  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,
$^8\text{Be}^* \to {}^8\text{Be} \, X$, and then decays through $X \to e^+
e^-$. The $X$ boson mediates a fifth force with a characteristic range of 12 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.
