Browsing by Author "López Castro, G."

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)%.

The ratios Rτ=P ≡ Γðτ → Pντ½γ Þ=ΓðP → μνμ½γ Þ (P ¼ π, K) provide sensitive tests of lepton universality jgτ=gμj ¼ 1 and are a useful tool for new physics searches. The radiative corrections to Rτ=P are computed following a large-NC expansion to deal with hadronic effects: Chiral Perturbation Theory is enlarged by including the lightest multiplets of spin-one heavy states such that the relevant Green functions are well behaved at high energies. We find δRτ=π ¼ ð0.18 0.57Þ% and δRτ=K ¼ ð0.97 0.58Þ%, which imply jgτ=gμjπ ¼ 0.9964 0.0038 and jgτ=gμjK ¼ 0.9857 0.0078, compatible with and at 1.8σ of lepton universality, respectively. We test unitarity and bind nonstandard effective interactions with the τ → Pντ½γ decays.