SOX : Short Distance Neutrino Oscillations with Borexino

Abstract

In the last years, several neutrino oscillation experiments reported results not compatible within the 3-neutrino model, that hint at the existence of light sterile neutrinos. To test this hypothesis, the SOX (Short distance neutrino Oscillations in BoreXino) experiment will search for oscillations from active to sterile neutrinos by placing a radioactive 144 Ce-ν e source underneath the liquid scintillator detector Borexino. Oscillations will be observed via a reduction of the detected interaction rate of the antineutrinos and an oscillatory pattern as a function of the neutrino energy and travelled distance. Data taking is going to start in beginning of 2018 and first results are expected within this year.

Status of light sterile neutrino searches

“…The 95% CL exclusion limits [138] for a (3+1) sterile neutrino model are shown in the right part of Fig. 17.…”

“…In the analysis, almost all scintillator could serve as the fiducial volume, thus providing the baselines L between 4.25 and 12.75 m. Fig. 17 shows in its left part the Monte Carlo simulations [138] of the expected signal (L over reconstructed νe energy) for different oscillation parameters. The lower pad gives the ratio between the oscillation hypothesis and the no-oscillation hypothesis.…”

“…Right: 95% CL exclusion limits for a 100-150 kCi source in the ∆m 2 41sin 2 (2θ 14 ) plane compared to the preferred regions of a global fit to the anomalies [13]. From [138].…”

Recent results from Borexino

“…By excluding the regions of the active volume close to the muon track or to the neutron capture point for a few half lives of 11 C (29 min), we select about 50% of the exposure where the 11 C background is only 10% of the total. Another powerful handle to distinguish neutrinos from 11 C is PSA as the scintillation due to annihilation gamma-rays accompanying the 11 C e + -decay may be delayed when released by ortho-positronium bound state. The rate of pep neutrinos has been extracted with a multivariate analysis based of the energy of the event, the distance from the center of the detector and a pulse shape parameter [6].…”

“…The Borexino backgrounds are unprecedented low. The Phase-II of the solar run up the beginning of the SOX experiment [11] features six years of excellent quality data. We are working to obtain a more stringent limit on the CNO neutrino flux, hoping for an observation, along with more stringent measurements of 7 Be , 8 B, pep, and geo-neutrinos.…”