Measurement of the decay η′→π0π0η at MAMI

An experimental study of the η0 → π0π0η → 6γ decay has been conducted with the best up-to-date statistical accuracy, by measuring η0 mesons produced in the γp → η0 p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The results obtained for the standard parametrization of the η0 → π0π0η matrix element are consistent with the most recent results for η0 → ππη decays, but have smaller uncertainties. The available statistics and experimental resolution allowed, for the first time, an observation of a structure below the πþπ− mass threshold, the magnitude and sign of which, checked within the framework of the nonrelativistic effective-field theory, demonstrated good agreement with the cusp that was predicted based on the ππ scattering length combination, a0 − a2, extracted from K → 3π decays.