Lepton Universality in Kaon Decays

TL;DR

This paper investigates how sterile neutrinos in an extended Standard Model can cause measurable violations of lepton universality in kaon and pion decays, with potential deviations observable in current experiments.

Contribution

It analyzes the impact of sterile neutrinos within the inverse seesaw framework on lepton universality ratios, showing current bounds can be saturated under existing constraints.

Findings

Sterile neutrinos can induce observable deviations in $R_K$ and $R_ au$ ratios.

Current experimental bounds are compatible with significant sterile neutrino effects.

Deviations from unitarity in leptonic mixing matrices are possible within the model.

Abstract

In the Standard Model extended by sterile neutrinos, modified W-l-nu couplings arise, which are able to induce a tree-level enhancement to lepton flavour universality violation in kaon decays. The additional mixing between the active neutrinos and the sterile ones can generate deviations from unitarity in the leptonic mixing matrix for charged currents. We reconsidered this idea in the context of the inverse seesaw and evaluated its impact on the well measured ratio $R_K$. We show that the current experimental bound can be saturated in agreement with the different experimental and observational constraints. Similar results can be obtained when considering the ratio $R_\pi$.