Protophobic Fifth Force Interpretation of the Observed Anomaly in ${}^8$Be Nuclear Transitions

Recently a 6.8$\sigma$ anomaly has been reported in the opening angle and invariant mass distributions of $e^{+}{e}^{-}$ pairs produced in ${}^8\text{Be}$ nuclear transitions. The data are explained by a 17 MeV vector gauge boson $X$ that is produced in the decay of an excited state to the ground state, ${}^8{\text{Be}}^{\ast }\to {}^8\text{Be}X$, and then decays through $X \to e^+ e^-$\mathrm{.}The$X$ boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The protophobic $X$ boson may also alleviate the current 3.6$\sigma$ discrepancy between the predicted and measured values of the muon’s anomalous magnetic moment.