Isospin Breaking in K → ππ Decays

We perform a complete analysis of isospin breaking in K → 2π amplitudes in chiral perturbation theory, including both strong isospin violation (mu 6= md) and electromagnetic corrections to next-to-leading order in the low-energy expansion. The unknown chiral couplings are estimated at leading order in the 1/Nc expansion. We study the impact of isospin breaking on CP conserving amplitudes and rescattering phases. In particular, we extract the effective couplings g8 and g27 from a fit to K → ππ branching ratios, finding small deviations from the isospin-limit case. The ratio ReA0/ReA2 measuring the ∆I = 1/2 enhancement is found to decrease from 22.2±0.1 in the isospin limit to 20.3±0.5 in the presence of isospin breaking. We also analyse the effect of isospin violation on the CP violation parameter ǫ′, finding a destructive interference between three different sources of isospin violation. Within the uncertainties of large-Nc estimates for the low-energy constants, the isospin violating correction for ǫ′ is below 15 %. * Work supported in part by HPRN-CT2002-00311 (EURIDICE) and by Acciones Integradas, Project No. 19/2003 (Austria), HU2002-0044 (MCYT, Spain).