Particle physics

Particle physics—the study of the elementary constituents of matter—is one of IP2I’s main research activities.

At the subatomic scale, matter is made up of elementary particles such as electrons and quarks. These particles interact through other particles called bosons, which mediate the fundamental forces (electromagnetism, the weak interaction and the strong interaction). Some of these particles can bind together to form more complex particles, such as protons and neutrons, which belong to the hadron family, then atomic nuclei and finally atoms.

The main tools used to study elementary particles are accelerators and colliders such as the LHC. Their ever-increasing collision energy makes it possible to probe increasingly massive particles, while higher beam intensities allow extremely rare processes involving these particles to be investigated. In addition, the detectors installed at these facilities enable increasingly precise measurements of all the relevant parameters in particle physics.

IP2I has been involved in the LHC programme at CERN from the very beginning. The institute contributed to the design and construction of several components of the current CMS experiment, including the calorimeter and tracking detector. Following the discovery of the Higgs boson, a major success in 2012, CMS continues to analyse the collected data, carrying out precision tests of the Standard Model of particle physics and searching for unknown particles and phenomena beyond the Standard Model, often referred to as “new physics”. The team is also preparing upgrades of the tracker and muon detectors for the High-Luminosity phase of the LHC, which is scheduled to begin in 2029.

It is also essential to look further ahead and prepare the next generations of detectors, as projects of this kind typically span several decades. IP2I is involved in future lepton-collider projects, where it plays a leading role in calorimetry. It is also contributing to the preparation of the circular lepton collider FCC-ee, notably by developing original ideas for particle tracking.

Finally, the AEGIS group, also working in partnership with CERN, is carrying out an experiment to test whether antimatter behaves gravitationally in the same way as matter, by directly measuring the acceleration of antihydrogen due to the Earth’s gravitational field.

The CMS group contributed to the design and construction of several sub-detectors of the CMS experiment. It also works on the analysis of the collected data and played a role in the discovery of a Higgs boson, which was a major success.

Analysis topics

In 2024, the CMS group at IP2I completed four analyses based on the 138 fb^-1 of data collected during proton-proton collisions at √s = 13 TeV in LHC Run 2 between 2016 and 2018. The results led to four papers submitted for publication by the CMS Collaboration.

• Search for a new resonance decaying into two spin-0 bosons in a final state with two photons and two bottom quarks in proton-proton collisions at √s = 13 TeV, JHEP 05 (2024) 316 and Searches for Higgs boson production through decays of heavy resonances, arXiv:2403.16926, submitted to Physics Reports (contacts: Maxime Gouzevich and Elise Jourd’huy)

The Brout-Englert-Higgs potential, better known as the “Mexican hat” potential, remains one of the least explored aspects of the Standard Model. Its exact shape and symmetry have never been measured directly; instead, the simplest possible form capable of producing spontaneous symmetry breaking is assumed. This phenomenon, known as the Higgs mechanism, was confirmed at CERN in 2012 through the discovery of the Higgs boson.

The “shape of the hat” may in fact be much more complex than originally assumed and may depend on the presence of other Higgs bosons, X and Y, in our Universe. It can only be probed directly by measuring the Higgs self-coupling H* → HH, or the coupling X → HY. For the past ten years, the CMS group at IP2I has been carrying out an extensive search for such phenomena and recently contributed to two publications based on data collected by the CMS experiment at the LHC between 2016 and 2018. In the first paper, strong constraints were placed on the possible existence of X and Y and on the associated theoretical models. The second paper is a major review presenting all CMS measurements in this field since 2012.

This measurement is of major importance for understanding how the masses of elementary particles emerged during the first fractions of a second after the Big Bang. If the electroweak phase transition was as violent as boiling water, it may also have generated the matter-antimatter asymmetry. An echo of collisions between such bubbles could potentially be detected by future gravitational-wave observatories.

• Search for a standard model-like Higgs boson in the mass range between 70 and 110 GeV in the diphoton final state in proton-proton collisions at √s=13 TeV arXiv:2405.18149, submitted to Physics Letters B. (Contact: Suzanne Gascon-Shotkin)

CMS recently completed a search for a Higgs boson in the mass range from 70 to 110 GeV, using the decay channel into two photons (γ).

The diphoton mass distributions, separated into different categories using a multivariate analysis, were fitted with a model combining the expected background processes with a hypothetical signal (“resonance”). The fit was repeated several times, shifting the assumed resonance position in steps of 0.1 GeV. The absence of a clear peak provides a measurement of the highest value (“upper limit”) that the production cross section of a new resonance, multiplied by its branching fraction into two photons, can have: higher values would have produced a visible signature in the data.

The observed distribution is higher than expected under the background-only hypothesis in the mass region around 95.4 GeV, providing an interesting indication that a signal beyond the Standard Model could be present in the event samples. The excess observed around 95.4 GeV has a local significance very close to the traditional threshold of 3 standard deviations often used to describe “evidence” for a new signal. In other words, the probability that this excess is caused by a statistical fluctuation of the background is below 1%. Taking into account the fact that a random excess could have appeared anywhere within the searched mass range leads to a “global significance” of 1.3 standard deviations. The team is looking forward to similar results from other experiments and to the much larger data samples that will be accumulated during Run 3.

• Searches for violation of Lorentz invariance in tt¯ production using dilepton events in proton-proton collisions at √s = 13 TeV, https://arxiv.org/abs/2405.14757 (contact: Nicolas Chanon)

Lorentz symmetry lies at the heart of Einstein’s special theory of relativity, which provides a foundation for the Standard Model of particle physics. However, some theories, such as string theory, predict that special relativity may no longer hold at extremely high energies. Traces of Lorentz-symmetry breaking could nevertheless be observable at lower energies, such as those reached at the LHC.

The CMS group at IP2I initiated a new search for Lorentz-symmetry violation at the LHC using top-quark pairs. Top quarks are the heaviest elementary particles ever observed, and many theories predict that they could exhibit phenomena departing from the Standard Model. The signature being searched for is a variation over time in the rate of top-quark pairs produced at the LHC. Finding a deviation from a constant rate would indicate an anisotropy, or preferred direction, in space-time. So far, the measurements are consistent with a constant rate, but the results improve the precision by up to a factor of 100 compared with the previous measurement performed at the Tevatron in Chicago in 2012.

The analysis teams are now turning to Run 3 data at √s = 13.6 TeV in order to address the questions left open after Run 2.

CMS continues to study the Standard Model of particle physics and to search for unknown particles or phenomena, commonly referred to as “new physics”, through interactions involving top quarks and/or Higgs bosons.

Preparation for the high-luminosity phase of the LHC

It is also essential to prepare for the future. The group is involved in upgrading the detectors for the next phase of LHC and CMS operation, which is expected to increase the amount of collected data by a factor of ten.

In a dedicated clean room, IP2I is preparing to become an assembly centre for silicon modules mounted on the D-shaped structures that will form the endcaps of the future tracker.

Production of the data-acquisition systems for the new muon detectors of the CMS experiment has begun at IP2I as part of the High-Luminosity Large Hadron Collider (HL-LHC) upgrade. When a muon passes through this type of detector, it generates an avalanche of electrons, producing a signal on copper strips. Electronic boards collect this signal, digitise it and measure its arrival time with an extremely high precision of 500 ps. These boards are then integrated into large data-acquisition systems more than one metre high. The entire system was designed at IP2I and manufactured in France. Production has now started, and more than 72 of these systems are expected to be installed at CERN by the end of 2025.

The team’s objective is to develop a high granularity hadronic calorimeter to equip future leptonic gas pedals. The calorimeter proposed for the FLC is entitled Semi-Digital Hadronic Calorimeter (SDHCAL). It uses RPC (resistive plate chamber) type gas detectors as active medium, read by integrated electronics with a granularity of 1cm2. The RPCs are inserted in a compact mechanical structure that acts as an absorber.

This type of calorimeter is used to measure the energy of the hadrons produced during collisions in particle gas pedals: on reaching the hadron calorimeter, they will deposit their energy in it by creating a sheaf of particles (called a jet) whose shape and size make it possible to identify it and measure its energy. The high granularity of the detector is therefore essential to increase its performance.

The team, with its partners, has built the first prototype of this new generation of calorimeters, as well as the associated electronics, and has designed Particle Flow Algorithms (PFA) to improve the reconstruction of the jets resulting from the interactions and to precisely measure their energy. Several tests at CERN have demonstrated the great power of SDHCAL.

The team contributed to the realization of the TOMUVOL detector for volcano tomography. It has developed a new reading scheme for large detectors with a reduced number of electronic channels while maintaining high granularity.

SDHCAL Prototype

Activities

• Organization of several beam tests of the SDHCAL prototype at CERN (2015, 16, 17 and 18) and exploitation of the data for the study of hadronic showers.
• Development of the full simulation of the SDHCAL prototype as well as the SDHCAL in ILD and CEPC.
• Design, construction and operation of the TOMUVOL detector with LPC.
• Design and realization of large RPC detectors (2 m2) and new reading electronics for module0 of SDHCAL for the ILD ILC project.
• Participation in the drafting of the ILD-ILC DBD and in the CDR of the CEPC project.
• Design and realization of the large reading cards for RPC chambers for the muon CMS upgrade project using timing.
• Design of a new ZDAQ acquisition system
• Design of a new reading card for gas detectors (PCT/EP2018/053561-EN3062926)
• Development of PFA algorithms (ArborPFA/APRIL)
• Development of a new material for RPC detectors to increase their detection rate by a factor of 1000.
• Design and realization of a detection system for homeland security for the company Smiths Detection based on the patent. Financing by PULSALYS.
• Co-organization of the CHEF conferences on calorimetry.

Interaction of a pion (left) and an electron (right) in SDHCAL

Measurement of hadron beam energy at CERN (left: linearity, right: resolution)

Knit Pads for Reading Gas Detectors (PCT/EP2018/053561-EN3062926)

The AEgIS (Antihydrogen Experiment: Gravity, Interferometry, Spectroscopy) team at IP2I is working on the experiment of the same name carried out at CERN. The aim is to make the first direct measurement of the effects of gravitation on antimatter by observing the fall of antihydrogen atoms.

Researchers use antiprotons to produce a beam of antihydrogen atoms that is sent into a device called a Moire deflectometer. Combined with a position detector, this allows them to measure the magnitude of the gravitational interaction between matter (Earth) and antimatter (antihydrogen) to an accuracy of 1%.

The deflectometer is equipped with a grid system that divides the antihydrogen beam into parallel beams, creating a periodic structure whose analysis allows the deflection of the antihydrogen beam during its horizontal flight to be determined. By combining this measurement with the time of flight, we can then measure the gravitational force exerted on the antihydrogen atoms.

The AEgIS collaboration brings together physicists from all over Europe and faces many technical challenges such as the use of very low temperatures (0.1° K), ultra-vacuum (10^-11 mbar), high magnetic fields (1 and 5 T), Lyman lasers α to characterize antihydrogen jets, the production of excited positronium, antiprotons, etc…

Our team built the hydrogen ion beam used to characterize the deflectometer’s moiré arrays.