Statistical significance of the sterile-neutrino hypothesis in the context of reactor and gallium data

TL;DR

This study assesses the statistical significance of the 3+1 sterile-neutrino hypothesis using reactor, solar, and gallium data, revealing strong evidence from gallium experiments but no significant support from reactor data.

Contribution

It provides a comprehensive statistical analysis using Monte Carlo simulations to evaluate sterile-neutrino evidence, accounting for uncertainties and recent experimental results.

Findings

Gallium data shows over 5σ evidence for sterile neutrinos.

Reactor data shows no significant indication of sterile neutrino oscillations.

Gallium results are in tension with solar neutrino experiments.

Abstract

We evaluate the statistical significance of the 3+1 sterile-neutrino hypothesis using $\nu_e$ and $\bar\nu_e$ disappearance data from reactor, solar and gallium radioactive source experiments. Concerning the latter, we investigate the implications of the recent BEST results. For reactor data we focus on relative measurements independent of flux predictions. For the problem at hand, the usual $\chi^2$-approximation to hypothesis testing based on Wilks' theorem has been shown in the literature to be inaccurate. We therefore present results based on Monte Carlo simulations, and find that this typically reduces the significance by roughly $1\,\sigma$ with respect to the na\"ive expectation. We find no significant indication in favor of sterile-neutrino oscillations from reactor data. On the other hand, gallium data (dominated by the BEST result) show more than $5\,\sigma$ of evidence supporting the sterile-neutrino hypothesis, favoring oscillation parameters in agreement with constraints from reactor data. This explanation is, however, in significant tension ($\sim 3\,\sigma$) with solar neutrino experiments. In order to assess the robustness of the signal for gallium experiments we present a discussion of the impact of cross-section uncertainties on the results.