Search for a hypothetical gauge boson and dark photons in charmonium transitions

We report a direct search for a new gauge boson, $X$, with a mass of $17\text{ MeV}\mathrm{/}{c}^2$, which could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the ${}^8\text{Be}$ nuclear transitions. The search is conducted in the charmonium decays ${\chi }_{cJ}\to XJ\mathrm{/}\psi (J=0,1,2)$ via the radiative transition $\psi (3686)\to \gamma {\chi }_{cJ}$ using $(2712.4\pm 14.3)\times {10}^6$ $\psi (3686)$ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed, and the updated upper limit on the coupling strength of charm quark and the new gauge boson, ${\epsilon }_c$, is set to be $\mathrm{\mid }{\epsilon }_c\mathrm{\mid }<1.2\times {10}^{-2}$ at $90\mathrm{\%}$ confidence level. We also report new constraints on the mixing strength $\epsilon$ between the Standard Model photon and dark photon ${\gamma }^{\mathrm{\prime }}$ in the mass range from $5\text{ MeV}\mathrm{/}{c}^2$ to $300\text{ MeV}\mathrm{/}{c}^2$. The upper limits at $90\mathrm{\%}$ confidence level vary within $(2.5-17.5)\times {10}^{-3}$ depending on the ${\gamma }^{\mathrm{\prime }}$ mass.