Searches for lepton flavour and lepton number violation in kaon decays

where GF is the Fermi constant, MP and Ml are meson and lepton masses, fP is the decay constant, and Vqq′ is the corresponding Cabibbo-Kobayashi-Maskawa matrix element. Although the SM predictions for the Pl2 decay rates are limited by hadronic uncertainties, their specific ratios do not depend on fP and can be computed very precisely. In particular, the SM prediction for the ratio RK = Γ(Ke2)/Γ(Kμ2) of kaon leptonic decay widths inclusive of internal bremsstrahlung (IB) radiation is 1