Nouvelles études sur le fond cosmogéniques de spallation pour la recherche de neutrinos reliques de Supernova dans l'expérience Super-Kamiokande

par Alice Coffani

Projet de thèse en Physique

Sous la direction de Michel Gonin.

Thèses en préparation à l'Institut polytechnique de Paris , dans le cadre de École doctorale de l'Institut polytechnique de Paris , en partenariat avec LLR - Laboratoire LEPRINCE-RINGUET (laboratoire) et de Neutrinos (equipe de recherche) depuis le 31-08-2018 .

  • Résumé

    Étudier le taux de formation des Supernovas depuis la formation de notre Univers. Expérience Super-Kamiokade au Japon

  • Titre traduit

    New studies on cosmogenic induced spallation background for Supernova relic neutrino search in the Super-Kamiokande experiment

  • Résumé

    In February of 1987, the Kamiokande detector detected the world's first neutrinos from a supernova burst. Since then, no supernova explosion has occurred in or near our galaxy, so we have not observed any neutrinos from a supernova burst since then. Supernova explosions in our galaxy may be fairly rare, but supernovae themselves are not. On average, there is one core collapse supernova somewhere in the universe each second. The neutrinos emitted from all of these since the onset of stellar formation have suffused the universe. We refer to this thus-far unobserved flux as the Diffuse Supernova Neutrino Background [DSNB], also known as the “relic” supernova neutrinos. The detection of the supernova relic neutrinos enables us to investigate the history of star formation, a key factor in cosmology, nucleosynthesis, and stellar evolution. Furthermore, the study of supernova bursts, which produce and disperse elements heavier than helium, is vital to understand many aspects of the present universe. Supernova bursts generate all types of neutrinos, however, because of its larger cross section, antielectron neutrinos are the most copiously detected neutrinos in a water Cherenkov detector like SuperKamiokande (Nobel Prize in physics, 2015). About 80% of the detectable supernova neutrino events are inverse beta interactions: an anti-electron neutrino interacts with a proton, ending up with a positron and a neutron in the final state. Super-K can detect the relativistic positron because it emits Cherenkov light. But to identify the signal as coming from an anti-electron neutrino, we need to detect not only the positron but also the neutron. We will dissolve a 0.2% concentration of a gadolinium compound in the Super-Kamiokande detector in order to detect the neutron. The cross section of gadolinium to capture neutrons is very large, and the gadolinium then emits a cascade of observable gamma rays after the capture reaction. The coincident detection of a positron's Cerenkov light, followed shortly thereafter in roughly the same place by a shower of gamma rays, will serve to positively identify inverse beta reactions in the detector. Once we add gadolinium to Super-Kamiokande, we expect to record relic neutrino signals with almost no background. This will be the world's first observation of the DSNB. The same coincidence technique will also allow Super-K to make a very high statistics measurement of the anti-electron neutrino flux and spectrum from all of Japan's nuclear power reactors, yielding the world's most accurate determination of the mixing parameters connecting the first two generations of neutrinos. This thesis gives the opportunity to participate to some outstanding project in the field of high energy at the frontier between cosmology and elementary particle physics, in addition to the discovery of the Japanese culture. I will first participate in the final stage of operation of adding gadolinium in the Super-K detector, in the participation of Monte Carlo simulations and finally in the analysis of the first data. I am is expected to spend some significant fraction of his time in Japan, in particular during the first year for the hardware operations and commissioning. Most of the time, I will be accompanied during my stay by members of our group Neutrinos. In parallel to these activities, Monte-Carlo simulations will be carry out in order to estimate the different sources of background and detection efficiencies for the “relic” neutrinos. It is expected that I will play a leading role in the performance studies and optimization of background reduction. First data with the new detector Super-Kamiokande will be taken middle of 2019. I will participate actively in the first analysis. The defense of my thesis is foreseen by summer 2021.