$\mathrm{Ab\; initio}$ investigation of the ${}_7\mathrm{Li}(p,e^{+}{e}^{-}){}_8\mathrm{Be}$ process and the X17 boson

Observations of anomalies in the electron-positron angular correlations in high-energy decays in ${}_4\mathrm{He}, {}_8\mathrm{Be}$, 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 $\approx 17\mathrm{MeV}$. 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 ${}_7\mathrm{Li}$. 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 ${}_8\mathrm{Be}$, the $p+{}_7\mathrm{Li}$ elastic scattering, the ${}_7\mathrm{Li}(p,\gamma ){}_8\mathrm{Be}$ cross section, and the internal pair creation ${}_7\mathrm{Li}(p,e^{+}{e}^{-}){}_8\mathrm{Be}$. 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 ${}_7\mathrm{Li}(p,X){}_8\mathrm{Be}$ cross sections for several proposed models of the hypothetical X17 particle.