Light Majorana Neutrinos in (Semi)invisible Meson Decays

TL;DR

This paper develops a model-independent framework for analyzing meson decays into neutrinos involving light Majorana neutrinos, and explores their implications for decay rates and experimental sensitivity.

Contribution

It introduces a new parametrization of neutrino mass matrices applicable to any number of neutrinos, bridging heavy Majorana and Dirac limits, and applies it to meson decay predictions.

Findings

Updated predictions for B meson decay branching ratios.

Analysis of decay sensitivity to neutrino mass and mixing.

Framework applicable to various neutrino mass scenarios.

Abstract

We reconsider decays of pseudoscalar mesons (P) to neutrino pairs and possibly additional photons in presence of (light) Majorana neutrinos. For this purpose we derive a model-independent general parametrization of neutrino mass matrices with physically interpretable and irreducible set of parameters. The parametrization is valid for any number of neutrinos and interpolates smoothly between the heavy Majorana and the (pseudo)Dirac neutrino limits. We apply the new parametrization to the study of $P \rightarrow \nu \nu$ and $P \rightarrow \nu \nu \gamma$ decays within the SM extended by additional singlet fermions. We update the SM predictions for the branching ratios of $B_{s,d} \rightarrow \nu \nu \gamma$ and discuss the sensitivity of the $B_{s,d} \rightarrow E_{\rm miss} (\gamma)$ decays to neutrino mass and mixing parameters.