Ab initio investigation of the $^7$Li($p,e^+e^-$)$^8$Be process and the X17 boson

Observations of anomalies in the electron-positron angular correlations in high-energy decays in $^4$He, $^8$Be, and $^{12}$C have been reported recently by the ATOMKI collaboration. These could be explained by the creation and subsequent decay of a new boson with a mass of ${\sim}17$ MeV. Theoretical understanding of pair creation in the proton capture reactions used in these experiments is important for the interpretation of the anomalies. We apply the ab initio No-Core Shell Model with Continuum (NCSMC) to the proton capture on $^7$Li. The NCSMC describes both bound and unbound states in light nuclei in a unified way with chiral two- and three-nucleon interactions as the only input. We investigate the structure of $^8$Be, the $p+^7$Li elastic scattering, the $^7$Li($p,\gamma$)$^8$Be cross section and the internal pair creation $^7$Li($p,e^+ e^-$)$^8$Be. We discuss the impact of a proper treatment of the initial scattering state on the electron-positron angular correlation spectrum and compare our results to available ATOMKI data sets. Finally, we calculate $^7$Li($p,X$)$^8$Be cross sections for several proposed models of the hypothetical X17 particle.