G-factor (physics)

A g-factor (also called g value) is a dimensionless quantity that characterizes the magnetic moment and angular momentum of a whole atom, a particle, or a nucleus. It is the ratio of the magnetic moment (or, equivalently, the gyromagnetic ratio) of a particle to that expected of a classical particle of the same charge and angular momentum. In nuclear physics, the nuclear magneton replaces the classically expected magnetic moment (or gyromagnetic ratio) in the definition. The two definitions coincide for the proton.

Because the g-factor can be measured very precisely, and also calculated very precisely from theoretical models, small discrepancies in particles' measured and predicted g-factors are used as tests for theories in particle physics, in particular the Standard Model.