where again the gauge field A only enters via the gauge covariant derivative operator D μ (i.e., it is only indirectly visible). The quantities γ μ are the Dirac matrices, and G ψ is the already-mentioned Yukawa coupling parameter for ψ. Now the mass-generation follows the same principle as above, namely from the existence of a finite expectation value | ⟨ ⟨ ϕ ϕ ⟩ ⟩ | . Again, this is crucial for the existence of the property mass.