  A spontaneously broken SU(2) theory is the simplest generalization of the
Abelian Higgs model, containing three equally massive vector bosons and a
single Higgs scalar. A strictly diagrammatic proof is presented of the
tree-level unitarity of this model in the unitary gauge, i.e. employing only
physical fields. A new Ward-like identity is used to show that the high-energy
behaviour of all amplitudes cannot be more than quadratic; the use of
generating functions for all off-shell amplitudes then leads to the unitarity
proof.
