Gauge links, TMD-factorization, and TMD-factorization breaking
  In this section, we discuss some basic features of transverse momentum dependent, or unintegrated, parton distribution functions. In particular, when these correlation functions are combined in a factorization formulae with hard processes beyond the simplest cases, there are basic problems with universality and factorization. We discuss some of these problems as well as the opportunities that they offer. 
P.J. Mulders and T.C. Rogers
hep-ph