Based on a sample of 1.31 × 10 9 J=ψ events collected with the BESIII detector, the matrix elements for the decays η 0 → ηπ þ π − and η 0 → ηπ 0 π 0 are determined using 351,016 η 0 → ðη → γγÞπ þ π − and 56,249 η 0 → ðη → γγÞπ 0 π 0 events with background levels less than 1%. Two commonly used representations are used to describe the Dalitz plot density. We find that an assumption of a linear amplitude does not describe the data well. A small deviation of the obtained matrix elements between η 0 → ηπ þ π − and η 0 → ηπ 0 π 0 is probably caused by the mass difference between charged and neutral pions or radiative corrections. No cusp structure in η 0 → ηπ 0 π 0 is observed.

Measurement of the matrix elements for the decays η→π^{+}π^{-}π^{0} and η/η^{′}→π^{0}π^{0}π^{0}Phys. Rev. D 92, 012014

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