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

The Standard Model (SM) of particle physics is known to be incomplete. It fails to explain dark matter, and why matter survived annihilation with antimatter following the Big Bang. Proposed extensions to the SM, such as weak-scale Supersymmetry (SUSY), may explain these phenomena by positing the existence of new particles and new interactions that are not symmetric under the time-reversal (T) transformation. These same theories nearly always predict a small, yet potentially measurable, asymmetric charge distribution directed along the spin ($\vec{S}$) of the electron, an electric dipole moment (EDM, $\vec{d_e}=d_e \vec{S}/(\hbar/2)$), which is also asymmetric under T. The predicted value of $d_e$ in these SM extensions is typically in the range of $10^{-27}$-$10^{-30}$ $e$ cm, orders of magnitude larger than is predicted by the SM. Here, we report a new search for the electron EDM using the polar molecule thorium monoxide (ThO). Our result, $d_e = (-2.1 \pm 3.7_\mathrm{stat} \pm 2.5_\mathrm{syst})\times 10^{-29}$ $e$ cm, 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 limit[Hudson et al., Nature 473, 493 (2011)]. Our result sets strong constraints on new physics (SM extensions) associated with T-violating interactions at the TeV energy scale.