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

TL;DR

This paper reports a precise lattice QCD calculation of the ratio of kaon to pion decay constants, carefully analyzing systematic uncertainties and confirming compatibility with Standard Model unitarity constraints.

Contribution

It provides a high-precision determination of $f_K/f_$ using 2+1 flavor lattice QCD with multiple lattice spacings and pion masses, emphasizing systematic error control.

Findings

Result agrees with Standard Model unitarity

Includes five lattice spacings for continuum extrapolation

Achieves pion masses below physical value

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.