Transverse parton distribution functions at next-to-next-to-leading   order: the quark-to-quark case
  We present a calculation of the perturbative quark-to-quark transverse parton distribution function at next-to-next-to-leading order based on a gauge invariant operator definition. We demonstrate for the first time that such a definition works beyond the first non-trivial order. We extract from our calculation the coefficient functions relevant for a next-to-next-to-next-to-leading logarithmic $Q_T$ resummation in a large class of processes at hadron colliders. 
Thomas Gehrmann, Thomas L\"ubbert and Li Lin Yang
hep-ph