Charm Production by Cosmic Muons
  Narrow muon bundles in underground detectors permit to study muoproduction reactions that take place in the surrounding rock. We analyze the relevance of a QED+QCD reaction, muoproduction of "open charm". The contribution to double muon events is estimated to be 4-8 % of the one due to QED "trident" process, for an ideal detector located under a rock depth of 3 km water equivalent, and an observation threshold of 1 GeV. 
Francesco Vissani (Gran Sasso)