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Anomalous magnetic dipole moment

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	Topics Anomalous magnetic dipole moment Topic Home In quantum electrodynamicsQuantum electrodynamics Quantum electrodynamics is a relativistic quantum field theory of electromagnetism.... , the anomalous magnetic moment of a particle is a contribution of effects of quantum mechanicsQuantum mechanics Quantum mechanics is a first quantized quantum theory that supersedes classical mechanics at the atomic and subatomic levels... , expressed by Feynman diagramFeynman diagram A Feynman diagram is a method for performing calculations in quantum field theory, invented by American physicist Richard Fe... s with loops, to the magnetic momentMagnetic moment In physics, the magnetic moment or magnetic dipole moment is a measure of the strength of a magnetic source.... of that particle. The "Dirac" magnetic momentMagnetic moment Summary In physics, the magnetic moment or magnetic dipole moment is a measure of the strength of a magnetic source.... , corresponding to tree-level Feynman diagrams, can be calculated from the Dirac equationDirac equation In physics, the Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac... . It is usually expressed in terms of the g-factorFacts About G-factor A g-factor is a dimensionless quantity which characterizes the magnetic moment and gyromagnetic ratio of a particle or ... ; the Dirac equation predicts . For particles such as the electronElectron The electron is a fundamental subatomic particle that carries an electric charge.... , this classical result differs from the observed value by a small fraction of a percent. The difference is the anomalous magnetic moment, denoted and defined as The one-loop contribution to the anomalous magnetic moment of the electron is found by calculating the vertex functionVertex function In quantum electrodynamics, the vertex function is the one particle irreducible correlation function involving ψ, and t... shown in the diagram on the right. The calculation is relatively straightforward and the one-loop result is: where is the fine structure constant. This result was first found by SchwingerJulian Schwinger Julian Seymour Schwinger was an American theoretical physicist.... in 1948. As of 1997, the coefficients of the QED formula for the anomalous magnetic moment of the electron have been calculated through order . The QED prediction agrees with the experimentally measured value to more than 10 significant figures, making the magnetic moment of the electron the most accurately verified prediction in the history of physicsPhysics Summary Physics , the most fundamental physical science, is concerned with the underlying principles of the natural world.... . (See precision tests of QEDPrecision tests of QED Quantum electrodynamics , a relativistic quantum field theory of electrodynamics, is among the most stringently tested theor... for details.) The anomalous magnetic moment of the muonMuon The muon is a fundamental particle with negative electric charge and a spin of 1/2.... is calculated in a similar way; its measurement provides a precision test of the Standard ModelStandard Model The Standard Model of particle physics is a theory which describes the strong, weak, and electromagnetic fundamental forces,... . The prediction for the value of the muon anomalous magnetic moment includes three parts: α_μ^SM = α_μ^QED + α_μ^EW + α_μ^had. The first two components represent the photon and lepton loops, and the W boson and Z boson loops, respectively, and can be calculated precisely from first principles. The third term represents hadron loops, and cannot be calculated accurately from theory alone. It is estimated from experimental measurements of the ratio of hadronic to muonic cross sections \|R]]) in e⁺e^- collisons. As of November 2006, the measurement disagrees with the Standard Model by 3.4 standard deviationStandard deviation Overview In probability and statistics, the standard deviation of a probability distribution, random variable, or population or multi... s, suggesting beyond the Standard ModelBeyond the Standard Model As of 2006, in particle physics, the predictions coming from the Standard Model—a model discovered in the early 1970s—... physics may be having an effect. Composite particles often have a huge anomalous magnetic moment. This is true for the protonProton In physics, the proton is a subatomic particle with an electric charge of one positive fundamental unit , a diameter of abo... , which is made up of charged quarkQuark In particle physics, quarks are one of the two basic constituents of matter .... s, and the neutronNeutron Summary In physics, the neutron is a subatomic particle with no net electric charge and a mass of 939.573 MeV/c .... , which has a magnetic moment even though it is electrically neutral. See also anomalous electric dipole momentAnomalous electric dipole moment Overview In particle physics, the anomalous electric dipole moment, or the electric dipole moment of a particle in short, is th... g-factorG-factor A g-factor is a dimensionless quantity which characterizes the magnetic moment and gyromagnetic ratio of a particle or ... External links