Hunt for $X(17)$

TL;DR

This work investigates the 17 MeV X(17) boson as a potential explanation for the Atomki anomalies under a Vector ± Axial-vector ($V \pm A$) coupling to electrons. It derives constraints on the $Xee$ interaction from Atomki data, electron AMM, beam-dump experiments, and KLOE-2, and identifies the surviving region in the coupling parameter space. The paper then evaluates two experimental channels: $e^+e^- \to X(17) \to e^+e^-$ at PADME and $e^+e^- \to X(17)\gamma \to e^+e^-\gamma$ at BESIII, concluding that PADME faces a pessimistic outlook whereas BESIII could provide a decisive test, given favorable couplings and detector resolution. If new data disfavors X(17) in the $V \pm A$ framework, the anomalies may instead originate from unrecognized nuclear effects, motivating further dedicated experiments and theoretical studies.

Abstract

The $^8Be$ anomaly reported by the Atomki experiments can be explained by the hypothesis of an $X(17)$ boson that interacts with electrons via the ``Vector $\pm$ Axial-vector'' ($V \pm A$) interaction. Using existing experimental data, we derive constraints on the couplings of the $X(17)$ boson to electrons within this $V \pm A$ framework. With this setup, we attempt to identify $X(17)$ signals in the $e^+e^- \longrightarrow X(17) \longrightarrow e^+e^-$ process at the PADME experiment and in the $e^+e^- \longrightarrow X(17)γ\longrightarrow e^+e^- γ$ process at the BESIII experiment. Our findings indicate that observing the $X(17)$ signal at the PADME experiment is pessimistic, whereas the BESIII experiment may provide a definitive answer regarding the $X(17)$ hypothesis.

Figures (11)

• Figure 1: Contour curves of the survived region for the coupling parameters $|\varepsilon_e^v|$ and $|\varepsilon_e^a|$ in the "$V \pm A$" model. The boundary of this survived region is marked by the red curve.
• Figure 2: The decay width of $X \rightarrow e^+e^-$ as a function of the coupling parameters $|\varepsilon_e^v|$ and $|\varepsilon_e^a|$. The parameters $|\varepsilon_e^{v/a}|$ are constrained within the red region shown in Fig. \ref{['fig:contoureconstrain']}. The red and blue spots denote the points corresponding to the maximum and minimum decay width values, respectively.
• Figure 3: The Feynman diagrams for the $e^{+} e^{-} \rightarrow X(17)/\gamma^* \rightarrow e^{+} e^{-}$ processes.
• Figure 4: The total cross section of the process $e^+e^- \longrightarrow X \rightarrow e^+e^-$ as a function of the coupling parameters $|\varepsilon_e^v|$ and $|\varepsilon_e^a|$. The parameters $|\varepsilon_e^{v/a}|$ are constrained within the red region shown in Fig. \ref{['fig:contoureconstrain']}. The red and blue spots denote the points corresponding to the maximum and minimum total cross section values, respectively.
• Figure 5: Left panel: Differential cross section as a function of $\cos\theta$. Right panel: Total cross section as a function of $\sqrt{s}$. Here, $\sigma_{B+S}$ includes the interference contribution between the background and signal processes, and the shaded area arises from the maximum and minimum coupling parameter values given in Eq. \ref{['eq:ee2X2eelocation']}. In the right panel, the solid, dashed, and dotted lines correspond to the angular cuts $|\cos\theta| < 0.9$, $|\cos\theta| < 0.6$, and $|\cos\theta| < 0.3$, respectively.