On the analysis of lepton scattering on longitudinally or transversely   polarized protons
  We discuss polarized lepton-proton scattering with special emphasis on the difference between target polarization defined relative to the lepton beam or to the virtual photon direction. In particular, this difference influences azimuthal distributions in the final state. We provide a general framework of analysis and apply it to the specific cases of semi-inclusive deep inelastic scattering, of exclusive meson production, and of deeply virtual Compton scattering. 
M. Diehl and S. Sapeta