Another look at the impact of an eV-mass sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays

TL;DR

This paper investigates how the existence of an eV-mass sterile neutrino could influence the effective neutrino mass in neutrinoless double-beta decay, revealing scenarios where this mass could be zero regardless of neutrino mass ordering.

Contribution

It provides a detailed analysis of the sterile neutrino's impact on neutrinoless double-beta decay, including parameter space implications and a comparison with the standard three-neutrino case.

Findings

Zero effective mass possible with sterile neutrino, independent of mass ordering.

A 3D view of effective mass in the (m1, rho)-plane with sterile neutrino parameters.

Comparison of sterile and non-sterile neutrino scenarios in effective mass predictions.

Abstract

The possible existence of an eV-mass sterile neutrino, slightly mixing with ordinary active neutrinos, is not yet excluded by neutrino oscillation experiments. Assuming neutrinos to be Majorana particles, we explore the impact of such a sterile neutrino on the effective neutrino mass of neutrinoless double-beta decays $\langle m \rangle^\prime_{ee} \equiv m^{}_1 |V^{}_{e1}|^2 e^{{\rm i}\rho} + m^{}_2 |V^{}_{e2}|^2 + m^{}_3 |V^{}_{e3}|^2 e^{{\rm i}\sigma} + m^{}_4 |V^{}_{e4}|^2 e^{{\rm i}\omega}$, where $m^{}_i$ and $V^{}_{ei}$ (for $i = 1, 2, 3, 4$) denote respectively the absolute masses and the first-row elements of the 4$\times$4 neutrino flavor mixing matrix $V$, for which a full parametrization involves three Majorana-type CP-violating phases $\{\rho, \sigma, \omega\}$. A zero effective neutrino mass $|\langle m \rangle^\prime_{ee}| = 0$ is possible no matter whether three active neutrinos take the normal or inverted mass ordering, and its implications for the parameter space are examined in great detail. In particular, given the best-fit values of $m^{}_4 \approx 1.3~{\rm eV}$ and $|V^{}_{e4}|^2 \approx 0.019$ from the latest global analysis of neutrino oscillation data, a three-dimensional view of $|\langle m \rangle^\prime_{ee}|$ in the $(m^{}_1, \rho)$-plane is presented and further compared with that of the counterpart $|\langle m \rangle^{}_{ee}|$ in the absence of any sterile neutrino.