Electron electric dipole moment

The electron electric dipole moment (EDM) d_e is an intrinsic property of an electron such that the potential energy is linearly related to the strength of the electric field:

$U=\mathbf {d} _{\rm {e}}\cdot \mathbf {E} .$

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

Implications for Standard Model and extentions

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 10⁻³³ e⋅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 d_e that ranges from 10⁻²⁷ to 10⁻²⁹ e⋅cm. Some supersymmetric models predict that |d_e| > 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.

Formal definition of the electron EDM

As the electron has a net charge, the definition of its electric dipole moment is ambiguous in that

$\mathbf {d} _{\rm {e}}=\int ({\mathbf {r} }-{\mathbf {r} }_{0})\rho ({\mathbf {r} })d^{3}{\mathbf {r} }$

depends on the point ${\mathbf {r} }_{0}$ about which the moment of the charge distribution $\rho ({\mathbf {r} })$ is taken. If we were to choose ${\mathbf {r} }_{0}$ to be the center of charge, then $\mathbf {d} _{\rm {e}}$ would be identically zero. A more interesting choice would be to take ${\mathbf {r} }_{0}$ as the electron's center of mass evaluated in the frame in which the electron is at rest.

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 $F_{i}(q^{2})$ appearing in the matrix element

$\langle p_{f}|j^{\mu }|p_{i}\rangle ={\bar {u}}(p_{f})\left\{F_{1}(q^{2})\gamma ^{\mu }+{\frac {i\sigma ^{\mu \nu }}{2m_{\rm {e}}}}q_{\nu }F_{2}(q^{2})+i\epsilon ^{\mu \nu \rho \sigma }\sigma _{\rho \sigma }q_{\nu }F_{3}(q^{2})+{\frac {1}{2m_{\rm {e}}}}\left(q^{\mu }-{\frac {q^{2}}{2m_{e}}}\gamma ^{\mu }\right)\gamma _{5}F_{4}(q^{2})\right\}u(p_{i})$

of the electromagnetic current operator between two on-shell states with Lorentz invariant phase space normalization in which

$\langle p_{f}\vert p_{i}\rangle =2E(2\pi )^{3}\delta ^{3}({\bf {p}}_{f}-{\bf {p_{i}}}).$

Here $u(p_{i})$ and ${\bar {u}}(p_{f})$ are 4-spinor solutions of the Dirac equation normalized so that ${\bar {u}}u=2m_{e}$, and $q^{\mu }=p_{f}^{\mu }-p_{i}^{\mu }$ is the momentum transfer from the current to the electron. The $q^{2}=0$ form factor $F_{1}(0)=Q$ is the electron's charge, $\mu =(F_{1}(0)+F_{2}(0))/2m_{\rm {e}}$ is its static magnetic dipole moment, and $-F_{3}(0)/2m_{\rm {e}}$ provides the formal definition of the electron's electric dipole moment. The remaining form factor $F_{4}(q^{2})$ would, if nonzero, be the anapole moment.

Experimental measurements of the electron EDM

To date, no experiment has found a non-zero electron EDM. As of 2020 the Particle Data Group publishes its value as |d_e| < 0.11×10⁻²⁸ e⋅cm. Here is a list of some electron EDM experiments after 2000 with published results:

List of Electron EDM Experiments

Year	Location	Principal Investigators	Method	Species	Experimental upper limit on |de|
2002	University of California, Berkeley	Eugene Commins, David DeMille	Atomic beam	Tl	1.6×10−27 e⋅cm
2011	Imperial College London	Edward Hinds, Ben Sauer	Molecular beam	YbF	1.1×10−27 e⋅cm
2014	Harvard-Yale (ACME I experiment)	David DeMille, John Doyle, Gerald Gabrielse	Molecular beam	ThO	8.7×10−29 e⋅cm
2017	JILA	Eric Cornell, Jun Ye	Ion trap	HfF+	1.3×10−28 e⋅cm
2018	Harvard-Yale (ACME II experiment)	David DeMille, John Doyle, Gerald Gabrielse	Molecular beam	ThO	1.1×10−29 e⋅cm

Future proposed experiments

Besides the above groups, electron EDM experiments are being pursued or proposed by the following groups:

• David Weiss (Pennsylvania State University): Cs atomic trap

• Daniel Heinzen at University of Texas at Austin: Cs atomic trap

• University of Groningen: BaF molecular beam

• John Doyle (Harvard University), Nicholas Hutzler (California Institute of Technology), and Timothy Steimle (Arizona State University): YbOH molecular trap

• Amar Vutha (University of Toronto), Eric Hessels (York University): oriented polar molecules in an inert gas matrix

See also

• Neutron electric dipole moment
• Electron magnetic moment
• Parity (physics) § Parity violation
• CP violation
• Charge conjugation
• T-symmetry
• Anomalous electric dipole moment