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Concept
Electron electric dipole moment
Summary
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). Within the Standard Model of elementary particle physics, such a dipole is predicted to be non-zero but very small, at most 10−38 e⋅cm, 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.
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.
If neutrinos are Majorana particles, a larger EDM (around e-33e⋅cm) is possible in the Standard Model.
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 that ranges from 10−27 to 10−29 e⋅cm. Some supersymmetric models predict that > 10−26 e⋅cm 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, would place further limits on which theory takes precedence.
As the electron has a net charge, the definition of its electric dipole moment is ambiguous in that
depends on the point about which the moment of the charge distribution is taken. If we were to choose to be the center of charge, then would be identically zero.
A more interesting choice would be to take as the electron's center of mass evaluated in the frame in which the electron is at rest.
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