Searching for Neutral State in the Rare Decay J/ψ → e + e − ϕ

We investigate the decay process J/ψ → e + e − ϕ , where the relatively clean electromagnetic (EM) transitions dominate at leading order at the tree level, while hadronic contributions arise only through hadronic loop transitions. The branching ratio of J/ψ → e + e − ϕ was estimated to be approximately 2.28 × 10-8 where the hadronic effects are negligible compared to the EM contributions. The upper limit on the branching fraction was set to be B( J/ψ → e + e − ϕ )1.2× 10 −7 by BESIII collaboration. In this paper, we investigate the existence of a neutral state such as dark photon and dark Z . Our analysis shows that the constraints on the dark photon and dark Z are stringent in the way that large enhancement to the SM branching ratio up to the present experimental limit is not possible. Therefore, observing a signal for the dark photon and dark Z in this decay channel is changeable.