The SOX project is an extension of the research program of Borexino that is dedicated to the search of oscillation from active to sterile neutrinos. SOX is motivated by a number of anomalies that have appeared in neutrino experiments at short oscillation baselines.

The SOX concept relies on the deployment of an intense radioactive neutrino source close to the Borexino detector. Two exposition phases are foreseen: CeSOX with a Cerium-144 electron antineutrino source (activitiy of 4 PBq or 100 kCi), and CrSOX with a mono-energetic Chromium-51 source based on Chrome-51 (200-400 PBq, 5-10 MCi). Based on the low neutrino energies (Ce/Pr-144 < 3MeV; Cr-51 750 keV) the expected oscillation length is of the order of one meter. Therefore, Borexino would not only observe a rate deficit but the projection of the oscillation pattern within the detection volume. A positive result in Borexino would thus provide an un-ambiguous signature for sterile neutrinos oscillations. Figure 9 shows the sensitivity of SOX dependent on the oscillation parameters realized for the sterile neutrinos. The currently preferred parameter space is almost fully covered []. Moreover, SOX will provide the opportunity of a measurement of the Weinberg angle at MeV energies as well as an improved limit on the neutrino magnetic moment.

The start of the CeSOX exposition phase is foreseen for the end of 2015.

Fig. 9: Projected sensitivity of the two SOX phases.