A photon-proton marriage
  The shapes of invariant differential cross section for charged hadron production as function of hadron's transverse momentum and rapidity in ep collisions at HERA machine are considered. The particle spectra shapes observed in pp and gamma-gamma collisions before have shown very different properties. This difference could be directly measured in the "mixed" type collisions of photon and proton at HERA experiments. 
A. A. Bylinkin, A. A. Rostovtsev (Institute for Theoretical and   Experimental Physics, ITEP, Moscow, Russia)
hep-ph hep-ex