Leptonic decay-constant ratio $f_K/f_{\pi }$ from lattice QCD using 2+1 clover-improved fermion flavors with 2-HEX smearing

Abstract

We present a calculation of the leptonic decay-constant ratio $f_K/f_{\pi }$ in 2+1 flavor QCD. Our data set includes five lattice spacings and pion masses reaching down below the physical one. Special emphasis is placed on a careful study of all systematic uncertainties, especially the continuum extrapolation. Our result is perfectly compatible with the first-row unitarity constraint of the Standard Model.

Comments: 19 pages, 6 figures, 3 tables; v2: added supplementary analysis, version published in Phys. Rev. D