Can a protophobic vector boson explain the ATOMKI anomaly?

Abstract

In 2016, the ATOMKI collaboration announced [Phys. Rev. Lett. 116, 042501 (2016)] observing an unexpected enhancement of the e+-e−pair production signal in one of the Be8 nuclear transitions induced by an incident proton beam on a 7Li target. Many beyond-standard-model physics explanations have subsequently been proposed. One popular theory is that the anomaly is caused by the creation of a protophobic vector boson (X) with a mass around 17 MeV [e.g. Phys. Rev. Lett. 117, 071803 (2016)] in the nuclear transition. We study this hypothesis by deriving an isospin relation between photon and X couplings to nucleons. This allows us to find simple relations between protophobic X-production cross sections and those for measured photon production. The net result is that X production is dominated by direct transitions induced by E1X and L1X (transverse and longitudinal electric dipoles) and C1X (charge dipole) without going through any nuclear resonance (i.e. Bremsstrahlung radiation) with a smooth energy dependence that occurs for all proton beam energies above threshold. This contradicts the experimental observations and invalidates the protophobic vector boson explanation.