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We calculate the ratios $R_{\tau / P}\equiv \Gamma \left( \tau \to P\nu_{\tau}[\gamma] \right) / \Gamma \left(P\to \mu \nu_{\mu}[\gamma]\right)$($P=\pi,K$) at one loop following a large-$N_C$ expansion where ChiralPerturbation Theory is enlarged by including the lightest resonances andrespecting the short-distance behavior dictated by QCD. We find $\deltaR_{\tau/\pi}=(0.18\pm 0.57 )\%$ and $\delta R_{\tau/K}=(0.97\pm 0.58 )\%$,where the uncertainties are induced fundamentally by the counterterms. We testthe lepton universality, obtaining $\left|g_\tau/g_\mu\right|_\pi=0.9964\pm0.0038$ and $\left|g_\tau/g_\mu\right|_K=0.9857\pm 0.0078$, and analyze the CKMunitarity, getting results at $2.1\sigma$ and $1.5\sigma$ from unitarity via$|V_{us}/V_{ud}|$ and $|V_{us}|$, respectively. We also update the search fornon-standard interactions in $\tau$ decays. As a by-product, we report thetheoretical radiative corrections to the $\tau \to P \nu_{\tau}[\gamma]$ decayrates: $\delta_{\tau \pi} = -(0.24 \pm 0.56) \%$ and $\delta_{\tau K} = -(0.15\pm 0.57) \%$.} \maketitle

One-loop determination of $\tau \to \pi (K) \nu_{\tau}[\gamma]$ branching ratios and new physics tests