Results from the Baksan Experiment on Sterile Transitions (BEST)

Abstract

The Baksan Experiment on Sterile Transitions (BEST) was designed to investigate the deficit of electron neutrinos, $\hat{I}{½}_e$, 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 $\hat{I}{½}_e$ and sterile neutrino ($\hat{I}{½}_s$) states. A 3.414-MCi \nuc{51}{Cr} $\hat{I}{½}_e$ 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}($\hat{I}{½}_e$,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 $R_{in}=0.79\pm 0.05$ and $R_{out}=0.77\pm 0.05$. The ratio of the outer to the inner result is 0.97$\pm$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 $\hat{I}{½}_e\to \hat{I}{½}_s$ oscillations with a relatively large $\hat{I}”m^2$ ($>$0.5 eV${}^2$) and mixing sin${}^{2}2\hat{I}¸$ ($\approx$0.4).

Paper updated to final version