Studying triangle singularity through spin observables

In this work, we study the spin density matrix element $\rho_{00}$ of $\phi$ in decay $J/\psi \rightarrow \eta \pi \phi$. previous studies, a band around 1.4 GeV on $\pi^0\phi$ distribution Dalitz plot was reported by BESIII Collaboration. This structure may be caused production resonance or triangle singularity mechanism. We find that predictions elements final based these mechanisms show distinct features. Thus measurement reaction offer an alternative way to and clarify mechanisms, i.e. kinematic effects. work also shows potential observables studying singularities.