Status of the SoLid experiment: Search for sterile neutrinos at the SCK$\cdot$CEN BR2 reactor

Abstract

The reactor antineutrino energy spectra and flux were reevaluated during the preparation of the recent experiments devoted to the measurement of $\theta_{13}$. Consequently some discrepancies between data and the theoretical predictions in reactor antineutrino experiments at short distances were observed when using the new predicted flux and spectra. This problem has been called the Reactor Antineutrino Anomaly (RAA), which together with the gallium anomaly, both show discrepancies with respect to the expectations at the $\sim$ 3 $\sigma$ level. Oscillations into a light sterile neutrino state ($\Delta m^{2} \sim 1eV^{2}$) could account for such deficits. The SoLid experiment has been conceived to give an unambiguous response to the hypothesis of a light sterile neutrino as the origin of the RAA. To this end, SoLid is searching for an oscillation pattern at short baselines (6-9 m) in the energy spectrum of the $\overline{\nu}_{e}$'s emitted by the SCK\raisebox{-0.9ex}{\scalebox{2.8}{$\cdot$}}CEN BR2 reactor in Belgium. The detector uses a novel technology, combining PVT (cubes of 5$\times$5$\times$5 cm$^3$) and $^6$LiF:ZnS (sheets $\sim$ 250 $\mu$m thickness) scintillators. It is highly segmented (modules of 10 planes of 16$\times$16 cubes), and it's read out by a network of wavelength shifting fibers and SiPMs. The fine segmentation and the hybrid technology of the detector allows the clear identification of the neutrino signals, reducing significantly backgrounds. Thus, a high experimental sensitivity can be achieved. A 288 kg prototype was deployed in 2015, showing the feasibility of the detection principle. A full scale detector (1.6 tons) is currently under construction, the data taking with the first detector modules is expected by the end of 2017.