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

Abstract

Abstract We calculate the ratios Rτ/P≡ Γ (τ → Pντ [γ]) /Γ (P → μνμ [γ]) (P = π, 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 δRτ/π = (0.18 ± 0.57)% and δRτ/K = (0.97 ± 0.58)%, where the uncertainties are induced fundamentally by the counterterms. We test the lepton universality, obtaining |gτ/gμ|π = 0.9964 ± 0.0038 and |gτ/gμ|K = 0.9857 ± 0.0078, and analyze the CKM unitarity, getting results at 2.1σ and 1.5σ from unitarity via |Vus/Vud| and |Vus|, respectively. We also update the search for non-standard interactions in τ decays. As a by-product, we report the theoretical radiative corrections to the τ → Pντ [γ] decay rates: δτπ = −(0.24 ± 0.56)% and δτK = −(0.15 ± 0.57)%.