Looking for the electron’s electric dipole moment

Since 1950, experimenters have been trying to measure the electron’s electric dipole moment d_e and getting increasingly stringent upper limits—but no clear signal. In the early days, the quest seemed dubious because a d_e would break symmetries that were then thought to be inviolate. By the mid 1960s it was clear that those symmetries are in fact broken. But the standard model of particle theory, which incorporates the known symmetry violations, predicts a d_e of about 10⁻³⁸ e·cm, far too small to measure. However, attempts to progress beyond the manifestly incomplete standard model predict much higher d_e magnitudes that should be measurable with molecular-beam techniques. Now the ACME collaboration reports a new null result from a particularly sensitive experiment that uses a thorium oxide beam. Polarizing a ThO molecule provides an enormously strong intramolecular electric field in which to look for energy shifts attributable to d_e. Having found none, the ACME team sets a new upper limit of 10⁻²⁸ e·cm on d_e. That’s an order of magnitude lower than the previous limit set just three years ago. The new limit puts severe constraints on supersymmetric theories and other proposed extensions of the standard model that posit a d_e attributable to new particles with masses of a few hundred GeV—the energy regime in which the underlying unity of the weak and electromagnetic interactions is broken. The new d_e limit pushes the masses of the new particles into the multi-TeV range, where they are unlikely to be found by CERN’s Large Hadron Collider or by the next generation of electron–positron colliders. (J. Baron et al., Science 343, 269, 2014 .)—Bertram M. Schwarzschild