The Gallium Anomaly and the Question of Sterile Neutrinos

The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos, ν_e, observed in previous gallium-based radiochemical measurements with high-intensity neutrino sources, commonly referred to as the gallium anomaly, which could be interpreted as evidence for oscillations between ν_e and sterile neutrino (ν_s) states. A 3.414-MCi ⁵¹Cr ν_e source was placed at the center of two nested Ga volumes and measurements were made of the production of ⁷¹Ge through the charged current reaction,⁷¹Ga(ν_e,e)⁷¹Ge, at two average distances. The measured production rates for the inner and the outer targets respectively are  $54.9^+2.5_-2.4(stat)±1.4 (syst) and  55.6^+2.7_-2.6 (stat)±1.4 (syst) atoms of ⁷¹Ge/d. The ratio (R) of the measured rate of ⁷¹Ge production at each distance to the expected rate from the known cross section and experimental efficiencies are  R_in=0.79±0.05  and R_out= 0.77±0.05. The ratio of the outer to the inner result is 0.97±0.07, which is consistent with unity within uncertainty. The rates at each distance were found to be similar, but 20-24% lower than expected, thus reaffirming the anomaly. These results are consistent with ν_e toν_s oscillations with a relatively large Δm² (>0.5 eV²) and mixing sin² 2θ (about 0.4).