Recently a 6.8σ 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, 8Be8Be X, and then decays through Xe+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 3.6σ discrepancy between the predicted and measured values of the muon’s anomalous magnetic moment.

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Published in Physical Review Letters, v. 117, issue 7, 071803, p. 1-6.

© 2016 American Physical Society

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The work of J. L. F., B. F., I. G., J. S., T. M. P. T., and P. T. is supported in part by NSF Grant No. PHY-1316792. The work of S. G. is supported in part by the DOE Office of Nuclear Physics under Contract No. DE-FG02-96ER40989. J. L. F. is supported in part by a Guggenheim Foundation Grant and in part by Simons Investigator Grant No. 376204.