Electron electric dipole moment
The electron's 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 existence of a non-zero 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. More precisely, a non-zero EDM does not arise until the level of four-loop Feynman diagrams and higher. An additional, larger EDM (around 10−33 e·cm) is possible in the standard model if neutrinos are majorana particles.
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. Supersymmetric models predict that |de| > 10−26 e·cm. The present experimental limit is therefore close to eliminating some of these theories. Further improvements, or a positive result, would place further limits on which theory takes precedence.
Experimental Measurements of the Electron EDM
A non-zero electron EDM has not yet been found in all experiments to date. The Particle Data Group publishes its value as ±0.07)×10−26 e·cm. An experiment performed in 2011 at (0.07Imperial College London by the group of Edward Hinds using the molecule ytterbium monofluoride placed an upper bound of |de| < ×10−28 e·cm10.5 (at 90% confidence level). In 2014, the ACME Collaboration at Harvard-Yale (led by David DeMille, John Doyle, and Gerald Gabrielse) performed an experiment measuring the electron EDM using a beam of thorium monoxide, further refining the limit to |de| < ×10−29 e·cm8.7. Several other groups around the world are also running experiments measuring the electron EDM, such as the groups of Eric Cornell and Jun Ye at JILA, David Weiss at Pennsylvania State University, Daniel Heinzen at University of Texas at Austin, and Steven Lamoreaux at Yale University, among others.
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