The viable parameter space for the hypothetic X17 particle (with a mass of about 17 MeV) proposed to explain some unexpected nuclear physics is very nearly null.
In recent years, the ATOMKI collaboration has performed a series of measurements of excited nuclei, observing a resonant excess of electron-positron pairs at large opening angles compared to the Standard Model prediction.
The excess has been hypothesized to be due to the production of a new spin-1 or spin-0 particle, X17, with a mass of about 17 MeV.
Recently, the PADME experiment has reported an excess in the e+e− cross section at center-of-mass energies near 17 MeV, perhaps further hinting at the existence of a new state. Studies of the spin-1 case have hitherto focused on either vector or axial-vector couplings to quarks and leptons, whereas UV theories more naturally produce both vector and axial-vector i.e. chiral couplings, analogous to the Standard Model weak interactions.
We consider the ATOMKI anomalies in the context of an X with chiral couplings to quarks and explore the parameter space that can explain the ATOMKI anomalies, contrasting them with experimental constraints.
We find that it is possible to accommodate the reported ATOMKI signals. However, the 99% CL region is in tension with null results from searches for atomic parity violation and direct searches for new low mass physics coupled to electrons. This tension is found to be driven by the magnitude of the reported excess in the transition of 12C(17.23), which drives the best-fit region towards excluded couplings.
Max H. Fieg, Toni Mäkelä, Tim M.P. Tait, Miša Toman, "The X17 with Chiral Couplings" arXiv:2602.11263 (February 11, 2026).
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