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 lattice spacings, large volumes and a simulated pion mass reaching down to about 190 MeV. We obtain FK/Fpi=1.192 +/- 0.007(stat) +/- 0.006(syst). This result is then used to give an updated value of the CKM matrix element |Vus|. The unitarity relation for the first row of this matrix is found to be well observed.
@article{arxiv.1001.4692,
title = {The ratio FK/Fpi in QCD},
author = {S. Durr and Z. Fodor and C. Hoelbling and S. D. Katz and S. Krieg and T. Kurth and L. Lellouch and T. Lippert and A. Ramos and K. K. Szabo},
journal= {arXiv preprint arXiv:1001.4692},
year = {2011}
}