Particle Physics Division

The GBAR collaboration, to which the IRFU makes a major contribution, presented the results of its first data collection at CERN at the end of 2022 at the Moriond conferences in March 2023. For the first time, it observed the production of anti-hydrogen atoms resulting from the interaction of a beam of antiprotons supplied by CERN's Antiproton Decelerator (AD) and decelerated to an energy of 6 keV, with a cloud of positronium produced locally in the experiment. GBAR thus joins the very select club of experiments that have successfully synthesised anti-hydrogen atoms!

The ultimate aim of the GBAR experiment is to measure the acceleration of an anti-hydrogen atom in the Earth's gravity field, and compare it with that of ordinary matter. The Equivalence Principle, the basis of Einstein's General Relativity, states that all forms of matter and energy behave in the same way with respect to gravity. Since Galileo, falling-body experiments have tested this principle for different chemical elements of ordinary matter, confirming it with increasingly precise agreement. Recently, the MICROSCOPE satellite experiment verified it with a remarkable uncertainty of one part in 3x10¹⁵. But the action of gravity on antimatter has never yet been measured! Several indirect arguments suggest that antimatter should respect the Principle of Equivalence, and should therefore "fall" towards the Earth like matter. However, the relationship between matter and antimatter is intrinsically quantum, and the theory of gravitation does not sit well with quantum theory. So only experimental measurement can remove this doubt. Of course, the first step is to measure the sign, i.e. whether antimatter 'rises' while matter 'falls'. But even a small quantitative difference between the acceleration of antimatter and matter in free fall would be a revolution in physics

Neutrino oscillations have confirmed that these mysterious particles have mass, contradicting the predictions of the Standard Model. The DPhP group at CEA/IRFU is seeking to solve this mystery by observing the very rare double-beta decay without neutrino emission of the Mo-100 nucleus using scintillating bolometers. Following the CUPID-Mo demonstration experiment at the Modane underground laboratory, the group has finalised a detailed background model that offers high precision for studying the 2v2β decay. The model thus achieves the lowest background index ever obtained by the scientific community for a 0ν2β bolometric experiment.

To achieve the target of 10^-4 counts/keV/kg/year needed to detect this extremely rare decay, the CUPID experiment has also adopted a new detector technology: Nefanov-Trofimov-Luke (NTL) light detectors to improve background rejection. A measurement, using 10 identical light detectors coupled to Li₂MoO₄ and TeO₂ crystals, was carried out at the Canfranc underground laboratory and demonstrated the applicability of this technology to CUPID detectors. Given the combination of low background, particle discrimination capability, high efficiency and high energy resolution, CUPID is recognised as one of the most promising next-generation 0ν2β search experiments. After a validation review, the experiment will begin its production and construction phase to obtain a complete detector from 2029.

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

The Universe is immensely big, and getting bigger all the time. To study dark energy, the mysterious force behind the accelerating expansion of our Universe, scientists are using the large Dark Energy Spectroscopic Instrument (DESI) survey to map over 40 million galaxies, quasars and stars. Today, the collaboration has released its first batch of data, with nearly 2 million astrophysical objects for researchers to study. It is also publishing 15 papers on the scientific study of these data, and on the instrument, operations and validation of the survey observation strategy.

The data set (80 terabytes) comes from 2,480 exposures taken over six months during the so-called "survey validation" phase in 2020 and 2021 and processed in Python language on the supercomputer at the National Energy Research Scientific Computing Center (NERSC, Berkeley, USA). During this period, between the instrument's commissioning and the start of the official scientific campaign, researchers ensured that the instrument's performance would meet their scientific objectives - for example, by checking the time needed to observe galaxies of different luminosities, and validating the selection of astrophysical objects such as galaxies, quasars and stars to be observed.

Read the Berkeley press release : https://newscenter.lbl.gov/2023/06/13/desi-early-data-release-holds-nearly-two-million-objects/

Reactor antineutrino anomalies are a decade-long puzzle in neutrino physics. They are manifested by deviations of the order of a few percent between measurements and predictions. These deviations have been observed in the number of antineutrinos measured by more than a dozen experiments at nuclear reactors, and in the shape of the kinetic energy distributions by the seven most recent ones. They could have been the way to a new physics beyond the standard model, but the recent experiments, including the STEREO experiment carried by IRFU, have closed this door.

In a work just published in Physical Review Letter [1], a team of physicists from IRFU and the Laboratoire National Henri Becquerel of DRT have shown that these anomalies could come from biases in the measurements of fission electrons used as a reference for the prediction. They have developed a beta strength function  model to reduce the biases in the calculation of the energy spectra of electrons from fission of fissile reactor nuclei. The two "anomalies" on the antineutrino flux and the "bump" at 5 MeV in the antineutrino energy spectrum are now reproduced by their model. This allows to propose an explanation to solve an enigma of more than 10 years.

On October 9, 2022, at 13:16 and 59.99 seconds, a gamma-ray burst (GRB) dazzled almost all the X-ray and gamma ray detectors available at the time. Since their discovery, multi-wavelength telescopes in space and on the ground have continuously monitored these events. This outburst, named GRB221009A, shook the world community of astrophysicists, who have since been analysing it to understand the physical phenomena that triggered this most intense burst of energy in our history.