MeV Sterile Neutrino in light of the Cabibbo-Angle Anomaly

TL;DR

This paper explores how a MeV-scale sterile neutrino could resolve the Cabibbo-angle anomaly by affecting CKM matrix element measurements, specifically through modifications in nuclear and neutron decay processes.

Contribution

It introduces a viable scenario within an extended inverse seesaw model where a MeV sterile neutrino explains the CKM unitarity deficit.

Findings

A MeV sterile neutrino can increase the measured |V_{ud}| value.

The sterile neutrino impacts nuclear and neutron decay measurements.

The proposed model remains consistent with existing constraints.

Abstract

A modified neutrino sector could imprint a signature on precision measurements of the quark sector because many such measurements rely on the semi-leptonic decays of the charged currents. Currently, global fits of the determinations of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements point to a $3\sigma$-level deficit in the first-row CKM unitarity test, commonly referred to as the Cabibbo-angle anomaly. We find that a MeV sterile neutrino that mixes with the electron-type neutrino increases the extracted $|V_{ud}|$, accommodating the Cabibbo-angle anomaly. This MeV sterile neutrino affects the superallowed nuclear $\beta$ decays and neutron decay, but it barely modifies the other measurements of the CKM elements. While various constraints may apply to such a sterile neutrino, we present viable scenarios within an extension of the inverse seesaw model.