Magnetic dipole moments of elementary particles

An elementary particle, besides its orbital angular momentum, may also have internal angular momentum, or spin, cf. p. 25. In Chapter 3, the Dirac theory led to a relation between the spin angular momentum s of the electron and its dipole magnetic moment Afspm,ei (eq. (3.62), p. 122)  

The relation is quite a surprise, because the gyromagnetic factor is twice as large as that appearing in the relation between the electron orbital angular momentum L and the associated magnetic dipole moment 

Quantum electrodynamics explains this effect qualitatively - predicting a factor very close to the experimental value 2.0023193043737, known with the breathtaking accuracy of 0.0000000000082. 

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Both the orbital and the effective spinning motions of the electron have associated angular moments quantized in units of ii = 1.055 x 10-34 Js. It is an elementary exercise in physics to show that the relationship between the magnetic dipole moment /< and the angular momentum L for a moving particle of mass m and charge Q is... 

An elementary particle has a magnetic dipole moment M proportional to its spin angular momentum I, i.e. M = yl, where y stands for what is called the gyromagnetic factor (characteristic for the kind of particle). 

The microscopic magnetic dipole moment, m, is expressed in ampere square meters (A.m ), or in J.T , and characterizes the magnetic properties of microscopic entities such as elementary particles (e.g., neutron, proton, electron), nuchdes, atoms, or molecules having a spin angular momentum value different from zero. 

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