Probing the dark axion portal via J/psi decays at BESIII and STCF

Large numbers of $J/\psi$ mesons can be resonantly produced at BESIII and STCF at the center-of-mass energy $\sqrt{s}=3.097$ GeV. Such $J/\psi$ mesons may undergo rare decays into an axionlike particle (ALP) $a$ and a dark photon $\gamma'$ in the theoretical framework of the dark axion portal. In this work, we investigate the exclusion reach of the existing BESIII dataset together with the projected sensitivity of STCF, focusing on the mono-photon signature. We perform Monte Carlo simulations and estimate the exclusion reach in the portal coupling $G_{a\gamma\gamma'}$ as a function of the ALP and dark-photon masses, taking background events into account. Our results indicate that the existing BESIII dataset already has exclusion sensitivity to previously unexplored regions of the dark axion portal parameter space, while the future STCF can further improve the sensitivity by roughly an order of magnitude.