Collins Function and the Single Transverse Spin Asymmetry

We study the Collins mechanism for the single transverse spin asymmetry in the collinear factorization approach. The corresponding twist-three fragmentation function is identified. We show that the Collins function calculated in this approach is universal. We further examine its contribution to the single transverse spin asymmetry of semi-inclusive hadron production in deep inelastic scattering and demonstrate that the transverse momentum dependent and twist-three collinear approaches are consistent in the intermediate transverse momentum region where both apply.