Gluon-Exchange in Spin-Dependent Fragmentation
  The fragmentation of an unpolarized quark into a transversely polarized spin-1/2 particle is studied in the framework of a simple model. Special attention is payed to the gluon exchange which is incorporated in the gauge link of the fragmentation function, and which we model by an abelian gauge field. The transverse single spin asymmetries in $e^+ e^-$ annihilation and semi-inclusive deep-inelastic scattering are calculated in the one-loop approximation. For $e^+ e^-$ annihilation one finds a cancellation between contributions from two on-shell intermediate states, which have no counterpart in deep-inelastic scattering. As a consequence of this cancellation, the model predicts the same spin asymmetry for both processes implying that, in the one-loop approximation, the corresponding fragmentation function is universal. 
A. Metz