Transversity and Single Spin Asymmetries

The transverse spin of interacting light quarks is described by two independent non-perturbative inputs, twist-2 transversity and twist-3 quark-gluon correlators, while for free massive quarks they are related by the equations of motion. Similar relation emerges also in a model, where quark-hadron transition is treated in a probabilistic way. The single spin asymmerty in SIDIS appears due to the interplay of transverity with Collins fragmentation function, which appears to be of twist 3, when defined in a Lorentz invariant way.