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We present a preliminary non-perturbative lattice calculation of the formfactors entering the processes $K\to \ell\,\nu_\ell\,\ell'^+\,\ell'^-$ and ofthe corresponding branching ratios. These form factors describe the interactionbetween the mediating virtual photon and the internal hadronic structure of themeson. By separating them from the point-like contribution to the matrixelement we are able to isolate and reconstruct the structure-dependentcontribution to the decay width. Our numerical analysis employs only one gaugeensemble and so it is affected by systematic uncertainties due to the missingcontinuum and physical point extrapolation. Despite this, we already find areasonable agreement with the experimental data and with the next to leadingorder Chiral Perturbation Theory predictions. The method is general and can beapplied to any pseudoscalar meson, though for heavier mesons the possibility ofinternal lighter states becomes problematic and still needs a proper study. Anon-perturbative, model-independent lattice evaluation of these processes wouldallow further progress in the theoretical predictions of SM hadronic quantitiesand in the search of New Physics.

Virtual Photon Emission in Leptonic Decays of Pseudoscalar Mesons