The X17 with Chiral Couplings

TL;DR

This paper investigates whether a 17 MeV vector boson X with chiral vector/axial couplings to quarks can account for the ATOMKI anomalies while remaining consistent with external constraints. It builds a nuclear EFT framework linking quark-level couplings to Be/He/C nuclear transitions, incorporates substantial uncertainties in axial-vector matrix elements, and scans neutron and electron couplings with the electron vector coupling tuned to satisfy SINDRUM-I bounds; the analysis also considers the PADME excess. The results show a 99% CL region that can fit Be and He data but is in tension with parity-violation and NA64/KLOE-2 constraints, driven mainly by the carbon transition ${}^{12}{\rm C}(17.23)$; removing this measurement opens viable parameter space that can accommodate the PADME signal with $|e\epsilon_e^V| \sim 5.6\times 10^{-4}$. The study highlights the need for improved nuclear matrix-element calculations and, if a consistent X17 model with chiral couplings exists, potential UV-completion considerations to address the competing experimental constraints.

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.

Figures (2)

• Figure 1: The result of the fit to the ATOMKI measurements in \ref{['tab:ATOMKImeasurements']}. For each point in the plane of the neutron couplings $\epsilon_n^V, \epsilon_n^A$, we determine the proton couplings $\epsilon_p^V, \epsilon_p^A$ best fitting the ATOMKI measurements, and the electron coupling $\epsilon_e^V$ canceling the constraints from SINDRUM-I. In the upper left plot, the bright green region with a solid black outline indicates agreement with ATOMKI at 99% CL; the opaque dark green region with a dotted outline uses the larger theory uncertainty on the carbon and helium axial matrix elements as discussed in the text. Regions compatible with individual ATOMKI measurements are shown by single dot-dashed lines ($^8{\rm Be}(18.15)$), double dot-dashed lines ($^8{\rm Be}(17.64)$), dashed lines ($^4{\rm He}$) and dotted lines ($^{12}{\rm C}(17.23)$). The gray region denotes exclusion regions driven by parity violation constraints and the NA64, KLOE-2 experiments. The other three panels show the corresponding values of $\epsilon_e^V$ (upper right), $\epsilon_p^V$ (lower left), and $\epsilon_p^A$ (lower right) at each point in the plane of the neutron couplings. The two green bands in the upper right panel indicate the $|\epsilon_e^V|$ necessary to to explain the PADME excess.
• Figure 2: The result of the fit to the ATOMKI measurements in \ref{['tab:ATOMKImeasurements']} with $^{12}$C(17.23) excluded from the fit. In the upper left panel, the blue shaded region shows where the ATOMKI beryllium and helium observations are consistent at the $99\%$ CL. In the upper left panel, the region outlined in blue shows where the ATOMKI beryllium and helium observations are consistent at the $99\%$ CL.