Particle Physics Division

The missing mass of the universe or non-baryonic dark matter is probably made up of particles that remain to be discovered. Massive and neutral, with very weak interactions, they still escape a detection that would identify them. While conventional photons are massless, dark matter could be made up of particles of a new type, similar to massive photons. New experimental results on the search for non-baryonic matter in this form, obtained by a team of three Irfu members, have just been published in Physical Review Letters[1].

After a decade-long search, scientists have for the first time detected a gamma-ray burst in very-high-energy gamma light. This discovery was made in July 2018 by the H.E.S.S. collaboration using the  huge 28-m telescope of the H.E.S.S. array in Namibia. Surprisingly, this Gamma-ray burst, an extremely energetic flash following a cosmological cataclysm, was found to emit very-high-energy gamma-rays long after the initial explosion. This discovery was published in Nature.

A team from IRFU's Department of Particle Physics (DPhP) has just conducted the most accurate study to date of the mass of cosmic neutrinos, including both standard model neutrinos and sterile neutrinos contributing to dark matter.

The researchers used the spectra of nearly 200,000 distant quasars measured by the Sloan Digital Sky Survey (SDSS) eBOSS project to map the distribution of hydrogen at very remote times in the history of our universe, ten to twelve billion years ago.

Neutrinos, propagating at relativistic speeds for billions of years, prevent gravity from acting on small scales and smooth the structures (clusters of galaxies, filaments, ...) revealed by the spectra of quasars. Thanks to the accuracy of the measurements, researchers have been able to narrow the possible range for the mass of cosmic neutrinos, to the point of being able to have their word on how the different masses of the three neutrinos of the Standard Model are ordered.

The Dark Energy Spectroscopic Instrument (DESI) is intended to make the spectroscopic survey of 35 million galaxies and quasars from 2020 onwards, to study precisely the properties of dark energy. Its installation, started in 2018, has recently entered a new phase with the reception and assembly of the first two spectrographs out of the 10 that the instrument will include. Irfu, partner of the project since the beginning and responsible for the cryogenic part of the spectrographs, has successfully completed this installation. The next 8 spectrographs will be installed next May and September under the coordination of the Irfu team, in partnership with the local teams.

vidéo réalisée par Victor Silva (Irfu/DIS)

Neutrinos from the Big Bang have been traveling the Universe for more than 13 billion years. They are almost undetectable but their footprint on the formation of large structures in the Universe, such as galaxies, can be detected. For the first time, this trace of the "diffuse neutrino background" from the Big Bang on the "baryonic acoustic oscillations" (BAO) has been deduced from the survey of 1.2 million galaxies of the "Sloan Digital Sky Survey" (SDSS). These data correspond to 5 years of observations from the Baryon Oscillation Spectroscopic Survey (BOSS) experiment, a ground-based telescope installed in New Mexico. The result, published in the journal Nature Physics, shows how the BAO phase can constrain the number of neutrino species in the Standard Model of Particle Physics.

The DPhP group has been involved in this project for more than 10 years and is currently working on its extension, the eBOSS project. In the very near future, the DESI project will be able to study even more precisely this cosmic neutrino background produced by the Big Bang.