Global Fit to Modified Neutrino Couplings and the Cabibbo-Angle Anomaly

TL;DR

This paper investigates whether lepton flavor universality violating new physics in the neutrino sector can explain the Cabibbo-angle anomaly, using a global Bayesian analysis that considers multiple observables and finds partial support for modified neutrino couplings.

Contribution

It provides a model-independent global Bayesian analysis showing that modified neutrino couplings can reduce the Cabibbo-angle tension and improve agreement with other data.

Findings

Non-zero electron and muon neutrino couplings are preferred at over 99% CL.

Simple right-handed neutrino models cannot fully explain the data.

The analysis reduces the Cabibbo-angle tension significantly.

Abstract

Recently, discrepancies of up to $4\,\sigma$ between the different determinations of the Cabibbo angle were observed. This "Cabibbo-angle anomaly" could be explained by lepton flavour universality (LFU) violating New Physics (NP) in the neutrino sector. However, modified neutrino couplings to Standard Model gauge bosons also affect many other observables sensitive to LFU violation which have to be taken into account in order to assess the viability of this explanation. Therefore, we perform a model-independent global analysis in a Bayesian approach and find that the tension in the Cabibbo angle is significantly reduced, while the agreement with other data is also mostly improved. In fact, non-zero modifications of electron and muon neutrino couplings are preferred at more than 99\% CL. Still, since constructive effects in the muon sector are necessary, simple models with right-handed neutrinos (whose global fit we update as a by-product) cannot fully explain data, pointing towards more sophisticated NP models.