Radiative corrections to Kl3± decays

The radiative corrections to the lepton spectrum and decay rate for Kl3± decays are calculated, assuming a phenomenological weak K-π vertex and using perturbation theory. The answer depends logarithmically on a cutoff. The result applied to pion beta decay yields a fractional change in lifetime of -1.2% and is not sensitive to reasonable variations in the cutoff. The radiative correction to the electron spectrum from Ke3± decays at rest is substantial, averaging about 5% in absolute magnitude over most of the measurable energy range, and is insensitive to the cutoff. The correction to the Ke3± lifetime, the Kμ3± lifetime and muon spectrum is probably small (a fraction of a percent) although the numerical estimates are sensitive to the cutoff.