Hadron-Pair Photoproduction in Longitudinally Polarized Lepton-Nucleon   Collisions
  We present a detailed phenomenological study of photoproduction of two hadrons, both with high transverse momentum, in longitudinally polarized lepton-nucleon collisions. We consistently include ``direct'' and ``resolved'' photon contributions and examine the sensitivity of the relevant spin asymmetries to the gluon polarization in the nucleon and to the completely unknown parton content of circularly polarized photons. Our results are relevant for the COMPASS and HERMES fixed-target experiments as well as for a possible future polarized lepton-proton collider like eRHIC at BNL. So far, all studies are limited to the lowest order approximation of QCD. 
C. Hendlmeier, M. Stratmann, and A. Schafer