Theory of transverse-momentum parton densities: solving the puzzle of   divergences
  The current status of the theoretical understanding of the transverse-momentum dependent parton densities (TMDs) is discussed. Special attention is payed to the difference between the operator definitions of TMDs proposed so far, the treatment of specific divergences, the geometry of the gauge links, and the role of the soft factors. 
I.O. Cherednikov
hep-ph