Rescattering effects in → 3 decays ∗

The isospin-breaking decay η → 3π is an ideal tool to extract information on light quark mass ratios from experiment. For a precise determination, however, a detailed description of the Dalitz plot distribution is necessary. In that respect, in particular the slope parameter α of the neutral decay channel causes some concern, since the one-loop prediction from chiral perturbation theory misses the experimental value substantially. We use the modified non-relativistic effective field-theory, a dedicated framework to analyze final-state interactions beyond one loop including isospin-breaking corrections, to extract charged and neutral Dalitz plot parameters. Matching to chiral perturbation theory at next-to-leading order, we find α = −0.025 ± 0.005, in marginal agreement with experi- mental findings. We derive a relation between charged and neutral decay parameters that points towards a significant tension between the most recent KLOE measurements of these observables.