ATLAS sets record limits on Higgs boson’s self-interaction A candidate collision event for a pair of Higgs bosons, with one boson decaying into two photons and the other into a pair of bottom quarks. The two particle jets originating from the bottom quarks are represented by turquoise cones and the two photons by yellow towers. (Image: ATLAS/CERN)

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 - 10:10

ATLAS acts as a cosmic-ray laboratory Event display showing nineteen charged-particle tracks (yellow lines) recorded by the ATLAS experiment during proton–oxygen collisions in July 2025. (Image: ATLAS)

Tens 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 - 10:29

CMS looks deep inside quarks Protons were shown to consist of quarks in 1968, but the question of whether quarks are made of even smaller particles is as yet unanswered. (Image: A. Iqbal/ CMS)

According 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 - 14:46

Mark Rayner (1983 – 2026) (Image: CERN)

It 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 - 12:59