Leptonic decay-constant ratio $f_K/f_\pi$ from lattice QCD using 2+1   clover-improved fermion flavors with 2-HEX smearing
  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. 
Stephan D\"urr, Zolt\'an Fodor, Christian Hoelbling, Stefan Krieg,   Laurent Lellouch, Thomas Lippert, Thomas Rae, Andreas S...
hep-lat hep-ph