The Standard Model of particle physics is known to be incomplete. Extensions to the Standard Model, such as weak-scale supersymmetry, posit the existence of new particles and interactions that are asymmetric under time reversal (T) and nearly always predict a small yet potentially measurable electron electric dipole moment (EDM), Formula, in the range of 10−27 to 10−30 e·cm. The EDM is an asymmetric charge distribution along the electron spin (Formula) that is also asymmetric under T. Using the polar molecule thorium monoxide, we measured Formula = (–2.1 ± 3.7stat ± 2.5syst) × 10−29 e·cm. This corresponds to an upper limit of Formula < 8.7 × 10−29 e·cm with 90% confidence, an order of magnitude improvement in sensitivity relative to the previous best limit. Our result constrains T-violating physics at the TeV energy scale.
The shape of the electron's charge distribution reflects the degree to which switching the direction of time impacts the basic ingredients of the universe. The Standard Model (SM) of particle physics predicts a very slight asphericity of the charge distribution, whereas SM extensions such as supersymmetry posit bigger and potentially measurable, but still tiny, deviations from a perfect sphere. Polar molecules have been identified as ideal settings for measuring this asymmetry, which should be reflected in a finite electric dipole moment (EDM) because of the extremely large effective electric fields that act on an electron inside such molecules. Using electron spin precession in the molecule ThO, Baron et al. (p. 269, published online 19 December; see the cover; see the Perspective by Brown) measured the EDM of the electron as consistent with zero. This excludes some of the extensions to the SM and sets a bound to the search for a nonzero EDM in other facilities, such as the Large Hadron Collider.