Universality of TMD distribution functions of definite rank

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 $p_{\text{st}}$, we can define a universal set of TMD correlators of definite rank with a well-defined operator structure.