Anomalous magnetic dipole moment

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In quantum electrodynamics, anomalous magnetic moment of a particle is a contribution of effects of quantum mechanics, expressed by Feynman diagrams with loops, to the magnetic moment of that particle. In the same fashion, an anomaly in physics is a quantum, loop diagram violating a classical symmetry.

One-loop correction

The "Dirac" magnetic moment, corresponding to tree Feynman diagrams, may be calculated from the Dirac equation. For particles such as electrons, this classical result differs from the observed value by a small fraction of a percent. The loop corrections add the anomalous moment and the total prediction of quantum electrodynamics, one that contains both the classical as well as the anomalous contributions, agrees with the experimental value to more than 10 decimal points. The magnetic moment of the electron is the most accurately verified prediction in the history of physics. The anomalous magnetic moment of the muon is measured in precision tests of the Standard Model.

Composite particles often have a huge anomalous magnetic moment. This is true for the proton, which is made up of charged quarks and the neutron, which has a magnetic moment even though it is electrically neutral.

Anomalous magnetic dipole moment

See also