Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron

Abstract: The Standard Model (SM) of particle physics fails to explain dark matter and why matter survived annihilation with antimatter following the Big Bang. Extensions to the SM, such as weak-scale Supersymmetry, may explain one or both of these phenomena by positing the existence of new particles and interactions that are asymmetric under time-reversal (T). These theories nearly always predict a small, yet potentially measurable ($10^{-27}$-$10^{-30}$ $e$ cm) electron electric dipole moment (EDM, $d_e$), which is an asymmetric charge distribution along the spin ($\vec{S}$). The EDM is also asymmetric under T. Using the polar molecule thorium monoxide (ThO), we measure $d_e = (-2.1 \pm 3.7_\mathrm{stat} \pm 2.5_\mathrm{syst})\times 10^{-29}$ $e$ cm. This corresponds to an upper limit of $|d_e| < 8.7\times 10^{-29}$ $e$ cm with 90 percent confidence, an order of magnitude improvement in sensitivity compared to the previous best limits. Our result constrains T-violating physics at the TeV energy scale.

• The paper reports a 12-fold improved electron EDM limit, setting |d_e| < 8.7×10⁻²⁹ e·cm.
• It employs ThO molecules with a cryogenic buffer gas beam to minimize systematic errors through precise electric field reversals.
• These findings impose stringent constraints on CP-violating theories, probing energy scales up to several TeV beyond the Standard Model.

Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron

This paper presents a significant advancement in the precision measurement of the electron electric dipole moment (EDM), setting a new upper limit that is an order of magnitude smaller than previous experiments. The ACME Collaboration employs a novel method using thorium monoxide (ThO) molecules and a cryogenic buffer gas beam source to achieve high sensitivity and reject systematic errors.

The experiment leverages the exceptionally large effective electric field within heavy polar molecules, specifically using the $^3\Delta_1$ electronic state of ThO, which provides an effective electric field of approximately 84 GV/cm. This field is instrumental in the measurement of the EDM, due to its ability to cause energy shifts detectable via spin precession measurements. The ACME Collaboration’s approach utilizes a cryogenic source to produce a beam of ThO molecules, which are subject to precisely controlled electric and magnetic fields. This is advantageous because it suppresses motional electric fields and geometric phases that can introduce significant systematic errors, while the small magnetic moment of the $^3\Delta_1$ state further minimizes sensitivity to external magnetic fields.

The experimental procedure involves creating a coherent superposition of spin states in ThO molecules and observing the resulting spin precession as they traverse parallel electric and magnetic fields. The core measurement is the precession angle, which varies with the orientation of these fields, allowing the team to infer the value of the electron EDM with high precision. The competitive advantage of this setup arises from its ability to reverse the orientation of the electric field without altering the magnetic context, thus enabling unambiguous determination of the EDM effect.

Key results from this investigation include a measured EDM limit of $|d_e| < 8.7 \times 10^{-29} \, e$ cm, reported with 90 percent confidence. This outcome represents a twelvefold improvement over the existing limits set by previous studies involving thallium and ytterbium fluoride. The precision of these findings is bolstered by robust systematic error analysis, including extensive parameter variation over 40 criteria, and substantial reductions in systematic uncertainties from previous techniques. For instance, the study minimizes the influence of AC Stark shifts, non-reversing electric fields, and other potential confounds that might skew results, as reflected in a comprehensive systematic error budget provided in the paper.

The implications of these findings are substantial for theoretical physics, particularly concerning extensions to the Standard Model that predict CP-violating phases. The absence of a large EDM at the observed sensitivity level suggests constraints on theories that propose new CP-violating sources, potentially probing energy scales up to several TeV. The results thus impose stringent limitations on CP violation mechanisms that cannot be directly measured at high-energy colliders, such as the LHC.

For future developments, this method illustrates a potential path forward in refining EDM measurements with even greater sensitivity. It is anticipated that similar methodologies, perhaps with further optimized molecule detection technologies or higher effective electric field magnitudes, could yield even more stringent constraints on the electron EDM, offering deeper insights into the fundamental symmetries of particle interactions. Through such progressive refinements, experiments like these continue to play a critical role in testing the boundaries of current physical theories and exploring possible new physics phenomena.