Universality of Single Spin Asymmetries in Hard Processes
  We discuss the use of time reversal symmetry in the classification of parton correlators. Specifically, we consider the role of (small) intrinsic transverse momenta in these correlators and the determination of the proper color gauge link. The transverse momentum weighted correlators in hard processes can be expressed as a product of universal gluonic pole matrix elements and gluonic pole cross sections. 
P. J. Mulders, C. J. Bomhof, F. Pijlman