Experimental search for electric dipole moments of light radioactive nuclei

TL;DR

This work advocates a direct search for electric dipole moments in light, beta-decaying nuclei using a compact frozen-spin ion-trap, aiming to bypass Schiff screening that limits neutral-atom experiments. By storing nearly fully stripped or paired-electron ions (notably $^8$Li) at sub-GeV scales and using beta-decay asymmetry, the method seeks to extract the EDM from spin precession in a controlled $\vec{E}$ and $\vec{B}$ environment, with systematic errors mitigated by a crossing-point analysis and CW/CCW reversals. The authors derive the EDM sensitivity scaling $\sigma(d_i) = \frac{\hbar}{2\tau\alpha P E_f}\frac{a}{a+1}\frac{Z}{z}\frac{1}{\sqrt{N}}$ and estimate a weekly reach near $1\times10^{-26}\ e\cdot\mathrm{cm}$ for $^8$Li, potentially surpassing current proton EDM limits and offering direct access to $d_p$ and $d_n$ with reduced model dependence. They further discuss the theoretical implications via naive shell-model estimates $d_{^8\mathrm{Li}} = \frac{2}{3}(d_n+d_p)$ and outline ab initio and EFT avenues to refine these predictions, highlighting the broader impact of light-ion EDM searches for constraining beyond-Standard-Model CP-violating physics.

Abstract

We discuss a search for the electric dipole moment (EDM) of a light beta-radioactive ion using a compact ion trap by adapting the "frozen-spin" method. The measurement will be done on ions stripped of their valence electrons, thereby bypassing the significant Schiff screening that hinders the application of successful contemporary EDM searches using heavy neutral atoms and molecules to light nuclei. We identified $^8$Li as the most promising candidate for a proof-of-concept EDM search and we estimate that the current indirect proton EDM limit of a few $10^{-25} e\cdot$cm set by $^{199}$Hg measurements can be surpassed with a week of measurement time at existing facilities.

Figures (4)

• Figure 1: Illustration of the connection between underlying fundamental theories to laboratory measurements, passing through potential effective $C\space P$-violating sources. Note that measurements on fully or partially stripped ions have a more direct and unambiguous connection to low-energy parameters than atoms/molecules. The muonEDM experiment Adelmann2025, which serves as a blueprint for the light ion search, would measure the EDM of an elementary particle providing a direct connection to fundamental theories. Figure adapted from Chupp2019.
• Figure 2: Constraints in the $d_n$--$d_p$ parameter space. Light blue and gray are the existing constraints on the mercury and neutron EDMs. The mercury EDM band includes a $\pm 2\sigma$ theoretical uncertainty. The intersection of these two bands sets the limit on the proton EDM, $|d_p|<4\times 10^{-25}\,e{\rm cm}$. Projected sensitivities from $^8$Li EDM measurements are shown assuming: one week of data collection (blue dashed line), 100days of data (green dash-dotted line), and 7days with fully stripped $^8$Li$^{3+}$ ions (red dotted line). Even with a one-week measurement, the $^8$Li EDM constraint improves existing bounds on $d_p$.
• Figure 3: Schematic cross-section of the experimental apparatus (components not to scale). Grayed-out components are already existing in the muonEDM experiment and those indicated in red lines are the new developments needed for storing light ions: an electrode system for stopping and storing ion bunches propagating clockwise (CW) or counter-clockwise (CCW) after injection through magnetically shielded channels, and scintillation beta-decay detectors.
• Figure 4: Daily sensitivity to the EDM of singly charged $^8$Li ions as a function of the strength of the confining magnetic field. The color gradient shows the kinetic energy of the particles for a given frozen-spin electric field. The three solid lines show the sensitivity at a fixed value of the orbit radius. A storage time of 1s is assumed.