Universality of TMD distribution functions of definite rank

Abstract: Transverse momentum dependent (TMD) distribution and fragmentation functions are described as Fourier transforms of matrix elementscontaining nonlocal combinations of quark and gluon fields. These matrix elements also contain a gauge link operator with a process dependent path, of which the process dependence that can be traced back to the color flow in the process. Expanding into irreducible tensors built from the transverse momenta $p_\st$, we can define a universal set of TMD correlators of definite rank with a well-defined operator structure.