Can a protophobic vector boson explain the ATOMKI anomaly?

In 2016, the ATOMKI collaboration announced [PRL \mathbf{116}, 042501 (2016)] observing an unexpected enhancement of the $e^+$-$e^-$ pair production signal in one of the $^8$Be nuclear transitions induced by an incident proton beam on a $^7$Li 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., PRL \mathbf{117}, 071803 (2016)] in the nuclear transition. In this talk, I will discuss our recent study (arXiv: 2008.11288) which disproves this hypothesis. I will start with an isospin relation between photon and $X$ couplings to nucleons, and then explain how these relationships suggest that the $X$ production must be dominated by direct transitions 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 energy dependence contradicts the experimental observations and invalidates the protophobic vector boson explanation.