Charm muoproduction by cosmic rays

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% 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. In recent year, there has been a certain experimental [1, 2] and theoretical interest [3, 4, 5, 6] around the topic of “narrow muon bundles” (multiple muons with a lateral separation less than a few meters) in underground detectors. These events have been observed as a peak at small lateral separation, and interpreted as a flux induced by the energetic muons produced in the atmosphere, which propagate underground. Thence, an analysis of these events requires to consider high energy muoproduction processes, in the rock surrounding the detector. Up to now, the process considered was the muon “trident” reaction [7] μZ → μμμZ, where a muon pair is formed in the field of the nucleus. For muons propagating in high Z materials an amplification factor Z/A (= 5.5 for standard rock, A=22 and Z=11) is present, due to coherent character of the reaction. This interpretation has been pursued since the first evidences obtained in cosmic ray experiments [9]. The trident reaction leads mostly to narrow bundles of three or two muons in an underground detector (one produced muon may stop before reaching the detector); a reference ratio in an ideal detector is of 3 double muons per triple muon, for a threshold of observation Eh = 1 GeV, and a depth h = 3 km w.e. of standard rock. Recent studies [5, 6], however, suggest that existing interpretations are insufficient to quantitatively account for the whole “narrow muon” data set. It is assumed that an effective rejection of photoproduced pions and photons can be achieved.