Triangle singularity in the $J/ψ\to ϕπ^+ a_0^-(π^- η),\; ϕπ^- a_0^+(π^+ η)$ decays

Abstract

We study the $J/ψ\to ϕπ^+ a_0(980)^- (a_0^- \to π^- η)$ decay, evaluating the double mass distribution in terms of the $π^- η$ and $π^+ a^-_0$ invariant masses. We show that the $π^- η$ mass distribution exhibits the typical cusp structure of the $a_0(980)$ seen in recent high statistics experiments, and the $π^+ a^-_0$ spectrum shows clearly a peak around $M_{\rm inv}(π^+ a^-_0)=1420 \,{\rm MeV}$, corresponding to a triangle singularity. When integrating over the two invariant masses we find a branching ratio for this decay of the order of $10^{-5}$, which is easily accessible in present laboratories. We also call attention to the fact that the signal obtained is compatible with a bump experimentally observed in the $ηπ^+π^-$ mass distribution in the $J/ψ\to ϕηπ^+π^-$ decay and encourage further analysis to extract from there the $ϕπ^+ a_0^-$ and $ϕπ^- a_0^+$ decay modes.