Electric dipole moments

Adam Ritz (CERN & Victoria) just gave a talk about the electric dipole moments of elementary particles and their constraints on new physics.

Do elementary particles have a dipole moment? Something analogous to two opposite charges separated by a distance? A dipole moment is a vector, and by rotational symmetry, the only direction that the dipole could take is the direction of the spin.

A special comment for Quantoken. By elementary particles, I mean leptons, quarks and gauge bosons (and possible the Higgs and graviton) - and their simple enough bound states that have no particle physics reason to have a significant (=beyond the CKM contribution) dipole moment, such as the neutron. More complicated structures such as molecules of course usually have a dipole moment whose origin we understand very well. That's also the case of some atomic states where the dipole arises from various relativistic corrections.

You know that the magnetic moment of a particle is always proportional to the spin with a fixed coefficient that depends on the particle type only. So if the particles also have an electric dipole, then there is some correlation between the electric moment and the magnetic moment. But which sign should this relative factor have? You know that E is a vector but B is a pseudovector, and therefore the electric dipole moments of the particles violate P (parity) much like CP (parity combined with charge conjugation).

The Standard Model predicts certain small electric dipole moments of particles such as neutron - because of the CP-violating phases in the CKM matrix used for quark masses. New theories of physics including supersymmetry typically predict more significant violations of the CP symmetry which also means significant dipole moments.




Experiments try to measure the dipole moments in various types of materials - liquid Xenon is a crazy example. No dipole moments of the elementary particles have been found so far which puts strong constraints on the parameters of new theories such as MSSM (minimal supersymmetric Standard Model) as well as baryogenesis (but no constraints on leptogenesis).

Various other experiments are underway and some of them are being prepared. One of them may cost as much as 10 million dollars, if you want to have an idea about the budget. Many of them want to access the limit

• d = 10^{-29} electron.centimeter

where "electron.centimeter" is a natural unit of the dipole moment. It is the same dipole as an electron-positron pair separated by 1 centimeter. You see that the corresponding distance "10^{-29}" centimeters is tiny; it is much shorter than the typical "radius" of the particles. Once the experiments get to this level, the subject of EDMs will be over because this is the scale of the dipole moment predicted by the Standard Model. Well, we should eventually see the basic dipole moments predicted by the Standard Model but it is virtually impossible to measure the EDMs accurately. In other words, it would be impossible to distinguish new contributions from the Standard Model background.

The negative results of the experiments looking for the dipole moments is a bad news for more or less any theory of new physics. No really convincing explanation why the CP violating terms should vanish has been given in the case of supersymmetry. It is also a bad news for the anthropic principle because the unnaturally small values of the dipole moments are apparently not required for any mechanism underlying life.

Of course, it is good news for everyone who is quite happy with the Standard Model. It is even better than we thought.