Electric dipole moments of hadrons and light nuclei

A non-zero permanent electric dipole moment of any subatomic particle is a clear signal for CP violation beyond the Kobayashi-Maskawa mechanism of the Standard Model of particle physics.

Electric dipole moments

Status of excluded regions of electric dipole moments (EDMs). Current status of excluded regions of electric dipole moments (EDMs). Shown are direct or derived EDM bounds of subatomic particles and a selection of atoms.
Copyright: Jörg Pretz

Both continuous and discrete symmetries, combined with possible breaking patterns, have been decisive in the development of the Standard Model (SM) of particle physics. As it is the case for all stationary states of finite and parity–non-degenerate quantum systems, the ground state of any known subatomic particle with spin, regardless whether the particle is elementary or composite and even if it is not its own antiparticle, can only support a non-zero permanent electric dipole moment (EDM), if both time-reversal (T) and parity (P) symmetries are violated explicitly. Assuming CPT conservation, CP violation is implied. In the SM, the CP violation generated by the Kobayashi–Maskawa mechanism of weak interactions induces only very tiny EDMs that are several orders of magnitude below current experimental limits. However, many models beyond the SM predict EDM values near these limits. Finding a nonzero EDM value for any subatomic particle would therefore be a signal that there exists a new source of CP violation. The latter is essential for explaining — within the framework of the Big Bang and inflation — the mystery of the observed baryon–antibaryon asymmetry of our Universe.

While currently there exist only direct EDM bounds on electrically neutral subatomic and atomic systems, the storage ring technology will allow to search for EDMs of charged hadrons including the proton and light nuclei. At IKP-3/IAS-4, a consistent and complete calculation of the matrix elements relevant to the electric dipole moments of deuteron, helion, and triton has been developed in the framework of chiral effective field theory. In addition, strategies for disentangling the underlying sources of CP violation, in particular involving charged hadrons, are investigated.