Results from the Baksan Experiment on Sterile Neutrinos (BEST)

B.T. Cleveland, S.N. Danshin, H. Ejiri, S.R. Elliott, D. Frekers, V.N. Gavrin, V.V. Gorbachev, D.S. Gorbunov, W.C. Haxton, et al.


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 \textit{gallium anomaly}, which could be interpreted as evidence for oscillations between \nu_e and sterile neutrino (\nu_s) states. A 3.414-MCi \nuc{51}{Cr} \nu_e source was placed at the center of two nested Ga volumes and measurements were made of the production of \nuc{71}{Ge} through the charged current reaction, \nuc{71}{Ga}(\nu_e,e^-)\nuc{71}{Ge}, at two average distances. The measured production rates for the inner and the outer targets respectively are (54.9^{+2.5}_{-2.4}(\mbox{stat})\pm1.4 (\mbox{syst})) and (55.6^{+2.7}_{-2.6}(\mbox{stat})\pm1.4 (\mbox{syst})) atoms of \nuc{71}{Ge}/d. The ratio (R) of the measured rate of \nuc{71}{Ge} production at each distance to the expected rate from the known cross section and experimental efficiencies are R_{in}=0.791\pm0.05 and R_{out}= 0.766\pm0.05. The ratio of the outer to the inner result is 0.97\pm0.07, 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 \rightarrow \nu_s oscillations with a relatively large \Delta m^2 (>1 eV^2) and mixing sin^2 2\theta (\approx0.4).