Violation of Universality in Spin-Dependent Fragmentation

The time-reversal odd fragmentation of an unpolarized quark into a transversely polarized spin-1/2 particle is studied in the framework of a simple model which, in particular, describes the gluon exchange incorporated in the gauge link of the fragmentation function. 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 that the spin asymmetries for the two processes have the same magnitude but the opposite sign. Keeping in mind that time-reversal odd fragmentation functions can be non-zero even if their gauge link is neglected, our result implies a violation of universality which goes beyond a sign change.