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

Recently a 6.8 σ anomaly has been reported in the opening angle and invariant mass distributions of e⁺e^- pairs produced in ⁸Be 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, ⁸Be^* → ⁸Be X , and then decays through X →e⁺e^-. The X boson mediates a fifth force with a characteristic range of 12 fm and has millicharged 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 σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment.