Ratio of kaon and pion leptonic decay constants with Nf = 2 + 1 + 1 Wilson-clover twisted-mass fermions.

We present a determination of the ratio of kaon and pion leptonic decay constants in isosymmetric QCD (ISOQCD), fK/fπ, making use of the gauge ensembles produced by the Extended Twisted Mass Collaboration with Nf = 2 + 1 + 1 flavors of Wilson-clover twisted-mass quarks, including configurations close to the physical point for all dynamical flavors. The simulations are carried out at three values of the lattice spacing ranging from ~0.068 to ~0.092 fm with linear lattice size up to L~5.5 fm. The scale is set by the particle data group (PDG) value of the pion decay constant, fISO QCDπ = 130.4(2) MeV, at the ISOQCD pion point, MISO QCDπ = 135.0(2) MeV, obtaining for the gradient-flow scales the values w 0 = 0.17383(63) fm, √t0 = 0.14436(61) fm and t0/w0 = 0.11969(62) fm. The data are analyzed within the framework of SU(2) chiral perturbation theory without resorting to the use of renormalized quark masses. At the ISOQCD kaon point MISO QCDK = 494.2(4) MeV we get (fK/fπ) ISO QCD = 1.1995(44), where the error includes both statistical and systematic uncertainties. Implications for the Cabibbo-Kobayashi-Maskawa matrix element |Vus| and for the first-row Cabibbo-Kobayashi-Maskawa unitarity are discussed. [ABSTRACT FROM AUTHOR]