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 ${\mathrm{cm}}^{-2\mathrm{}}$ ${\mathrm{s}}^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$ ${\mathrm{GeV}}^{-1\mathrm{}}$)$E^{-2.70\mathrm{}}\mathrm{dE}$ ($E$ 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.