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Recherche d'un neutrino stérile avec l'expérience STEREO : détermination des spectres neutrinos et caractérisation du bruit de fond

Abstract : The reactor antineutrino anomaly (RAA) was revealed in 2011 following the reevaluationof the predicted flux. The 6% deficit in the measured rate near nuclear reactors can beexplained either by a bias in the prediction, or the introduction of a sterile neutrino at the eV massscale, participating in the mixing with the other neutrinos. In order to probe the existence of such aneutrino, and in order to be independant from the prediction, the Stereo experiment compares the¯ve energy spectra detected at di↵erent propagation lengths thanks to a segmented target located at[9-11] m from the highly enriched 235U reactor core of the ILL, Grenoble, France. The detection usesan inverse β decay (IBD) on the protons of a liquid scintillator. The light yield allows for a goodenergy resolution, which is important for the measurement of the ¯ve spectra at the percent level. Inorder to limit the impact of an uncertainty on the energy scale, this thesis describes an optimizationof the cuts used for the ¯ve selection. One of the major challenge of this surface experiment isthe control of the cosmic background, the latter being estimated from the reactor-OFF periodsand subtracted from the reactor-ON periods. As described in this thesis, it mostly generates fastneutrons, and it can be strongly reduced by the key observable of PSD (pulse shape discrimination).The calibration of the PSD is essential for the good subtraction of the background and is the centralpoint of the creative ¯ve spectra extraction method that is described in this thesis. The spectra areused to perform the oscillation analysis : the best fit of the RAA is rejected at 99% C.L. Also, apreliminary first pure 235U ¯ve spectra is presented, revealing tensions with the prediction at highenergies and no sign of the 5 MeV bump.
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Laura Bernard. Recherche d'un neutrino stérile avec l'expérience STEREO : détermination des spectres neutrinos et caractérisation du bruit de fond. Physique des Hautes Energies - Expérience [hep-ex]. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAY029⟩. ⟨tel-02471164⟩

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