Where to find $X(17)$?

Abstract

The Atomki group puts forward the hypothesis of an $X(17)$ particle to explain its anomaly observation in ${}^8{\text{Be}}^{\ast }\to {}^8\text{Be}{e}^{+}{e}^{-}$ transition. Utilizing experimental results from the Atomki experiments, measurements of the electron's anomalous magnetic moment (AMM), the parity-violating Møller scattering experiment, the NA64 experiment, the KLOE-2 experiment, and the PADME experiment, we derive constraints on the $Xee$ coupling of the hypothetical $X(17)$ boson to electrons. A joint ${\chi }^2$ analysis of the electron's AMM (Cs and Rb measurements) and the parity-violating Møller scattering yields two mutually exclusive solutions. The Cs and Rb AMM constraints restrict the combination $({\epsilon }_e^v{)}^2-5({\epsilon }_e^a{)}^2$ with opposite signs. Cs+Møller combination favours a dominant axial-vector coupling $|{\epsilon }_e^a|\approx 5.3\times 10^{-4}\gg |{\epsilon }_e^v|$, whereas Rb+Møller favours a dominant vector coupling $|{\epsilon }_e^v|\approx 6.9\times 10^{-4}\gg |{\epsilon }_e^a|$. The Møller bound suppresses the subdominant coupling for both solutions. Incorporating NA64 and KLOE-2 limits on $({\epsilon }_e^v{)}^2+({\epsilon }_e^a{)}^2$ introduces mild tension for the Cs+Møller solution, while the Rb+Møller solution remains nearly unchanged.

arXiv comment25 pages, 6 figures, 2 table