Model independent analysis of gluonic pole matrix elements and universality of transverse-momentum-dependent fragmentation functions

Gluonic pole matrix elements explain the appearance of single spin asymmetries (SSA) in high-energy scattering processes. They involve a combination of operators which are odd under time reversal (T-odd). Such matrix elements appear in principle both for parton distribution functions and parton fragmentation functions. We show that for parton fragmentation functions, these gluonic pole matrix elements vanish as a consequence of the analytic structure of scattering amplitudes in quantum chromodynamics. This result is important in the study of the universality of transverse-momentum-dependent (TMD) fragmentation functions.