Factorization and universality in spin-dependent SIDIS

The QCD factorization in SIDIS is considered in close analogy to an analysis of the Drell-Yan process. The special role of the (weighted) average over produced hadron transverse momenta is stressed. The case of single spin asymmetry (SSA) due to the Collins-type fragmentation function is analyzed, and its twist-3 nature is uncovered. The analysis of the sources of imaginary phases and respective cuts in hadronic kinematic variables leads to the effective character (nonuniversality) of T-odd distribution functions, in contrast to the universality of T-odd fragmentation functions.