  The gauge-links connecting the parton field operators in the hadronic matrix
elements appearing in the transverse momentum dependent distribution functions
give rise to T-odd effects. Due to the process-dependence of the gauge-links
the T-odd distribution functions appear with different pre-factors. A
consequence is that in the description of single spin asymmetries the parton
distribution and fragmentation functions are convoluted with gluonic pole cross
sections rather than the basic partonic cross sections. In this paper we
calculate the gluonic pole cross sections encountered in single spin
asymmetries in hadron-hadron scattering. The case of back-to-back pion
production in polarized proton-proton scattering is worked out explicitly. It
is shown how T-odd gluon distribution functions originating from gluonic pole
matrix elements appear in twofold.
