Pseudoscalar decay constants of kaon and D-mesons from NfN_f= 2 twisted mass Lattice QCD

We present the results of a lattice QCD calculation of the pseudoscalar meson decay constants fπ, fK, fD and fDs, performed with Nf = 2 dynamical fermions. The simulation is carried out with the tree-level improved Symanzik gauge action and with the twisted mass fermionic action at maximal twist. We have considered for the final analysis three values of the lattice spacing, a simeq 0.10 fm, 0.09 fm and 0.07 fm, with pion masses down to mπ simeq 270 MeV. Our results for the light meson decay constants are fK = 158.1(2.4) MeV and fK/fπ = 1.210(18). From the latter ratio, by using the experimental determination of Γ(K → μbar nuμ(γ))/Γ(π → μbar nuμ(γ)) and the average value of |Vud| from nuclear beta decays, we obtain |Vus| = 0.2222(34), in good agreement with the determination from semileptonic Kl3 decays and the unitarity constraint. For the D and Ds meson decay constants we obtain fD = 197(9) MeV, fDs = 244(8) MeV and fDs/fD = 1.24(3). Our result for fD is in good agreement with the CLEO experimental measurement. For fDs our determination is smaller than the PDG 2008 experimental average but in agreement with a recent improved measurement by CLEO at the 1.4 σ level.We thank all the members of the ETM Collaboration for fruitful discussions. D.P. thanks the Dipartimento di Fisica,Universita di Roma Tre, and C.T. thanks the Laboratoire de Physique Th´eorique , Universite de Paris XI, for the hospitality. V.L., R.F., F.M. and S.S thank MIUR (Italy) for partial financial support under the contract PRIN06. D.P. thanks MEC (Spain) for partial financial support under grant FPA2005-00711. This work has been supported in part by the EU Contract No. MRTN-CT-2006-035482, “FLAVIAnet”and by the DFG Sonderforschungsbereich/Transregio SFB/TR9-03. We also acknowledge the Consolider-Ingenio 2010 Program CPAN (CSD2007-00042).Peer reviewe