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The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos, νe, 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 νe and sterile neutrino (νs) states. A 3.414-MCi 51Cr νe 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(νe,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 Rin=0.791±0.05 and Rout=0.766±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 νe→νs oscillations with a relatively large Δm2 (>1 eV2) and mixing sin22θ (≈0.4).
Meeting ID: 678 0686 2818
Angelo Di Canto