Geometry of gauge links in operator definitions of transverse-momentum dependent PDFs: A comparative view

Different approaches to the problem of a gauge-invariant operator definition of transverse-momentum dependent parton densities (TMDs) are reviewed and compared from the point of view of their compatibility with the operator definition of collinear (integrated) parton distribution functions. In particular, geometry of the longitudinal and transverse gauge links (Wilson lines) both in unsubtracted quark TMDs and in the associate soft factors is addressed. A possible connection between different operator definitions is also discussed.