Results from the Baksan Experiment on Sterile Transitions (BEST)

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_-2.4^+2.5(stat )±1.4 (syst ) ] and [55.6_-2.6^+2.7(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→ν_s oscillations with a relatively large Δ m² (>0.5 eV² ) and mixing sin²2 θ (≈0.4 ).