= Slavnov-Taylor identity
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A <Slavnov-Taylor identity> is a <BRST symmetry> constraint relating <correlation functions> or <interaction vertices> in a gauge-fixed <Non-Abelian gauge theory>. Unlike a simple statement that every open gauge index is transverse, the general identities include <Faddeev-Popov ghost fields>. After putting external legs <on shell> and making all the other external states physical, they imply decoupling when one massless vector <polarization vector> is replaced by its <four-momentum>. They also control cancellations of auxiliary covariant components needed for physical <unitarity>.
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