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

Abstract

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($p,{\hat{I}}^3$)${}^8$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.

32 pages, 13 figures. Matches more closely the published version