Notice that the addition of fermion mass terms into the electroweak Lagrangian is forbidden, since terms of the form m ψ ψ ¯ ¯ ψ ψ do not respect U(1) × SU(2) L gauge invariance. Neither is it possible to add explicit mass terms for the U(1) and SU(2) gauge fields. The Higgs mechanism is responsible for the generation of the gauge boson masses, and the fermion masses result from Yukawa-type interactions with the Higgs field.