The decay of an eta meson into three pions is exactly forbidden in the isospin limit.
In fact it can be shown that the amplitude is proportional to a ratio of quark masses involving the difference of the squared up and down quark masses making it an ideal process to study the strength of isospin breaking in the strong sector. In this talk I will introduce three different formalisms to describe the decay amplitude: chiral perturbation theory, dispersion relations and non‐relativistic EFT. I will point out the strengths and weaknesses of the different formalisms and show how one can ideally combine them to describe the momentum dependence and extract the quark mass ratio.
Jan Lüdtke (Vienna): Matching non‐relativistic EFT and dispersion relations for η → 3π decays
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