Isospin breaking in Kl4 decays

Data on K-e4 decays allow one to extract experimental information on the elastic pp scattering amplitude near threshold, and to confront the outcome of the analysis with predictions made in the framework of QCD. These predictions concern an isospin symmetric world, while experiments are carried out in the real world, where isospin-breaking effects-generated by electromagnetic interactions and by the mass difference of the up and down quarks are always present. We discuss the corrections required to account for these, so that a meaningful comparison with the predictions becomes possible. In particular, we note that there is a spectacular isospin-breaking effect in K-e4 decays. Once it is taken into account, the previous discrepancy between NA48/2 data on K-e4 decays and the prediction of pp scattering lengths disappears.