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

ACME Collaboration, Jacob Baron, Wesley C. Campbell, David DeMille, John M. Doyle, Gerald Gabrielse, Yulia V. Gurevich, Paul W. Hess, Nicholas R. Hutzler, Emil Kirilov, Ivan Kozyryev, Brendon R. O'Leary, Cristian D. Panda, Maxwell F. Parsons, Elizabeth S. Petrik, Ben Spaun, Amar C. Vutha, Adam D. West

TL;DR

Spin precession measurements in the polar molecule thorium monoxide indicate a nearly spherical charge distribution of an electron, which constrains T-violating physics at the TeV energy scale.

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.

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

TL;DR

Spin precession measurements in the polar molecule thorium monoxide indicate a nearly spherical charge distribution of an electron, which constrains T-violating physics at the TeV energy scale.

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 (- cm) electron electric dipole moment (EDM, ), which is an asymmetric charge distribution along the spin (). The EDM is also asymmetric under T. Using the polar molecule thorium monoxide (ThO), we measure cm. This corresponds to an upper limit of 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.

Paper Structure

This paper contains 1 section, 5 equations, 4 figures, 1 table.

Table of Contents

  1. Supplementary Materials

Figures (4)

  • Figure 1: Schematic of the apparatus (not to scale). A collimated pulse of ThO molecules enters a magnetically shielded region. An aligned spin state (smallest red arrows), prepared via optical pumping, precesses in parallel electric and magnetic fields. The final spin alignment is read out by a laser with rapidly alternating linear polarizations, $\hat{X},\hat{Y}$, with the resulting fluorescence collected and detected with photomultiplier tubes (PMTs).
  • Figure 2: Energy level diagram showing the relevant states. The state-preparation and readout lasers (double lined blue arrows) drive one molecule orientation $\tilde{\mathcal{N}}=\pm1$ (split by $2D\mathcal{E}\sim100^{\:}\mathrm{MHz}$) in the $H$ state to $C$, with parity $\tilde{\mathcal{P}}=\pm1$ (split by $50^{\:}\mathrm{MHz}$). Population in the $C$ state decays via spontaneous emission, and we detect the resulting fluorescence. $H$ state levels are accompanied by cartoons displaying the orientation of $\vec{\mathcal{E}}_\mathrm{eff}$ (blue arrows) and the spin of the electron (red arrows) that dominantly contributes to the $d_e$ shift.
  • Figure 3: (A) Histogram of $\omega^{\mathcal{NE}}$ measurements for each time point (within molecule pulse) and for all blocks. Error bars represent expected Poissonian fluctuations in each histogram bin. (B) Measured $\omega^{\mathcal{NE}}$ values grouped by the states of $\left|\mathcal{B}_{z}\right|,$$\left|\mathcal{E}_{z}\right|$, $\hat{k}\cdot\hat{z}$, and each superblock switch, before systematic corrections.
  • Figure 4: (A) Tuning out laser polarization gradient and $\partial \omega^{\mathcal{NE}}/\partial \mathcal{E}^\mathrm{nr}$ (see text for details). The red (black) points were taken with the polarization misaligned (aligned) with the birefringence axes of the electric field plates. (B) Microwave spectroscopic measurement of $\mathcal{E}^\mathrm{nr}$ along the molecule beam axis, $x$.