Constraints on Exotic Dipole-Dipole Couplings between Electrons at the Micrometer Scale

New constraints on exotic dipole-dipole interactions between electrons at the micrometer scale are established, based on a recent measurement of the magnetic interaction between two trapped ${}^{88}{\mathrm{Sr}}^{+}$ ions. For light bosons ($\text{mass}\le 0.1\mathrm{eV}$) we obtain a 90% confidence interval for an axial-vector-mediated interaction strength of $|g_A^e{g}_A^e/4\pi \hslash c|\le 1.2\times {10}^{-17}$. Assuming $CPT$ invariance, this constraint is compared to that on anomalous electron-positron interactions, derived from positronium hyperfine spectroscopy. We find that the electron-electron constraint is 6 orders of magnitude more stringent than the electron-positron counterpart. Bounds on pseudoscalar-mediated interaction as well as on torsion gravity are also derived and compared with previous work performed at different length scales. Our constraints benefit from the high controllability of the experimental system which contained only two trapped particles. It therefore suggests a useful new platform for exotic particle searches, complementing other experimental efforts.

Figure 1

Constraints (at the 90% confidence level) on the dimensionless coupling constants, $g_A^e{g}_A^e/(4\pi \hslash c)$ and $g_A^e{\overline{g}}_A^e/(4\pi \hslash c)$, as a function of the corresponding boson mass. Filled areas correspond to excluded values. The solid line and dark blue fill show the constraints derived from the possible contribution of an axial-vector-mediated interaction to the dipole-dipole interaction between electrons measured in Ref. [21]. The dashed line and light gray region show the constraints on the strength of axial-vector-mediated electron-positron interaction, $g_A^e{\overline{g}}_A^e/(4\pi \hslash c)$, by comparing the measurement of the ground-state hyperfine interval for positronium [24] to recent QED calculations [25].

Figure 2

Same as Fig. 1, for the case of a pseudoscalar-mediated interaction, constraining $g_P^e{g}_P^e/(4\pi \hslash c)$ for electron-electron (dark blue region) and $g_P^e{\overline{g}}_P^e/(4\pi \hslash c)$ for electron-positron (light gray region) interactions.