  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.
