scientists on the LHCb experiment.”
Further information:
LHCb presentation at Moriond is available here.
LHCb news article.
The Hellenic Society for the Study of High Energy Physics (HeSSHEP) promotes the scientific work of the Greek scientists, informs the general public and the Greek state on matters concerning the subject of High Energy Physics, organizes an workshop and conferences.
2020-2025 © HeSSHEP All Rights Reserved.
Αυτός ο ιστότοπος είναι συμβατός με τον GDPR. Δεν χρησιμοποιούνται περιττά cookies.
Website built by Aristeidis Repoulias
