The Search for a Permanent Electric Dipole Moment

initiated a search for an electric dipole moment (EDM) of the neutron and obtained what was considered at the time a remarkably precise result, consistent with zero.1 Thus began a long line of ever more sensitive EDM experiments on neutrons, atoms, and molecules. Although no EDM has yet been found, the limits set have had decisive influences on elementary particle theories. Now, experiments under way or being planned may at last find an EDM and, in any event, are expected to have a far-reaching impact on theoretical particle physics. EDM experiments assume such a key role because an EDM of a fundamental particle implies that time reversal symmetry T is violated. In nearly all current theories, violation of T implies a violation of CP symmetry, a combination of charge conjugation C and parity P (that is, inversion through the origin). Indeed, CP violation was discovered around 40 years ago in the decay of the K0 meson.2 Most of the theories suggested to explain that violation, though, were eventually ruled out because they predicted relatively large EDMs that were excluded by experiment. Today, the accepted explanation for the K0 meson CP violation is contained within the standard model of particle physics, a theory that leads to EDMs too small to be seen in any current or contemplated experiments. By contrast, in supersymmetry (SUSY), currently the most favored theory of new physics beyond the standard model, the natural size of EDMs is large enough to be seen by experiments now under way. Thus, elusive as EDMs have been, interest in them continues unabated. The concept of an EDM is familiar to all students of classical electromagnetism, and so it may seem remarkable that a particle can have an intrinsic EDM only if T (and P) are violated. Recall that a charge q displaced from a charge ⊗q by a distance r creates an EDM d [ qr. A particle’s EDM would necessarily lie along its spin axis, because all components perpendicular to that axis would average to zero; the alignment of spin and EDM is what leads to violations of T and P. As pictured in figure 1, reversing time would reverse the spin direction but leave the EDM direction unchanged, which gives a particle with the opposite direction of EDM relative to spin. Thus, if time reversal were a good symmetry, particles with spin would be doubly degenerate according to whether the EDM is aligned parallel or antiparallel to the spin. Nature provides no such degeneracy for elementary particles or atoms, so the existence of an EDM in such simple systems would be a violation of T. (More complicated systems, such as polar molecules, can and do have such degenerate pairs of states; for a thorough discussion of the subtleties, see reference 3.) Figure 1 also depicts inversion through the origin and shows how an intrinsic EDM violates P. Physicists have known since 1956 that P is violated in the weak interactions; it is the T violation associated with EDMs that makes the experimental hunt interesting.