Tracking down ghostly particles

An interview of Julien BILLARD, Researcher, IN2P3

What is dark matter? What are neutrinos?

 80% of our universe is made up of a material called dark matter, about which we know nothing. Its existence is indicated only by its gravitational attraction. Since it is invisible, scientists assume it consists of exotic particles that interact only very weakly with ordinary matter, and that have no electric charge. This assumption would explain why it has, for the moment, never been detected. In this respect, dark matter has some aspects in common with the neutrino, a very mysterious particle in the Standard Model which is still very difficult to detect because of its weak interaction with matter, and because it, too, has no charge

 How can we detect these ghostly particles?

 Particles such as neutrinos, or those that make up dark matter, pass through the earth as if it were transparent, almost never interacting with the atoms of which it is composed. To have any hope of "catching" one, it is necessary to build very sensitive and massive detectors of suitable materials and having low radioactivity. Germanium is just such a material, a crystal that, at very low temperatures, has a very good ability to vibrate and warm up as a result of interactions with these mysterious particles.

Thanks to funding from the LabEX LIO, the Institut de Physique des 2 Infinis (IP2I) of Lyon has installed a state-of-the-art cryogenic laboratory enabling the construction and testing of detectors requiring very low temperatures (from the order of 10 thousandths of degrees above absolute zero). Another characteristic - the IP2I cryostat is distinguished by having the lowest level of vibration in the world, thus optimizing the performance of the detectors being used in the studies.

A record energy threshold of 50 eV (electronvolt), on a massive 33.4 g crystal, was obtained in January 2019, enabling scientists at IP21 to probe new dark matter models.

The teams working on the IP2I LIO cryostat are part of two international collaborations: the EDELWEISS experiment and the Ricochet experiment.

EDELWEISS: towards the first direct detection of dark matter?

Specifically, the MANOIR team at IP2I is undertaking R&D to design ever more powerful detectors. They are then sent to the Modane Underground Laboratory (Laboratoire Souterrain de Modane (LSM)), in Savoie, where teams from the EDELWEISS experiment are trying to detect the presence of dark matter particles, beneath 1,780 meters of rock, in the shelter of cosmic rays that would otherwise shadow the putative dark matter signal.

 Ricochet: towards a better understanding of neutrinos

 Ricochet is an experiment using the same detection technology, but as part of the study of the interaction between a neutrino and a nucleus. Observing these interactions will provide a new way of exploring the still mysterious properties of the neutrino.

It will be built 8 meters from the ILL nuclear reactor in Grenoble, as soon as 2023.

The MANOIR team at IP2I has benefited from European Research Council (ERC) funding since 2019 to develop the future generation of cryogenic detectors that will enable this little-known interaction to be studied in detail.

These investigations, being carried out by nine institutes in France, Russia and USA, including IP2I, could shed light on the many still unexplained areas concerning the composition and origins of the universe.