One-loop determination of τ ; π(K)ντ [γ] branching ratios and new physics tests

Abstract

We calculate the ratios R /P 􀀀 ( ! P [ ]) /􀀀 (P ! μ μ[ ]) (P = ,K) at one loop following a large-NC expansion where Chiral Perturbation Theory is enlarged by including the lightest resonances and respecting the short-distance behavior dictated by QCD. We find R / = (0.18±0.57)% and R /K = (0.97±0.58)%, where the uncertainties are induced fundamentally by the counterterms. We test the lepton universality, obtaining |g /gμ| = 0.9964±0.0038 and |g /gμ|K = 0.9857±0.0078, and analyze the CKM unitarity, getting results at 2.1 and 1.5 from unitarity via |Vus/Vud| and |Vus|, respectively. We also update the search for non-standard interactions in decays. As a by-product, we report the theoretical radiative corrections to the ! P [ ] decay rates: = −(0.24 ± 0.56)% and K = −(0.15 ± 0.57)%.