16 Décembre – Thesis defense - Axel Pin

15 h Seminars room - CENBG (Gradignan)

Recherche de la nature du neutrino via la décroissance double bêta sans émission de neutrinos.
Search of the neutrino nature throught the neutrinoless double beta decay.
Characterisation and optimisation of the SuperNEMO calorimeter and impact on the search of the 82Se decay.

The search for neutrinoless double-beta decay (bb0nu) is currently the only known way of determining the nature of the neutrino. The SuperNEMO detector aims to detect this decay, which would prove the neutrino's Majorana nature. It is currently in its commissioning phase at the Laboratoire Souterrain de Modane. Thanks to its unique technology - which combines a calorimeter to measure particles' energies and times of flight, and a tracker to identify particles and reconstruct their trajectories - SuperNEMO aims to reach a sensitivity of 10^26 years (corresponding to an effective neutrino mass of 50 meV) for the bb0nu half-life of 82Se, through V-A light neutrino exchange. This technique also allows a search for new physics mechanisms, and the study of decays to excited states of the daughter nucleus.
The work documented in this thesis contributes to improved modelling of the SuperNEMO calorimeter's response, by taking into account nonlinear light-production effects (Birks, Cerenkov), as well as geometrical effects on the photon-collection efficiency of the optical modules (an assembly consisting of a plastic scintillator block and a photomultiplier tube (PMT)) that make up the calorimeter. This study, which is based on a GEANT4 optical simulation, generated correction factors to be used when reconstructing the energy deposited by particles in SuperNEMO's optical modules. In addition, the impact of these corrections on the detector's sensitivity to bb0nu from 82Se to excited states of its daughter nucleus was studied. Using SuperNEMO's initial data, characterisation studies of the calorimeter response were performed, with a focus on the evolution and equalisation of PMT gains.
In parallel, R&D work on the LiquidO project was carried out, to study the feasibility of a next-generation double-beta decay experiment  using a new opaque liquid scintillator. The first measurements with an electron beam, completed by optical simulations, allowed an initial validation of this new calorimetric approach.

Event localization