Lepton Universality in Kaon Decays

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 $R_\pi$.