Figure 1: Schematic layout of the SOX experiment.

Important Notice: As of February 2018, the SOX project has been cancelled due to the impossibility of realising the source with the required characteristics.

The measurement of decays of Z0 boson at LEP has confirmed that there are 3 families of light neutrinos interacting through weak interactions. In recent years, however, several experimental results indicate that the 3-flavour picture might not be complete: deficit observed in reactor antineutrino short-baseline experiments, the so called reactor anomaly, the deficit observed in calibration of Gallex and SAGE experiments with neutrino sources, the famous appearance of ${\bar \nu}_e$ in ${\bar \nu}_\mu$ beam seen by LSND and further tested by MiniBooNe. All these anomalies could be at least partially accommodated if there would exist at least one sterile neutrino. It is called sterile because it would not interact through the forces described by the Standard Model.

The very low radioactive background of the Borexino detector, its large size, and the well proved capability to detect both low energy electron neutrinos and antineutrinos make an ideal case for the study of short distance neutrino oscillations with artificial sources at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy. Short-distance neutrino Oscillation with BoreXino, the SOX project, aims at the complete confirmation or at a clear disproof of the existence of such a light sterile neutrino.

A 4 to 5 PBq (>100 kCi) 144Ce-144Pr antineutrino source, producing about 1015 ${\bar \nu}_e /$ s, will be placed below the Borexino detector at a distance of 8.3 m from the detector's centre in spring 2018. The Mayak Production Association, one of the biggest nuclear facilities in the Russian Federation, will produce the source based on the contract, which was successfully signed after a long period of negotiations at the end of 2017. The data taking will continue for about 1.5 years during 2018 and 2019.

The isotope of 144Ce decays with lifetime of 285 days to 144Pr, which then with much shorter lifetime decays to 144Nd. SOX will measure antineutrinos above the IBD threshold, emitted by 144Pr with the end-point at about 3 MeV. The 2.2 MeV gammas from the de-excitation of 144Nd will be efficiently shielded by a thick tungsten case. Two independent calorimeters are developed in order to measure to released heat with 1% precision. Currently, the focus is on the optimisation of the IBD selection criteria, background estimations and sensitivity studies. A significant effort will be devoted to the estimation of different systematic effect on the final sensitivity.

Figure 2: Sensitivity plot.

A global analysis of short-baseline neutrino oscillation data in 3+1 parameter space leads to $\Delta m^2_{41}$ around 1 eV2. In order to probe this region with MeV (anti)neutrinos, the oscillation experiments at distances of few meters have to be performed. In such an experiment, the hypothetical existence of sterile neutrino could be observed not only through an absolute disappearance (rate) but also through the oscillation pattern in (L, E) parameter space (shape) deviating from the standard 3-flavour scenario. That latter would provide a "smoking gun" signature of the sterile neutrino, a clear indication for physics beyond the standard model. Figure 2 shows the sensitivity of the SOX experiment dependent on the oscillation parameters of the sterile neutrinos. The currently preferred parameter space is almost fully covered by the so-called rate+shape analysis.