Charge Asymmetries in charm photoproduction
  Charm quarks are expected to be produced mainly by the photon-gluon fusion mechanism in photon-Nucleon interactions. However, a small part of the total charm cross section originates in similar processes to those appearing in charm hadroproduction through the resolved (hadronic) component of the photon. Although the contribution of the resolved part of the photon is small at fixed target energies, it can help to understand the small but sizeable charge asymmetries measured in charm hadron photoproduction experiments. 
E. Cuautle, G. Herrera, J. Magnin and A. Sanchez-Hernandez