Presentation Open Access
The average neutrino rate of previous neutrino oscillation experiments at short distance from reactors shows a deficit of neutrinos with respect to predictions. This deficit, known as the Reactor Antineutrino Anomaly (RAA), could be explained either by incomplete input in the flux predictions or by the existence of a new neutrino state, a light sterile neutrino. The STEREO experiment aims at answering this question. Located at ten meters from the compact nuclear core of the Institut Laue-Langevin (ILL) in France, the STEREO detector is longitudinally divided in six identical cells filled with Gd-loaded liquid scintillator. Via the inverse beta decay reaction, it allows to measure the antineutrino energy spectrum at six distances from the core. The comparison of the energy spectra offers an unambiguous test of the oscillation to the hypothetical sterile neutrino. Since the end of 2016 when the detector started to take data, more than 130 days of data with reactor in operation have been accumulated. Moreover, more than 150 days with reactor stopped, essential for cosmic background measurements, have been acquired. The detector performances in terms of detector response and energy reconstruction meet the initial expectations. Thanks to an efficient detector shielding and dedicated analysis methods, the background could be reduced to reach a signal to background ratio close to 1 with about 400 neutrinos detected per day. To avoid the dependence on spectrum predictions and to get rid of systematic uncertainties related to the flux normalization, the first oscillation analysis is based on spectra ratios using the first cell as a reference.