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We present the results of a lattice QCD calculation of the pseudoscalar mesondecay constants fpi, fK, fD and fDs, performed with Nf=2 dynamical fermions.The simulation is carried out with the tree-level improved Symanzik gaugeaction and with the twisted mass fermionic action at maximal twist. We haveconsidered for the final analysis three values of the lattice spacing, a~0.10fm, 0.09 fm and 0.07 fm, with pion masses down to mpi~270 MeV. Our results forthe light meson decay constants are fK=158.1(2.4) MeV and fK/fpi=1.210(18).  From the latter ratio, by using the experimental determination ofGamma(K-->mu nu_mu (gamma))/ Gamma(pi--> mu nu_mu (gamma)) and the averagevalue of |Vud| from nuclear beta decays, we obtain |Vus|=0.2222(34), in goodagreement with the determination from semileptonic Kl3 decays and the unitarityconstraint. 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 withthe CLEO experimental measurement. For fDs our determination is smaller thanthe PDG 2008 experimental average but in agreement with a recent improvedmeasurement by CLEO at the 1.4 sigma level.

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