Equivalence theorem redux

The equivalence theorem states that amplitudes involving longitudinal vector bosons are equal to those with the corresponding unphysical scalars in the limit MW²s→0. There are two ways to approach this limit, depending on whether MWMH→0 or MH²s→0. We show that the theorem has a different physical interpretation in each limit, but its validity in both depends only on the wave-function renormalization of the unphysical Goldstone bosons. We derive a condition that the renormalization parameters must satisfy in order for the theorem to hold. We show that this condition is satisfied in the first limit, appropriate to the heavy-Higgs-boson regime, if momentum subtraction at a scale mMH is used. With this prescription, the theorem is true to lowest nonzero order in g and to all orders in the Higgs-boson coupling.