Electric Dipole Tests of Time Reversal Symmetry

Abstract

If there is either T or P symmetry, there can be no electric dipole moment for particles whose orientations are fully specified by the orientations of their spin angular momenta. As a result, there have been extensive searches for electric dipole moments as tests of these symmetries. CP non‐conservation has been observed, which implies non‐conservation of T if CPT is conserved, but it has only been seen in the decay of the artificially produced K0L. The history of the searches for neutron and atomic electric dipole moments is summarized. The earliest experiments utilized a neutron beam magnetic resonance apparatus, but the present neutron experiments study ultra cold neutrons trapped in bottles whose walls totally reflect the neutrons. Recent atomic and molecular searches are sensitive both to particle electric dipole moments and to other related T non‐conserving quantities. The various neutron, atomic and molecular experiments are described. The results of these searches are compared with the predictions of theories that account for the known CP non‐conservation in the decay of the K0L. The experiments directly set limits to T‐odd, P‐odd interaction terms, but through eleectroweak radiative corrections they also set limits to T‐odd, P‐even interactions.