Search for a sterile neutrino with the STEREO experiment: construction of a muon veto and estimation of the correlated background

The re-evaluation of the theoretical antineutrino flux emitted by nuclear reactors revealed a deficit of about 6% between the observed flux and the expected one. This so-called reactor antineutrino anomaly has a statistical significance of 2.7σ, and one possible explanation for this deficit could be the existence of a light sterile neutrino state into which reactor antineutrino oscillate at very short distances. The STEREO project, which will be presented in this thesis, aims to find an evidence of such oscillation. The measurement will take place at only few meters (∼ 10 m) from the compact core of the Institut Laue- Langevin (ILL) research reactor in Grenoble, France, which provides a large flux of electron antineutrinos with an energy range from 1 to 10 MeV. The sensitive volume of the STEREO detector is 2 m³ of organic liquid scintillator doped with Gadolinium, segmented in 6 cells stacked along the direction of the core and detecting anti-neutrinos via inverse beta decay. This setup will provide excellent sensitivity to short-baseline oscillations effects by precisely measuring any relative distortion of the antineutrino spectrum as a function of both energy and baseline. The close proximity to the reactor yield a high background environment from the reactor and also generated in nearby experiments which is managed by the means of heavy shielding surrounding the detector. Many measurements campaign have been performed to characterize this background and had allowed the improvement of the passive shielding design installed on-site. Moreover, an additional detector is settled above STEREO to tag the presence of an incoming cosmic muon and define a muon veto. These muons will produce fast neutrons during their interaction through the shieldings of STEREO and these neutrons can mimic the expected antineutrino signal. Therefore it is crucial to define the most efficient and homogenous veto to these muons. It consists of a tank of 2.5 m³ of water detecting muons by their Cerenkov radiation. The light emitted is collected thanks to 20 photomultipliers positioned at the top of the tank and a chemical compound is added into the water to improve the light collection on the photomultipliers. This muon veto fully covers the STEREO detector along with its shieldings. Several configurations of the muon veto system using prototypes before the final instrumental have allowed to define a trigger system which allows an excellent efficiency to cosmic muons whereas having a little sensitivity to others particles. These numerous tests were also used to check and optimize the dedicated electronics built at LPSC, which is the same for both the STEREO detector and muon veto. After characterization of the muon flux on site and with respect to the muon veto efficiency, a simulation has been developed to evaluate the rate of fake antineutrino events expected from the muon-induced fast neutrons. Thus, this estimation has confirmed the scientific requirements of the experience, which is the ability to probe the reactor antineutrino anomaly in 300 days of reactor data. STEREO is at the moment being integrated at the ILL and should deliver his first physics results early 2017. (author)