An upper limit on the muon flux at energies above 100 TeV determined from horizontal air showers observed at Akeno

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Abstract
Determination of the cosmic-ray muon energy spectrum above 100 TeV by observing extensive air showers from the horizontal direction (HAS) has been carried out at Akeno for four years. No definite shower of muon origin with size above 105 has been observed at zenith angles above 60 degrees . The upper limit of HAS intensity is 5*10-12 m-2 s-1 sr-1 (90% confidence level) for sizes greater than 105. This value indicates that the upper limit of the muon flux above 100 TeV is about 1.3*10-8 m-2 s-1 sr-1, which is in agreement with that expected from the primary spectrum with a 'knee', assuming scaling in the fragmentation region and 40% protons in the primary beam. It also implies that the critical energy at which a muon flux from prompt processes (decay of charmed particles) takes over from that due to conventional process (decay of pions and kaons) is higher than 100 TeV in the horizontal direction.

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