Cosmic-ray muon spectrum up to 20 TeV at 89° zenith angle
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Abstract
The 800-ton cosmic-ray spectrograph (MUTRON) has been used to measure the sea-level energy spectrum of cosmic-ray muons arriving from 86\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{} 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}}^{\ensuremath{-}2}$${\mathrm{s}}^{\ensuremath{-}1}$${\mathrm{sr}}^{\ensuremath{-}1}$${\mathrm{GeV}}^{\ensuremath{-}1}$)${E}^{\ensuremath{-}2.70}\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.