where S = ℏ ℏ σ σ / 2 is the spin of the particle. The factor 2 in front of the spin is known as the Dirac g -factor. The term in B , is of the form − − μ μ ⋅ ⋅ B which is the usual interaction between a magnetic moment μ μ and a magnetic field, like in the Zeeman effect.