Combined Evidence for the $X_{17}$ Boson After PADME Results on Resonant Production in Positron Annihilation

Abstract

The Positron Annihilation into Dark Matter Experiment at the Laboratori Nazionali di Frascati has reported an excess of $e^{+}{e}^{-}$ final-state events from positron annihilation on fixed-target atomic electrons. While the global significance remains at the $(1.77\pm 0.15)\ddot{I}ƒ$ level, the excess is centered around $\sqrt{s}\sim 17\text{MeV}$, coinciding with the invariant mass at which anomalous $e^{+}{e}^{-}$ pair production has previously been observed in nuclear transitions from excited to ground states in ${}^8$Be, ${}^4$He and ${}^{12}$C, thereby strengthening the case for a common underlying origin, possibly involving a hypothetical new $X_{17}$ boson. We discuss the significance of this independent accelerator-based evidence. Combining it with existing nuclear physics results, we obtain a value for the $X_{17}$ mass of $m_{X_{17}}=16.88\pm 0.05\text{MeV}$, reducing the uncertainty from nuclear physics determinations by more than a factor of two, and mitigating the impact of poorly known correlations among their systematic errors.

12 pages, 2 figures, 1 table. Matched to the published version