A fundamental consequence of the gauge structure of the Standard Model (SM) is the universality of the coupling constants. The high experimental precision achieved by lepton universality tests offers a unique opportunity to probe deviations from the SM.
We consider a tree-level enhancement to the violation of lepton flavour universality in meson decays arising from modified Wlν couplings in the Standard Model minimally extended by fermionic singlets (present in many SM extensions).
Due to the presence of additional mixings between the active neutrinos and the new sterile states, the deviation from unitarity of the leptonic mixing matrix might lead to a tree-level enhancement of, for instance, RP = Γ(P → eν)/Γ(P → μν), with P = K, π.
We illustrate these enhancements in the case of the inverse seesaw model, showing that one can saturate the current experimental bounds on ∆rK (and ∆rπ), while complying with different experimental and observational constraints.