Operator Structure of TMDs

The focus of this talk is on the transverse components of parton momenta. Like for collinear parton distribution functions (PDFs), we are also in the case of transverse momentum dependent (TMD) PDFs, talking about forward matrix elements. While the collinear PDFs describe only spin-spin correlations, the TMD PDFs (or in short TMDs) include spin-momentum correlations, including also time-reversal-odd (T-odd) correlations. The latter are important in the description of single spin asymmetries. In this way TMDs open up new ways of studying the spin structure of hadrons or they can be used as tools that incorporate hadronic structure also in non-collinear situations. The operator structure of TMDs within QCD, in particular the structure of Wilson lines, is more complex than that for collinear functions leading to various ways of breaking of universality. This breaking of universality, however, can be handled within QCD.