Compatibility of the Updated $(g-2)_\mu$, $(g-2)_e$ and PADME-Favored Couplings with the Preferred Region of ATOMKI X17

TL;DR

This paper assesses whether a kinetically mixed dark photon can simultaneously explain the muon and electron g-2 anomalies and the ATOMKI X17 nuclear anomaly, identifying a narrow parameter region consistent with recent experimental constraints.

Contribution

It provides a detailed comparison of the allowed dark photon parameter space with recent experimental results, focusing on the X17 anomaly and muon/electron g-2 discrepancies, and highlights a narrow overlapping region.

Findings

A narrow overlapping region of kinetic mixing parameter is identified.

Recent experimental constraints are consistent with the dark photon explanation within this region.

Future experiments, including the 2026 Jefferson Lab X17 search, are crucial for validation.

Abstract

We re-evaluate the viability of a kinetically mixed dark photon ($A^{\prime}$) as a solution to the muon anomalous magnetic moment $(g-2)\mu$ discrepancy and the ATOMKI nuclear anomalies near 17~MeV, using the final FNAL measurement and the latest theory predictions (BMW21, WP25). For $m_{A^{\prime}} = 17$~MeV, the allowed kinetic mixing parameter narrows to $\varepsilon_\mu = 7.03(58)\times10^{-4}$ (WP25). We then directly compare the allowed region for the muon and X17 bands to those preferred by the electron magnetic moment measurements. For the electron, we obtain $\varepsilon_e = 1.19(15)\times10^{-3}$ (Cs, 2018) and $\varepsilon_e = 0.69(15)\times10^{-3}$ (Rb, 2020), based on two recent measurements of the fine structure constant compared to the most recent experimental value determined using a one-electron quantum cyclotron. This study focuses on the protophobic vector interpretation of X17 and assumes $\varepsilon_\nu<<\varepsilon_l$. While a mild tension persists, we identify a narrow overlapping region, $6.8\times10^{-4} \lesssim \varepsilon \lesssim 9.6\times10^{-4}$, between recent PADME results, the NA64 exclusion, and within the 2$\sigma$ preferred coupling region given by the Rb 2020 determination of $\alpha_\varepsilon$. These results provide well-defined targets for future experimental searches and motivate further theoretical refinements, both of which will play a decisive role in assessing the validity of the ATOMKI anomaly claims. Of particular note is the fixed target X17 experiment to be conducted in Hall-B of Thomas Jefferson National Accelerator Facility in Summer of 2026.