The leading-order electromagnetic and strong isospin-breaking corrections to the ratio of Kmu2 and Pimu2 decay rates are evaluated for the first time on the lattice, following a method recently proposed. The lattice results are obtained using the gauge ensembles produced by the European Twisted Mass Collaboration with Nf=2+1+1 dynamical quarks. Systematic effects are evaluated and the impact of the quenched QED approximation is estimated. Our result for the correction to the tree-level Kmu2/Pimu2 decay ratio is -1.22(16)%, to be compared to the estimate of -1.12(21)% based on chiral perturbation theory and adopted by the Particle Data Group.
Giusti, D., Lubicz, V., Tarantino, C., Martinelli, G., Sachrajda, C.T., Sanfilippo, F., et al. (2018). First Lattice Calculation of the QED Corrections to Leptonic Decay Rates. PHYSICAL REVIEW LETTERS, 120(7) [10.1103/PhysRevLett.120.072001].