Electromagnetic radiative corrections to pionic beta decay \pi^+ to \pi^0 e^+ \nu_e

Pionic beta decay \pi^+ to \pi^0 e^+ \nu_e is analyzed in chiral perturbation theory with virtual photons and leptons. All electromagnetic corrections up to order e^2 p^2 are taken into account. Theoretical results are confronted with preliminary data from a PSI measurement and a value for the CKM matrix element |Vud| is given. Although the precision is presently still below the one of existing determinations of |Vud|, an analysis of pionic beta decay, based on a systematic treatment within a low-energy effective field theory, may become a useful alternative.