Gauge-invariance in hadronic scattering processes
  High energy hadronic scattering processes can be described in terms of partonic scattering processes and parton distribution and fragmentation functions. These are bilocal matrix elements of quark operators. Colour gauge-invariance requires inclusion of a gauge link, which is characterized by its path. For transverse momentum dependent distribution and fragmentation functions, appearing in single spin asymmetries, different paths appear depending on the process. The difference is related to a time-reversal odd gluonic pole matrix element. We discuss briefly the path structure of the gauge link in more complicated processes. 
C. J. Bomhof