Can a protophobic vector boson explain the ATOMKI anomaly?

TL;DR

This paper critically examines the hypothesis that a protophobic vector boson explains the ATOMKI anomaly, deriving relations between photon and boson couplings, and finds that the predicted production mechanism contradicts experimental data.

Contribution

The study derives isospin relations between photon and protophobic boson couplings, showing that the predicted production mechanism does not match observations, thus invalidating this explanation for the anomaly.

Findings

X boson production is dominated by direct electromagnetic transitions.

Predicted energy dependence of X production is smooth and inconsistent with experimental data.

The protophobic vector boson hypothesis is invalidated by the derived relations and energy dependence analysis.

Abstract

In 2016, the ATOMKI collaboration announced [PRL {\bf 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\ {\bf 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 $E1^X$ and $L1^X$ (transverse and longitudinal electric dipoles) and $C1^X$ (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.