Partonic Pole Matrix Elements for Fragmentation

A model-independent analysis of collinear three-parton correlation functions for fragmentation is performed. By investigating their support properties it is shown, in particular, that the so-called partonic pole matrix elements vanish. This sheds new light on the understanding of transverse single spin asymmetries in various hard semi-inclusive reactions. Moreover, it gives additional strong evidence for the universality of transverse-momentum-dependent fragmentation functions.