Hunting for a 17 MeV particle coupled to electrons

TL;DR

The paper investigates a 17 MeV boson $X$ coupled to electrons as a possible explanation for the PADME $e^+e^-$ excess and its relation to ATOMKI anomalies. It analyzes four coupling structures—scalar, pseudoscalar, vector, and axial-vector—with a focus on a vector benchmark $m_X=16.9\,\mathrm{MeV}$ and $g_{eV}=5.6\times10^{-4}$, and confronts these with constraints from the electron anomalous magnetic moment, exotic pion decays, and future searches by Mu3e and PIONEER. The study finds that the PADME best-fit in the vector case is in tension with electron $g-2$ unless the current is conserved; exotic pion-decay limits are particularly powerful for non-conserved vector/axial couplings, while conserved-current scenarios remain less constrained but highly testable. The results emphasize the complementarity of electron $g-2$, pion decays, and muon-based probes, and project that upcoming data will decisively test the X17 electron-coupled hypothesis across several coupling structures. Overall, the framework provides a robust set of observables to validate or falsify an electron-coupled $X_{17}$ and can be extended to other light spin-0 or spin-1 mediators.

Abstract

We discuss a set of precision observables that can probe the existence of a light particle $X$ coupled to electrons in the mass range of 1-100 MeV. As a case study, we consider the recent excess of $e^+e^-$ final-state events at $\sqrt{s} = 16.9$ MeV reported by the PADME collaboration. Interestingly, this mass is tantalizingly close to the invariant mass at which anomalous $e^+e^-$ pair production has previously been observed in nuclear transitions from excited to ground states by the ATOMKI collaboration. For the scenario in which the new particle has a vector coupling to electrons, we show that the PADME excess is already in tension with constraints from the anomalous magnetic moment of the electron. Further improvements in the measurement of the electron $g$-2, together with upcoming results from PIONEER (searching for $π^+\to e^+ νX$) and Mu3e (searching for $μ^+ \to e^+ \barν_μν_e X$), are expected to definitively probe this scenario in the near future. We also explore alternative possibilities where the new particle has scalar, pseudoscalar, or axial-vector couplings.

Figures (1)

• Figure 1: Present constraints (solid lines) and expected sensitivities (dashed lines) for a light $X=S,P,V,A$ particle coupled to electrons in the 1--100 MeV mass range. The vertical, red line at $m_X = 16.9$ MeV denotes the $X_{17}$ benchmark, with the red star (in the vector case) representing the PADME best-fit value $Q_e\,g_{eV} = 5.6 \times 10^{-4}$. The burgundy (orange) lines illustrate the constraints from the exotic pion decays under the hypothesis that $X=V,A$ couples to a conserved (non-conserved) current.