Electric Dipole Moments of Fundamental Particles

The search for electric dipole moments (EDM) of fundamental particles started approximately fifty years with Ramsey’s search for a neutron EDM. Even though the techniques have been improved and the sensitivities has reached an unprecedented small level no EDM of a fundamental particle has been observed so far. Non-the-less EDM experiments have put strict limits and constrained the parameter space of models beyond the standard model. The permanent EDM of fundamental particles would violate both time (T) and parity (P) symmetries: an EDM vector would have to be along the spin vector since there is no other defining vector. Phenomenologically any component in any other direction would average out to zero due to the particle’s spin rotation. The interaction Hamiltonian is given by