Radiative corrections to K l 3 decays ∗

We present a complete calculation of the Kl3 decays K + → πlνl and K → πlνl to O(p4, (md −mu)p, ep) in chiral perturbation theory with virtual photons and leptons. We introduce the concept of generalized form factors and kinematical densities in the presence of electromagnetism, and propose a possible treatment of the real photon emission in K l3 decays. We illustrate our results by applying them to the extraction of the Kobayashi–Maskawa matrix element |Vus| from the experimental K e3 decay parameters. PACS. 13.20.Eb Decays of K mesons 13.40.Ks Electromagnetic corrections to strong and weak-interaction processes 12.15.Hh Determination of Kobayashi–Maskawa matrix elements 12.39.Fe Chiral Lagrangians * Work supported in part by TMR, EC-Contract No. ERBFMRX-CT980169 (EURODAΦNE).