Electron electric dipole moment

The electron electric dipole moment de is an intrinsic property of an electron such that the potential energy is linearly related to the strength of the electric field: The electron's electric dipole moment (EDM) must be collinear with the direction of the electron's magnetic moment (spin).

[1] Within the Standard Model, such a dipole is predicted to be non-zero but very small, at most 10−38 e⋅cm,[2] where e stands for the elementary charge.

The discovery of a substantially larger electron electric dipole moment would imply a violation of both parity invariance and time reversal invariance.

[3][4] In the Standard Model, the electron EDM arises from the CP-violating components of the CKM matrix.

The moment is very small because the CP violation involves quarks, not electrons directly, so it can only arise by quantum processes where virtual quarks are created, interact with the electron, and then are annihilated.

[2][a] If neutrinos are Majorana particles, a larger EDM (around 10−33 e⋅cm) is possible in the Standard Model.

[2] Many extensions to the Standard Model have been proposed in the past two decades.

These extensions generally predict larger values for the electron EDM.

For instance, the various technicolor models predict |de| that ranges from 10−27 to 10−29 e⋅cm.

[5] Some supersymmetric models predict that |de| > 10−26 e⋅cm[6] but some other parameter choices or other supersymmetric models lead to smaller predicted values.

The present experimental limit therefore eliminates some of these technicolor/supersymmetric theories, but not all.

Further improvements, or a positive result,[7] would place further limits on which theory takes precedence.

[8] As the electron has a net charge, the definition of its electric dipole moment is ambiguous in that depends on the point

[9] Classical notions such as the center of charge and mass are, however, hard to make precise for a quantum elementary particle.

In practice the definition used by experimentalists comes from the form factors

appearing in the matrix element[10] of the electromagnetic current operator between two on-shell states with Lorentz invariant phase space normalization in which Here

is the momentum transfer from the current to the electron.

is its static magnetic dipole moment, and

provides the formal definition of the electron's electric dipole moment.

as a slight shift of spectral lines.

scales approximately with the nuclear charge cubed.

[11] For this reason, electron EDM searches almost always are conducted on systems involving heavy elements.

[6] To date, no experiment has found a non-zero electron EDM.

As of 2020 the Particle Data Group publishes its value as |de| < 0.11×10−28 e⋅cm.

Here is a list of some electron EDM experiments after 2000 with published results: The ACME collaboration is, as of 2020, developing a further version of the ACME experiment series.

The latest experiment is called Advanced ACME or ACME III and it aims to improve the limit on electron EDM by one to two orders of magnitude.