The color structure of gluon emission

The color structure of an arbitrary QCD diagram is studied. A basis for the color form factors of a generic amplitude is suggested, in which all the coherence effects are automatically and simply accounted for to the leading order in Nc. Some applications of this formalism are given for tree-level amplitudes describing processes with one or two pairs of fermions and radiation of gluons and photons. The different coherence properties of the abelian and non-abelian radiation are exhibited with some analytic and explicit examples, leading to the appearance of string-effect-like correlations for hard non-abelian radiation. (orig.)