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 nu(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 nu(e) and sterile neutrino (nu(s)) states. A 3.414-MCi Cr-51 nu(e) source was placed at the center of two nested Ga volumes and measurements were made of the production of Ge-71 through the charged current reaction, Ga-71(nu(e), e(-)) Ge-71, 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 Ge-71/d. The ratio (R) of the measured rate of Ge-71 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 nu(e) -> nu(s) oscillations with a relatively large Delta m(2) (> 0.5 eV(2)) and mixing sin(2) 2 theta (approximate to 0.4).