Tree-level lepton universality violation in the presence of sterile neutrinos: impact for RK and Rπ

We consider a tree-level enhancement to the violation of lepton flavour universality in light meson decays arising from modified Wl nu couplings in the standard model minimally extended by sterile neutrinos. Due to the presence of additional mixings between the active (left-handed) neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix intervening in charged currents might lead to a tree-level enhancement of R-P = Gamma(P -> e nu)/Gamma(P -> mu nu), with P = K, pi. We illustrate these enhancements in the case of the inverse seesaw model, showing that one can saturate the current experimental bounds on Delta r(K) (and Delta r(pi)), while in agreement with the different experimental and observational constraints.