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Anomalie des antineutrinos de réacteurs : recherche d’un état stérile avec l’expérience STEREO

Abstract : The Reactor Antineutrino Anomaly (RAA) was highlighted in 2011, when a reevaluated prediction of reactor antineutrinos spectra showed a 6% deficit in the rates observed by previous reactor experiments. A possible explanation for this anomaly consists in introducing a sterile neutrino state at the eV mass scale, participating to the neutrino oscillations mecanism only. The STEREO experiment has been designed to probe the phase-space region indicated by the RAA, by placing a segmented antineutrino target at 10m from the virtually pure uranium 235 compact core of the ILL research reactor facility, in Grenoble. Antineutrinos are detected via inverse beta decay (IBD) in gadolinium-doped liquid scintillator. An oscillation pattern - if any - would develop along the six identical target cells. The relative comparison of their energy distributions allows to test the sterile neutrino hypothesis with reduced systematic uncertainties and without referring to an external prediction. High light yields of the liquids enable to reach good energy resolutions, needed for the sensibility of the measurement. Cells are optically separated thanks to highly reflective plates; this technology has been validated on a prototype cell in the first stage of this thesis. An effective model of these walls has been included in the simulation, and details of the geometry have been carefully implemented. The second part of this work focuses on data analysis. The IBD signature is a two-fold signal, requiring a proper pair search algorithm for extracting the antineutrinos candidates. The dedicated software is optimized to apply specific temporal isolation selections, which allow to get rid of the majority of the background produced by cosmic events. The residual correlated background in the region of interest comes from muon decays and neutron induced reactions. A modelisation of the Pulse Shape Discrimination (PSD) distributions of this background is used to extract the antineutrino signal. Based on the resulting spectra, the first STEREO oscillation analysis rejects the RAA best fit point at 98% of confidence level.
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  • HAL Id : tel-01931309, version 1

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Aurélie Bonhomme. Anomalie des antineutrinos de réacteurs : recherche d’un état stérile avec l’expérience STEREO. Physique des Hautes Energies - Expérience [hep-ex]. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS319⟩. ⟨tel-01931309⟩

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