Semi-inclusive deep inelastic scattering at small transverse momentum
  We study the cross section for one-particle inclusive deep inelastic scattering off the nucleon for low transverse momentum of the detected hadron. We decompose the cross section in terms of structure functions and calculate them at tree level in terms of transverse-momentum-dependent parton distribution and fragmentation functions. Our results are complete in the one-photon exchange approximation at leading and first subleading twist accuracy, with both beam and target polarization. 
Alessandro Bacchetta (DESY), Markus Diehl (DESY), Klaus Goeke (Ruhr-U.   Bochum), Andreas Metz (Ruhr-U. Bochum), Piet J. Mul...