Unitarity and universality with kaon physics at KLOE

Abstract

All relevant inputs for the extraction of the CKM matrix element \vus from \kl, \ks and \kpm decays have been measured at KLOE. From a global fit using only KLOE results, but \ks lifetime, a value of $|\vus|\fzero = 0.2157 \pm 0.0006$ is obtained, where \fzero is the form factor parametrizing the hadronic matrix element evaluated at zero momentum transfer. Comparison of the values of \fVus for $K_{e3}$ and $K_{\mu3}$ modes provides a test of lepton universality at 0.8% level of accuracy. The value $\vus/\vud=0.2323(15)$ has been obtained from the ratio $\Gamma(K\to\mu \nu)/\Gamma(\pi\to\mu \nu)$ using KLOE measurement of $\BR(\kpm \to \mudue)$ and lattice calculation of the ratio of decay constants $f_K/f_{\pi}$. These results, together with $\vud = 0.97418(26)$, are compatible at 0.6$\sigma$ level with CKM matrix unitarity. The universality of lepton and quark weak couplings can be tested and constraints on new physics extensions of the Standard Model can be set using these very precise measurements from kaon decays.