A g-factor (also called g value or dimensionless magnetic moment) is a dimensionless quantity which characterizes the magnetic moment and gyromagnetic ratio of a particle or nucleus. It is essentially a proportionality constant that relates the observed magnetic moment μ of a particle to the appropriate angular momentum quantum number and the appropriate fundamental quantum unit of magnetism, usually the Bohr magneton or nuclear magneton.
There are three magnetic moments associated with an electron: One from its spin angular momentum, one from its orbital angular momentum, and one from its total angular momentum (the quantum-mechanical sum of those two components). Corresponding to these three moments are three different g-factors:
Electron spin g-factor
The most famous of these is the electron spin g-factor (more often called simply the electron g-factor), ge, defined by
where μS is the total magnetic moment resulting from the spin of an electron, S is its spin angular momentum, and μB is the Bohr magneton. In atomic physics, the electron spin g-factor is often defined as the absolute value or negative of ge:
The z-component of the magnetic moment then becomes
The value gS is roughly equal to 2.002319, and is known to extraordinary precision. The reason it is not precisely two is explained by quantum electrodynamics calculation of the anomalous magnetic dipole moment.
Electron orbital g-factor
Secondly, the electron orbital g-factor, gL, is defined by
where μL is the total magnetic moment resulting from the orbital angular momentum of an electron, L is the magnitude of its orbital angular momentum, and μB is the Bohr magneton. The value of gL is exactly equal to one, by a quantum-mechanical argument analogous to the derivation of the classical magnetogyric ratio. For an electron in an orbital with a magnetic quantum number ml, the z-component of the orbital angular momentum is
which, since gL = 1, is just μBml
Total angular momentum (Landé) g-factor
Thirdly, the Landé g-factor, gJ, is defined by
where μ is the total magnetic moment resulting from both spin and orbital angular momentum of an electron, J = L+S is its total angular momentum, and μB is the Bohr magneton. The value of gJ is related to gL and gS by a quantum-mechanical argument; see the article Landé g-factor.
Nucleon and nucleus g-factors
Protons, neutrons, and many nuclei have spin and magnetic moments, and therefore associated g-factors. The formula conventionally used is
where μ is the magnetic moment resulting from the nuclear spin, I is the nuclear spin angular momentum, μN is the nuclear magneton and g is the effective g-factor.
The muon, like the electron has a g-factor from its spin, given by the equation
where μ is the magnetic moment resulting from the muon’s spin, S is the spin angular momentum, and mμ is the muon mass.
The fact that the muon g-factor is not quite the same as the electron g-factor is mostly explained by quantum electrodynamics and its calculation of the anomalous magnetic dipole moment. Almost all of the small difference between the two values (99.96% of it) is due to a well-understood lack of a heavy-particle diagrams contributing to the probability for emission of a photon representing the magnetic dipole field, which are present for muons, but not electrons, in QED theory. These are entirely a result of the mass difference between the particles.
However, not all of the difference between the g-factors for electrons and muons are exactly explained by the Standard Model. The muon g-factor can, in theory, be affected by physics beyond the Standard Model, so it has been measured very precisely, in particular at the Brookhaven National Laboratory. In the E821 collaboration final report in November 2006, the experimental measured value is Template:Val, compared to the theoretical prediction of Template:Val. This is a difference of 3.4 standard deviations, suggesting beyond-the-Standard-Model physics may be having an effect. The Brookhaven muon storage ring has been transported to Fermilab where the g−2 experiment will use it to make more precise measurements of muon g-factor.
Measured g-factor values
The electron g-factor is one of the most precisely measured values in physics, with a relative standard uncertainty of 2.6 x 10−13.