For decades, experimenters have been using atomic and molecular beams to measure the electric dipole moment (EDM) of the electron. As yet they’ve found no clear signal—just increasingly stringent upper limits. Those modest tabletop searches are addressing an issue crucial to particle physics, a discipline whose usual search tools are gargantuan. It’s been argued that such EDM searches are the fastest and cheapest route to the discovery of new physics beyond the standard model of particle theory.
The electron can have a nonvanishing EDM only if nature violates symmetry under time reversal (T) and under the combined operations of charge conjugation (C), which replaces particles by their antiparticles, and parity inversion (P). The standard model incorporates the small violations of CP and T symmetry that experiments at high-energy accelerators have revealed (see Physics Today, November 2012, page 16). But its prediction for...
