Recherche de neutrinos lourds avec l'expérience T2K

Non-zero neutrino mass requires the introduction of new physic beyond the Standard Model. For instance, the nuMSM (Neutrino Minimal Standard Model) predicts the existence of new heavy neutrinos, with masses at GeV-scale, that would explain both neutrino masses and baryon asymmetry in the Universe. A particularly interesting mass domain is between 140 and 493 MeV/c$^2$, which is accessible to current experiments, in particular with the near detector ND280 of the T2K experiment, which is a long baseline neutrino oscillation experiment located in Japan. The analysis aims at the selection of a sample containing heavy neutrino decays after they have been produced in kaon decays along the standard T2K neutrino beam. Background coming from standard neutrino interactions has been reduced to few events in the current dataset (2010-2017), which allow constraining heavy neutrino parameter space quite well with respect to dedicated experiments such as PS191. An important step in the analysis consists of the validation of the reconstruction algorithms and of the expected background. It has been done with the study of active neutrino interactions in the walls of the TPCs. This sample is also interesting per se for physics. The last topic in this thesis concerns the upgrade of the near detector, that is scheduled for 2021. The goal is to improve neutrino cross-section measurements in order to get more precise determination of oscillation parameters.