Vector and scalar form factors for K- and D-mesonsemileptonic decays from twisted mass fermionswith Nf = 2

Abstract

We present lattice results for the formfactors relevant in the K→p ℓnℓ and D→p ℓnℓ semileptonic decays, obtained from simulations with two flavors of dynamical twisted-mass fermions and pion masses as light as 260 MeV. For K →p ℓn decays we discuss the estimates of the main sources of systematic uncertainties, including the quenching of the strange quark, leading to our final result f+(0) = 0.9560(57)stat.(62)syst.. Combined with the latest experimental data, our value of f+(0) implies for the CKMmatrix element |Vus| the value 0.2267(5)exp.(20) f+(0) consistent with the first-row CKM unitarity. For D →p ℓnℓ decays the application of Heavy Meson Chiral Perturbation Theory allows to extrapolate our results for both the scalar and the vector form factors at the physical point with quite good accuracy, obtaining a nice agreement with the experimental data. In particular at zero-momentumtransfer we obtain f+(0) = 0.64(5). A preliminary analysis of the discretization effects is presented and discussed.