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

BESIII Collaboration, M. Ablikim, M. N. Achasov +678

We report a direct search for a new gauge boson, $X$, with a mass of $17\text{MeV}/c^2$, which could explain the anomalous excess of $e^{+}{e}^{-}$ pairs observed in the ${}^8\text{Be}$ n…