i.e. contribution from formula_61 and formula_62 terms do not appear. We see that the mass-generating interaction is achieved by constant flipping of particle chirality. The spin-half particles have no right/left chirality pair with the same representations and equal and opposite weak hypercharges, so assuming these gauge charges are conserved in the vacuum, none of the spin-half particles could ever swap chirality, and must remain massless. Additionally, we know experimentally that the W and Z bosons are massive, but a boson mass term contains the combination e.g. , which clearly depends on the choice of gauge. Therefore, none of the standard model fermions "or" bosons can "begin" with mass, but must acquire it by some other mechanism.