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Ratio of kaon and pion leptonic decay constants withNf=2+1+1Wilson-clover twisted-mass fermions
Physical Review D  (IF5.296),  Pub Date : 2021-10-26, DOI: 10.1103/physrevd.104.074520
C. Alexandrou, S. Bacchio, G. Bergner, P. Dimopoulos, J. Finkenrath, R. Frezzotti, M. Garofalo, B. Kostrzewa, G. Koutsou, P. Labus, F. Sanfilippo, S. Simula, M. Ueding, C. Urbach, U. Wenger (Extended Twisted Mass Collaboration)

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 0.092fm with linear lattice size up to L5.5fm. The scale is set by the particle data group (PDG) value of the pion decay constant, fπISOQCD=130.4(2)MeV, at the ISOQCD pion point, MπISOQCD=135.0(2)MeV, obtaining for the gradient-flow scales the values w0=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 MKISOQCD=494.2(4)MeV we get (fK/fπ)ISOQCD=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.