Searching for neutral state in the rare decay $J/\psi \rightarrow e^+ e^- \phi$

Abstract

We investigate the decay process $J/\psi \rightarrow e^+ e^-\phi$, 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 $J/\psi \rightarrow e^+ e^-\phi$ was estimated to be approximately $2.28 \times 10^{-8}$ keV where the hadronic effects are negligible compared to the EM contributions. The upper limit on the branching fraction was set to be $B(J/\psi \rightarrow e^+ e^-\phi) < 1.2 \times 10^{-7}$ 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.