BEST Impact on Sterile Neutrino Hypothesis

Abstract

Recently the Baksan Experiment on Sterile Transitions (BEST) has presented results confirming the gallium anomaly -- a lack of electron neutrinos $\nu_e$ at calibrations of SAGE and GALLEX -- at the statistical significance exceeding 5$\,\sigma$. This result is consistent with explanation of the gallium anomaly as electron neutrino oscillations into sterile neutrino, $\nu_s$. Within this explanation the BEST experiment itself provides the strongest evidence for the sterile neutrino among all the previous anomalous results in the neutrino sector. We combine the results of gallium experiments with searches for sterile neutrinos in reactor antineutrino experiments (assuming CPT-conservation in the $3+1$ neutrino sector). While the "gallium" best fit point in the model parameter space (sterile neutrino mass squared $m_{\nu_s}^2\approx 1.25\,$eV$^2$, sterile-electron neutrino mixing $\sin^22\theta\approx 0.34$) is excluded by these searches, a part of the BEST-favored 2$\,\sigma$ region with $m^2_{\nu_s}>5\,$eV$^2$ is consistent with all of them. Remarkably, the regions advertised by anomalous results of the NEUTRINO-4 experiment overlap with those of the BEST experiment: the best fit point of the joint analysis is $\sin^22\theta\approx 0.38$, $m_{\nu_s}^2\approx7.3\,$eV$^2$, the favored region will be explored by the KATRIN experiment. The sterile neutrino explanation of the BEST results would suggest not only the extension of the Standard Model of particle physics, but also either serious modifications of the Standard Cosmological Model and Solar Model, or a specific modification of the sterile sector needed to suppress the sterile neutrino production in the early Universe and in the Sun.