2026/07/24 by Peter Boyle, Norman H. Christ, Xu Feng +4
#hep-lat #hep-ph
The ratio of electronic to muonic leptonic decay widths, Re/μ, for the light mesons π and K, provides a clean test of lepton flavor universality (LFU) and a sensitive probe of physics beyond the Standard Model. Its Standard-Model prediction is exceptionally precise, with the leading uncertainty associated with the structure-dependent (SD) radiative correction of O(0.1%). As experiments such as PIONEER and NA62 aim for unprecedented precision, this SD correction has become an essential ingredient in precision experiment--theory comparisons. We present the first lattice QCD+QED calculation of this SD correction at the physical pion mass and in the continuum limit. We employ the infinite-volume reconstruction (IVR) method with Coulomb-gauge photons, significantly reducing both statistical errors and finite-volume effects. We obtain the Standard-Model predictions, Re/μ=1.23501(10)×10-4 for π and Re/μ=2.47653(34)×10-5 for K. Our results reduce the hadronic uncertainty in Re/μ, provide the most precise Standard-Model predictions to date, and establish first-principles benchmarks for future high-precision tests of LFU.