Spin-dependent Fragmentation Functions

A confusing variety of new and exotic fragmentation functions (FFs) is discussed recently, in particular in the context of the extraction of the third, leading twist – hitherto unknown – nucleon distribution function. This transversity distribution h1(x) (frequently also denoted δq(x) or ∆Tq(x)) involves a flip of quark chirality, and therefore, requires a second chiral-odd function to form a chiral-even observable. There is a number of FFs describing specific physical situations which offer themselves as possible chiral-odd partners for h1(x). These FFs are not only indispensable tools for the extraction of the transversity distribution, but of interest in themselves, since they bear witness to the process of hadronisation, or in other words, how confinement comes about. This holds true also for the chiral-even FFs. The present contribution is intended to serve as a reminder that there is a systematic and unambiguous way to classify the FFs according to the physical situations they describe, and to exclude those not in accordance with general constraints from hermiticity of Dirac fields and their well-known behaviour under the parity transformation. In this contribution I will restrict myself to the consideration of jets initiated by quarks.