Cosmic-ray muon spectrum up to 20 TeV at 89° zenith angle

The 800-ton cosmic-ray spectrograph (MUTRON) has been used to measure the sea-level energy spectrum of cosmic-ray muons arriving from 86^∘{} to 90^∘{} zenith angles in the momentum region of 100-20 000 GeV/c. The measured muon energy spectrum can be interpreted by using a cosmic-ray primary spectrum of (1.80 cm^-2{s}^{{-}1}sr^-1{GeV}^{{-}1}$)${E}^{{-}2.70}dE$ ($E$ in GeV) and a scaling model incorporating an increasing interaction cross section for meson production in hadron-hadron interaction. The muon charge ratio at energies up to 15 TeV in the same zenith-angle range has been measured. It shows a small enhancement with increasing energy. By combining both results we may conclude that the cosmic-ray primary particle composition stays the same up to about 100 TeV as that obtained by direct measurements in the energy range below 1 TeV.