Constraints on Sterile Neutrinos in the MeV to GeV Mass Range

TL;DR

This paper analyzes recent experimental data to set improved upper bounds on sterile neutrino couplings in the MeV to GeV mass range, using nuclear decay ratios and other experimental constraints.

Contribution

It provides new constraints on sterile neutrino couplings by combining multiple experimental data sources and decay ratio analyses, extending the mass range of bounds.

Findings

Stronger upper bounds on $|U_{e4}|^2$ and $|U_{4}|^2$ for sterile neutrinos.

Constraints cover the mass range from MeV to GeV.

Improved limits from nuclear beta decay and meson decay ratios.

Abstract

A detailed discussion is given of the analysis of recent data to obtain improved upper bounds on the couplings $|U_{e4}|^2$ and $|U_{\mu 4}|^2$ for a mainly sterile neutrino mass eigenstate $\nu_4$. Using the excellent agreement among ${\cal F}t$ values for superallowed nuclear beta decay, an improved upper limit is derived for emission of a $\nu_4$. The agreement of the ratios of branching ratios $R^{(\pi)}_{e/\mu}=BR(\pi^+ \to e^+ \nu_e)/BR(\pi^+ \to \mu^+ \nu_\mu)$, $R^{(K)}_{e/\mu}$, $R^{(D_s)}_{e/\tau}$, $R^{(D_s)}_{\mu/\tau}$, and $R^{(D)}_{e/\tau}$, and the branching ratios $BR(B^+\rightarrow e^+\nu_e)$ and $BR(B^+\rightarrow \mu^+\nu_\mu)$ decays with predictions of the Standard Model, is utilized to derive new constraints on $\nu_4$ emission covering the $\nu_4$ mass range from MeV to GeV. We also discuss constraints from peak search experiments probing for emission of a $\nu_4$ via lepton mixing, as well as constraints from pion beta decay, CKM unitarity, $\mu$ decay, leptonic $\tau$ decay, and other experimental inputs.