Constraints on very light sterile neutrinos from $\theta_{13}$-sensitive reactor experiments

TL;DR

This paper uses data from reactor experiments to set new limits on the existence of very light sterile neutrinos, refining constraints on their mixing with electron neutrinos and assessing their impact on neutrino physics and cosmology.

Contribution

It provides the first stringent reactor-based constraints on very light sterile neutrinos in the mass-squared difference range [$10^{-3}$, $10^{-1}$] eV$^2$, using a 4-flavor analysis.

Findings

Established upper bounds on $|U_{e4}|^2$ for VLS$ u$

Confirmed robustness of $ heta_{13}$ measurement against sterile neutrino mixing

Discussed implications for cosmological dark radiation

Abstract

Three dedicated reactor experiments, Double Chooz, RENO and Daya Bay, have recently performed a precision measurement of the third standard mixing angle $\theta_{13}$ exploiting a multiple baseline comparison of $\nu_e \to \nu_e$ disappearance driven by the atmospheric mass-squared splitting. In this paper we show how the same technique can be used to put stringent limits on the oscillations of the electron neutrino into a fourth very light sterile species (VLS$\nu$) characterized by a mass-squared difference lying in the range [$10^{-3} - 10^{-1}$] eV$^2$. We present accurate constraints on the admixture $|U_{e4}|^2$ obtained by a 4-flavor analysis of the publicly available reactor data. In addition, we show that the estimate of $\theta_{13}$ obtained by the combination of the three reactor experiments is rather robust and substantially independent of the 4-flavor-induced perturbations provided that the new mass-squared splitting is not too low ($\gtrsim 6 \times 10^{-3}$ eV$^2$). We briefly comment on the possible impact of VLS$\nu$'s on the rest of the neutrino oscillation phenomenology and emphasize their potential role in the cosmological "dark radiation" anomaly.