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The origin of ultra-high energy cosmic rays (UHECRs) has been a long-standing mystery. One of the uncertainties in UHECR observation derives from the hadronic interaction model used for air shower Monte-Carlo (MC) simulations. The number of muons observed at ground level from UHECR induced air showers is expected to depend upon the composition of primary cosmic rays. The MC prediction also depends on hadronic interaction models. One may test the hadronic interaction models by comparing the measured number of muons with the MC prediction. The Telescope Array (TA) is the largest experiment in the northern hemisphere observing UHECR in Utah, USA. It aims to reveal the origin of UHECR by studying the energy spectrum, mass composition and anisotropy of cosmic rays by utilizing an array of surface detectors (SDs) and fluorescence detectors. We studied muon densities in the UHE extensive air showers by analyzing the signal of TA SD stations for highly inclined showers which should have high muon purity. A high muon purity condition is imposed that requires the geometry of the shower and relative position of the given station and implies that muons dominate the signal. On condition that the muons contribute about 65% of the total signal, the number of particles from air showers is typically 1.88 ± 0.08(stat:) ± 0.42(syst:) times larger than the MC prediction with the QGSJET II-03 model for protons. The same feature was also obtained for other hadronic models, such as QGSJET II-04.

Study of muons in extensive air showers from ultra-high energy cosmic rays measured with the Telescope Array experiment