Evidence for the Cusp Effect in η′ Decays into ηπ0π0

Abstract

Using a sample of 4.3×105 η′→ηπ0π0 events selected from the ten billion J/ψ event dataset collected with the BESIII detector, we study the decay η′→ηπ0π0 within the framework of nonrelativistic effective field theory. Evidence for a structure at π+π- mass threshold is observed in the invariant mass spectrum of π0π0 with a statistical significance of around 3.5σ, which is consistent with the cusp effect as predicted by the nonrelativistic effective field theory. After introducing the amplitude for describing the cusp effect, the ππ scattering length combination a0-a2 is determined to be 0.226±0.060stat±0.013syst, which is in good agreement with theoretical calculation of 0.2644±0.0051.