Revealing di-pion correlations for the observed substructure near the $π^+π^-$ mass threshold in $ψ(3686)\to J/ψπ^+π^-$

Abstract: Based on the world's largest $ψ(3686)$ data sample, the BESIII Collaboration recently reported a substructure near the $π^+π^-$ mass threshold in the decay $ψ(3686) \to J/ψπ^+π^-$, challenging the established understanding of the di-pion invariant mass spectrum. We propose that this substructure arises directly from di-pion correlations. Using a chiral unitary approach, we successfully reproduce the observed anomaly, thereby providing strong evidence of di-pion correlation in heavy quarkonium decays. This approach also allows us to predict the corresponding di-pion correlation function.