FK/Fpi in full QCD

Abstract

We determine the ratio FK/Fpi in QCD with Nf=2+1 flavors of sea quarks, based on a series of lattice calculations with three different couplings, large volumes and a simulated pion mass reaching down to about 190 MeV. We obtain FK/Fpi = 1.192 +- 0.007(stat) +- 0.006(sys) with all the sources of systematic uncertainty under control.