The Fermi-Watson theorem is generalized to the case of two coupled channels with different masses and applied to final state interaction in Ke4 decays. The impact of considered effect on the phase of the ππ scattering is estimated and shown that it can be crucial for scattering lengths extraction from experimental data on Ke4 decays.
@article{arxiv.0704.2675,
title = {The isospin symmetry breaking effects in $K_{e4}$ decays},
author = {S. R. Gevorkyan and A. N. Sissakian and A. V. Tarasov and H. T. Torosyan and O. O. Voskresenskaya},
journal= {arXiv preprint arXiv:0704.2675},
year = {2011}
}