Estimates of the subthreshold production of charm
  Charm photoproduction rates off nuclei below the nucleon threshold are estimated using the phenomenologically known structure functions both for $x<1$ and $x>1$. The rates rapidly fall from values of the order 1 nb at the threshold to values of the order 1 pb at 6 GeV for Pb target. 
M.A.Braun and B.Vlahovic