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 $$e^+e^-$$ e + e - final-state events from positron annihilation on fixed-target atomic electrons. While the global significance remains at the $$(1.77\pm 0.15)\,\sigma $$ ( 1.77 ± 0.15 ) σ level, the excess is centered around $$\sqrt{s} \sim 17\,\text {MeV}$$ s ∼ 17 MeV , coinciding with the invariant mass at which anomalous $$e^+e^-$$ e + e - pair production has previously been observed in nuclear transitions from excited to ground states in $$^8$$ 8 Be, $$^4$$ 4 He and $$^{12}$$ 12 C, thereby strengthening the case for a common underlying origin, possibly involving a hypothetical new $$X_{17}$$ 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 $$X_{17}$$ X 17 mass of $$m_{X_{17}}= 16.88 \pm 0.05\,\text {MeV}$$ m X 17 = 16.88 ± 0.05 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.