Production cross-section of very narrow muon bundle

Ahnost 18 years ago, the big cloud chamber located at Mt. Norikura Cosmic-Ray Laboratory (2770 m) captured the very beautiful muon pencil beam event shown in fig. 1 (1). 5 muons (more correctly, 5 charged particles) have passed parallel to one another through the 21 layers of 1 cm thick lead plates. They were bunched withiu a 10 cm diameter cylinder. The average number of ,normal muons (or hadrons) in this zone is only 3 muons (or hadrons) per m 2. The parallelness indicates that each muon (charged particle) energy was highex than 50 GeV. The dimensions of the cloud chamber (fig. 2) were 200 • 130 • cm a. Why is this event strange? Can the event be caused by a simple statistical fluctuat ion of normal muons (hadrons)? Since the average density of the nluon ill tile area is 3 muons/m 2, the probabili ty that five mu ns (hadrons) pass within a 80 cm 2 area is only 3.0.10 -7, which is negligibly small. For the hadrons, we must further multiply the punch-through possibility of 5 hadrons, which leads to (3.10-7) 9 (2.10 -5) = 6.10 -~2. Are they the simple decay products from pions or k'mns? The decay length of pions and kaons increases with the incident momentum. The 10 cm restriction indicates a strong limitation for the transverse momentum of the produced hadrons. The production height H of a 50GeV muon with t ) T = 200MeV is 12.5m. H inc rease s to 25m for a 100 GeV muon and 10 km (the standard ninon production height) for a 40 TeV muon. (If 1)T = 400 MeV, a 20 TeV muon is produced 2.5 km above the chamber.) Thus, as the muon momentum increases, the production height increases. Due to tho increase of pion lifetime with energy, only a decay with a PT of a few MeV is allowed (fig. 3). In other words, it is impossible to make 5 muons from =/K decay with normal I~T ( --~ 200 MeV). At present, we do not yet have a concrete knowledge of hadron production with such a low t)T, corresponding to aa electromagnetic interaction (magnetic monopole),