Search for electron-neutrino transitions to sterile states in the BEST experiment
The Baksan Experiment on Sterile Transitions (BEST) probes the gallium anomaly and its possible connections to oscillations between active and sterile neutrinos. Based on the Gallium-Germanium Neutrino Telescope (GGNT) technology of the SAGE experiment, BEST employs two zones of liquid Ga target to explore neutrino oscillations on the meter scale. Oscillations on this short scale could produce deficits in the 71 Ge production rates within the two zones, as well as a possible rate difference between the zones. From July 5th to October 13th 2019, the two-zone target was exposed to a primarily monoenergetic, 3.4-MCi 51 Cr neutrino source 10 times for a total of 20 independent 71 Ge extractions from the two Ga targets. The 71 Ge production rates from the neutrino source were measured from July 2019 to March 2020. At the end of these measurements, the counters were filled with 71 Ge doped gas and calibrated during November 2020. In this paper, results from the BEST sterile neutrino oscillation experiment are presented in details. The ratio of the measured 71 Ge production rates to the predicted rates for the inner and the outer target volumes are calculated from the known neutrino capture cross section. Comparable deficits in the measured ratios relative to predicted values are found for both zones, with the 4𝜎 deviations from unity consistent with the previously reported gallium anomaly. Finally, if interpreted in the context of neutrino oscillations, the deficits give best-fit oscillation parameters of Δ𝑚 2 = 3.3$^{+∞}{−2.3}$ eV 2 and sin 2 2𝜃 = 0.42$^{+0.15}{−0.17}$, consistent with 𝜈 𝑒 →𝜈 𝑠 oscillations governed by a surprisingly large mixing angle.