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Recently a 6.8$\sigma$ anomaly has been reported in the opening angle andinvariant 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,$^8\text{Be}^* \to {}^8\text{Be} \, X$, and then decays through $X \to e^+e^-$. The $X$ boson mediates a fifth force with a characteristic range of 12 fmand has milli-charged couplings to up and down quarks and electrons, and aproton coupling that is suppressed relative to neutrons. The protophobic $X$boson may also alleviate the current 3.6$\sigma$ discrepancy between thepredicted and measured values of the muon's anomalous magnetic moment.

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