Universality and Gluonic Pole Matrix Elements

We investigate the spectral properties of quark-gluon-quark and quark-quark-gluon correlators in order to study gluonic pole matrix elements. Such matrix elements appear both for distribution functions such as the Sivers function, and fragmentation functions such as the Collins function. We find that for a large class of spectator models, the contribution of the gluonic pole matrix element for fragmentation functions vanishes. This result is important in the study of universality for fragmentation functions.