Consistency conditions for gauge theory S matrices from requirements of generalized unitarity.

In the modern on-shell approach, the perturbative S matrix is constructed iteratively using on-shell building blocks with manifest unitarity. As only gauge invariant quantities enter in the intermediate steps, the notion of a gauge anomaly is absent. In this Letter, we rephrase the anomaly cancellation conditions in a purely on-shell language. We demonstrate that while the unitarity methods automatically lead to a unitary S matrix, the rational terms that are required to enforce locality, invariably give rise to inconsistent factorization channels in chiral theories. In four dimensions, the absence of such inconsistencies implies the vanishing of the symmetric trace of three generators. In six dimensions, if the symmetric trace of four generators does not vanish, the rational term develops a factorization channel revealing a new particle in the spectrum: the 2-form of the Green-Schwarz mechanism. Thus, in the purely on-shell construction, the notion of a gauge anomaly is replaced by the difficulty to consistently impose locality on the unitary S matrix.