With more than 5,000 scientists, engineers, technicians, administrators and students, CMS is one of the largest scientific collaborations in the world. With members from more than 240 institutes and universities in nearly 50 countries around the world, the collaboration exploits the data provided by the CMS experiment, one of the two giant general-purpose detectors installed along the circumference of the LHC, CERN's Large Hadron Collider.

Gautier Hamel de Monchenault, a physicist in the particle physics department at CEA-IRFU, was elected on Monday 12 February 2024 as the spokesperson for the CMS collaboration at CERN. He will hold this prestigious position from 1 September 2024 to 31 August 2026. He will be the 10th spokesperson for the CMS collaboration and the second French spokesperson to lead one of the four LHC experiments.

These intense years will see the end of the third data-taking period of the LHC as we know it, and the start of the installation of detector upgrades in preparation for the high-luminosity HL-LHC data, as well as the update of the European particle physics strategy.

The ATLAS collaboration announced at the Moriond conference the observation of simultaneous production of four top quarks. This is one of the rarest and heaviest processes ever observed at the Large Hadron Collider (LHC). This measurement, coordinated by IRFU, allows to test the Standard Model of particle physics in its most complex predictions.

Link to the ATLAS collaboration publication

As part of the luminosity increase of the Large Hadron Collider (LHC), the first phase of the ATLAS experiment upgrade is coming to an end, before a restart planned for early 2022. To meet the requirements of physics in a highly radiative environment with a high particle flux, the two internal wheels of the muon spectrometer will be replaced by new devices: the New Small Wheels (NSW).
After several years of R&D and production, IRFU has just sent the last of the 32 detection modules that will be integrated into the NSW at CERN. This corresponds to the assembly of about 400 m² of gas detectors based on Micromegas technology: a record!

The muon spectrometer of the ATLAS experiment has been an important contribution of IRFU since the design, and IRFU is still in charge of the alignment today. The spectrometer plays a key role in the reconstruction of high-energy muons, the detection of which is crucial for the search of phenomena beyond the Standard Model. The spectrometer consists of about 1200 muon chambers that form a gigantic edifice 44 m long and 24 m in diameter. Despite these imposing proportions, the relative positions of the chambers must be known with an accuracy of the order of 50 μm in order to achieve the optimum performance of the spectrometer. To this end, a network of optical lines is used to continuously monitor the positions of the chambers relative to each other, as well as their deformations. An elaborate procedure for reconstructing the 56,000 parameters determining the alignment (for the central part) has been developed by a team at IRFU in order to obtain the precision required for the reconstruction of high-energy muons. This method, adopted by the ATLAS collaboration, is presented for the first time in an ATLAS note.

The CMS collaboration presented its most achieved measurement of the Higgs boson properties in the two-photon decay channel at the ICHEP conference in August 2020. The results are based on the complete LHC Run 2 data recorded between 2016 and 2018 and show a level of accuracy never achieved before.

Thanks to this increased sample size, to sophisticated analysis methods using artificial intelligence and developed in part by the CMS group at IRFU, previously unimaginable measurements are now possible: the study of rare modes of production becomes possible. This painstaking work has made it possible to carry out increasingly precise measurements of the properties of the Higgs boson, making it possible to test the Standard Model of particle physics ever further. The latter has once again triumphed in this confrontation.

But with the restart of the LHC collider in 2022, and then its luminosity increase in 2027, the amount of data will be significantly larger, allowing the Standard Model to be examined from every angle.

Photon-photon elastic scattering is a very rare phenomenon in which two real photons interact producing a new real photon pair. The direct observation of this process at high energy, impossible during decades, was done by ATLAS [1] and CMS [2] experiment at CERN between 2016 and 2019. These successes have led the two collaborations to strengthen their involvement in this new field, leading to a new measurement, currently being published by the ATLAS experiment [3]. Presented for the first time at the LHCP conference in May 2020, the new idea is to use photon collisions to search for a hypothetical axion-like particle. As with the first publications on the subject, IRFU members are at the origin of the ideas at work in the analyses carried out at CERN.

The Tevatron CDF and D0 collaborations have just received the 2019 Prize for Particle and High Energy Physics awarded by the European Physical Society for the discovery of the top quark in 1995 and the detailed measurements of its properties from 1995 to the present. This prize thus rewards the physicists and engineers of the Irfu who contributed to the construction of the D0 detector, the discovery of the quark top, and conducted numerous studies on top quark physics.

The ATLAS and CMS collaborations, involving teams from CEA/IRFU and CNRS/IN2P3, announced on 4 June 2018 at the LHCP conference the direct observation of the coupling of the quark top to the Higgs boson. Studying the interaction between the Higgs boson and the heaviest elementary particle known, the quark top, is a way of investigating the effects of new physics, which must take over from the standard model.

The results of the analyses, orchestrated by IRFU/DPHP physicists, led to the observation of this rare process and are in agreement with the predictions of the standard model. In the coming years, both experiments will collect much more data and improve the accuracy of their measurements, which could reveal a deviation from the prediction of the standard model.

CMS article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.231801
arXiv link for the ATLAS article submitted to publication : https://arxiv.org/abs/1806.00425

The LHC's Atlas collaboration at Cern has observed a rare process: the production of Higgs bosons in association with a top quark and top antiquark pair. This work, supervised by an Irfu researcher, opens up perspectives on the study of the Higgs mechanism that gives mass to particles.

Data collected at the LHC (Cern) were processed to provide the most accurate assessment of an asymmetry in top quark and top antiquark production. The result is that the measured value is compatible with the prediction of the standard particle model.

The ATLAS collaboration at CERN's LHC has found the first direct evidence for the rare process of high-energy light-by-light scattering, where two photons interact and change direction. This phenomenon was predicted several decades ago by quantum electrodynamics, i.e. the quantum theory of electromagnetism.

Physicists from IRFU have announced that no "big brother" of the Higgs boson has been detected at the ATLAS experiment at CERN's LHC. Their results rely on new analyzes with higher sensitivity.

Light-by-light scattering, predicted in 1936, was observed for the first time by the ATLAS experiment at the LHC, thanks to "ultra-peripheral" collisions of lead ions. It is of particular interest to physicists, as it is the result of interactions between a vacuum and intense electromagnetic fields.

Les collaborations ATLAS et CMS ont présenté pour la première fois leur mesure combinée de la masse du boson de Higgs, parvenant à une mesure précise à 0,2% près. Présentée lors de la 50^e  édition des «Rencontres de Moriond» en Italie le 17 mars 2015, cette mesure est l’une des plus précises obtenues au LHC à ce jour. Les physiciens du CEA-Irfu ont joué un rôle majeur pour obtenir ce résultat, à travers leurs contributions sur l’étalonnage des calorimètres électromagnétiques d’ATLAS et de CMS ainsi que sur la reconstruction des muons dans le spectromètre d’ATLAS.

Physicist working on the CDF and D0 experiments using Fermilab's Tevatron accelerator in Chicago, including scientists from IN2P3/CNRS and IRFU/CEA, announced their latest results on 26 July at the International Conference on High-Energy Physics (ICHEP 2010) in Paris. Their measurement further constrain the Higgs boson mass domain still open within the standard model of particle physics. This means that CDF and D0 have ruled out a Higgs Boson with a mass between 158 and 175 GeV/c².

An increasing amount of experimental results points to a low mass for this famous boson; will a solution to this puzzle be found sometime in the next two years strong?

The D0 experiment at the Tevatron accelerator at Fermilab (Chicago), in which physicists from CEA/IRFU and CNRS/IN2P3 are involved, has measured a significant matter-antimatter _asymmetry_ in the behaviour of particles containing b quarks, known as B mesons (or beauty mesons) beyond the predictions of the standard model (the current theory of particle physics). This result has been submitted for publication in the Journal Physical Review D.

Since researchers have been confronting the standard model of particle physics with experimentation, nothing has been able to shake it. Of all particles it describes, only the Higgs Boson has not yet been discovered. But the standard model is probably not the ultimate theory: it does not cover gravitation and numerous experimental observations remain unexplained.

A new invariance, called supersymmetry, was suggested during the 1970s. It associates particles with different spins (integer spin bosons and half-integer spin fermions). It is possible to create supersymmetric extensions of the standard model, elegantly resolving the mathematical problems that emerge during calculation of the Higgs Boson mass.

D0¹ experiment accumulating data from Fermilab's Tevatron (United States) just published² results relating to the Higgs Boson research needed for supersymmetric extensions to the standard model. All currently available data has been analysed, representing more than one and a half billion events.

Finding a supersymmetric light Higgs 

In the Tevatron, a high-energy proton-antiproton collider, large quantities of Higgs Bosons could be produced if they are sufficiently light. A useful channel for detecting them is their production associated with a bottom quark³ (b), H⁰b. In 90% of cases, supersymmetric light Higgs Bosons are supposed to disintegrate into two bottom quarks. That is why research in this area is based on identifying those events involving at least three jets⁴ resulting from bottom quarks in the final state.

Until the advent of the LHC, the Tevatron at the Fermi National Accelerator Laboratory, Fermilab (close to Chicago, USA), will remain the world's most powerful collider and the only location where the top¹ quark can be produced.

The DØ experiment recently published² results on the measurement of the rate of production of top-antitop quark pairs. This quantity, which is dependent on the value taken for the mass of the top quark, enables a prediction to be made for that mass using the standard model³. The top quark, which was discovered at Fermilab in 1995, remains the subject of very active research. Methods of analysis and the quantity of data are forever improving, which is resulting in subsequent improvements in the accuracy of the measurement of the top quark mass.  The precise measurement of this value, combined with results from other precision measurements, enables the most probable mass of the Higgs Boson to be estimated. Hence improved measurements of the mass of the top quark is tightening the vice in the search for the Higgs boson.

Scientists from the CDF and DZero collaborations at the U.S. Department of Energy's Fermilab have combined Tevatron data from the two experiments to advance the quest for the long-sought Higgs boson. They have presented their results on August 3rd at the International Conference on High Energy  Physics in Philadelphia indicating that they have for the first time excluded, with 95 percent probability, a mass for the Higgs  of 170 GeV (about 170 proton masses). This value lies near the middle of the possible mass range for the particle established by earlier experiments. This result not only restricts the possible masses where the Higgs might lie, but it also demonstrates that the Tevatron experiments are sensitive to potential Higgs signals.