Precise measurements of semileptonic kaon decay rates at KLOE provide the measurement of the CKM mixing matrix element vus and information about lepton universality. Leptonic kaon decays provide an independent measurement of $abs{vus}^2/abs{vud}^2$,
through the ratio $Gamma(Ktomu u)/Gamma(pitomu u)$. These measurements, together with the result of $|vud|$ from nuclear $beta$ transitions, provide the most precise test of CKM unitarity, allowing the universality of lepton and quark weak couplings to be tested. After the completion of the KLOE data taking, the proposal of a new run with an upgraded KLOE detector, KLOE-2, at an upgraded Dafne machine has been accepted by INFN and it is now starting. Present results from KLOE and future perspectives from KLOE-2 are reported.
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 fzer
o 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 $Gamma(Ktomu u)/Gamma(pitomu u)$ 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.
