Testing Exotic Electron-Electron Interactions with the Helium Ionization-Energy Anomaly

Abstract

Precision atomic spectroscopy provides a sensitive probe of physics beyond the Standard Model. A recently reported $9σ$ theory-experiment discrepancy in the ionization energy of metastable helium has motivated the hypothesis of a new boson mediating exotic electron-electron interactions. Using a model-independent sign-consistency analysis of the induced energy shifts, we show that the sign requirement alone excludes vector-vector and pseudoscalar-pseudoscalar interactions as possible explanations of the anomaly. Incorporating existing constraints together with improved limits obtained here further excludes axial-vector scenarios. Within the single-boson framework considered in this work, only a narrowly constrained scalar-mediated interaction remains viable. The remaining parameter space could be probed, for example, by modest improvements in the determination of the electron gyromagnetic ratio.

Figures (3)

• Figure 1: Schematic illustration of the assumed exotic interaction, mediated by a new boson ($X$), between electrons within $^{4}$He and $^{3}$He. Only electron--electron exotic interactions are considered, as they generate identical leading-order energy shifts in both isotopes despite their different nuclear structures. The shaded regions indicate schematic electron probability distributions.
• Figure 2: Axial-vector new-boson coupling product $g_A^e g_A^e$ inferred from the helium discrepancy (pink band), shown together with existing constraints (gray curves) and the new constraints obtained in this work (green dash-dotted curves) as a function of the interaction range $\lambda$ (bottom axis). The corresponding boson mass $M$ is shown on the top axis. For an axial-vector boson, the coupling implied by the helium discrepancy is excluded by our new constraints, in combination with previous limits from Ref. [6].
• Figure 3: Scalar new-boson coupling product $g_s g_s$ inferred from the helium discrepancy (pink band), shown together with the new constraints obtained in this work (green dash-dotted curve) and existing constraints from Ref. delaunay_probing_2017 (gray curves) as a function of the interaction range $\lambda$. The corresponding boson mass $M$ is shown on the top axis. In this case, the coupling implied by the helium discrepancy remains possible for $M < 800\,\mathrm{eV}$, while the higher-mass range is excluded by existing and new constraints.