The X17 with Chiral Couplings

TL;DR

This paper investigates the possibility that the X17 particle with chiral couplings explains recent anomalies observed in nuclear and particle physics experiments, analyzing the parameter space and experimental constraints.

Contribution

It explores the chiral coupling scenario for X17, contrasting it with previous models and experimental constraints, providing a comprehensive analysis of its viability.

Findings

Chiral couplings can explain ATOMKI anomalies within certain parameters.

Experimental constraints from atomic parity violation challenge the chiral X17 explanation.

Tension arises mainly from the $^{12}{ m C}(17.23)$ transition data.

Abstract

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 {\em or} axial-vector couplings to quarks and leptons, whereas UV theories more naturally produce {\em both} vector and axial-vector (\textit{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 $^{12}{\rm C}(17.23)$, which drives the best-fit region towards excluded couplings.