Comparison of lattice QCD+QED predictions for radiative leptonic decays of light mesons with experimental data

We present a comparison of existing experimental data for the radiative leptonic decays P -> lv(l gamma), where P = K or pi and l = e or mu, from the KLOE, PIBETA, E787, ISTRA+ and OKA collaborations with theoretical predictions based on the recent non-perturbative determinations of the structure-dependent vector and axial-vector form factors, F-V and F-A respectively. These were obtained using lattice QCD + QED simulations at order O(alpha(em)) in the electromagnetic coupling. We find good agreement with the KLOE data on K -> ev(e gamma) decays from which the form factor F+ = F-V + F-A can be determined. For K -> mu v(mu gamma) decays we observe differences of up to -34 standard deviations at large photon energies between the theoretical predictions and the data from the E787, ISTRA+ and OKA experiments and similar discrepancies in some kinematical regions with the PIBETA experiment on radiative pion decays. A global study of all the kaon-decay data within the Standard Model results in a poor fit, largely because at large photon energies the KLOE and E787 data cannot be reproduced simultaneously in terms of the same form factor F+. The discrepancy between the theoretical and experimental values of the form factor F- = F-V - F-A is even more pronounced. These observations motivate future improvements of both the theoretical and experimental determinations of the structure-dependent form factors F+ and F-, as well as further theoretical investigations of models of "new physics" which might for example, include possible flavor changing interactions beyond V - A and/or nonuniversal corrections to the lepton couplings.