Electron electric dipole moment
Encyclopedia
The electron electric dipole moment
(EDM) de is an intrinsic property of an electron
such that the potential energy is linearly related to the strength of the electric field: U=de·E. Within the standard model
of elementary particle physics, such a dipole
is predicted to be non-zero but very small, at most , 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 diagram
s and higher. An additional, larger EDM (around ) is possible in the standard model if neutrinos are majorana particles.
Experimentally, the electric dipole moment is too small to measure in all experiments to date. The Particle Data Group
publishes its value as . The most recent experiment performed at Imperial College London, placed an upper bound on (with a 90% confidence level) of .
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 . 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.
Electric dipole moment
In physics, the electric dipole moment is a measure of the separation of positive and negative electrical charges in a system of charges, that is, a measure of the charge system's overall polarity with SI units of Coulomb-meter...
(EDM) de is an intrinsic property of an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
such that the potential energy is linearly related to the strength of the electric field: U=de·E. Within the standard model
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
of elementary particle physics, such a dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
is predicted to be non-zero but very small, at most , where e stands for the elementary charge
Elementary charge
The elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
. The existence of a non-zero electron electric dipole moment would imply a violation of both parity invariance
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
and time reversal invariance
T-symmetry
T Symmetry is the symmetry of physical laws under a time reversal transformation: T: t \mapsto -t.Although in restricted contexts one may find this symmetry, the observable universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics.Time asymmetries...
. In the Standard Model, the electron EDM arises from the CP-violating
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...
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
Virtual particle
In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
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 diagram
Feynman diagram
Feynman diagrams are a pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, first developed by the Nobel Prize-winning American physicist Richard Feynman, and first introduced in 1948...
s and higher. An additional, larger EDM (around ) is possible in the standard model if neutrinos are majorana particles.
Experimentally, the electric dipole moment is too small to measure in all experiments to date. The Particle Data Group
Particle Data Group
The Particle Data Group is an international collaboration of particle physicists that compiles and reanalyzes published results related to the properties of particles and fundamental interactions. It also publishes reviews of theoretical results that are phenomenologically relevant, including...
publishes its value as . The most recent experiment performed at Imperial College London, placed an upper bound on (with a 90% confidence level) of .
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 . 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.