The 17 MeV Anomaly in Beryllium Decays and $U(1)$ Portal to Dark Matter

The experiment of Krasznahorkay $\mathit{\text{et}}\mathit{\text{al}}$ observed the transition of a ${}^8\mathrm{B}\mathrm{e}$ excited state to its ground state and accompanied by an emission of $e^{+}{e}^{-}$ pair with 17 MeV invariant mass. This 6.8$\sigma$ anomaly can be fitted by a new light gauge boson. We consider the new particle as a $U(1)$ gauge boson, $Z^{\mathrm{\prime }}$, which plays as a portal linking dark sector and visible sector. In particular, we study the new $U(1)$ gauge symmetry as a hidden or non-hidden group separately. The generic hidden $U(1)$ model, referred to as dark $Z$ model, is excluded by imposing various experimental constraints. On the other hand, a non-hidden $Z^{\mathrm{\prime }}$ is allowed due to additional interactions between $Z^{\mathrm{\prime }}$ and Standard Model fermions. We also study the implication of the dark matter direct search on such a scenario. We found the search for the DM-nucleon scattering excludes the range of DM mass above 500 MeV. However, the DM-electron scattering for MeV-scale DM is still allowed by current constraints for non-hidden $U(1)$ models. It is possible to test the underlying $U(1)$ portal model by the future Si and Ge detectors with $5e^{-}$ threshold charges.