Study of muons from ultrahigh energy cosmic ray air showers measured with the Telescope Array experiment

Abstract

One of the uncertainties in interpretation of ultra-high energy cosmic ray\n(UHECR) data comes from the hadronic interaction models used for air shower\nMonte Carlo (MC) simulations. The number of muons observed at the ground from\nUHECR-induced air showers is expected to depend upon the hadronic interaction\nmodel. One may therefore test the hadronic interaction models by comparing the\nmeasured number of muons with the MC prediction. In this paper, we present the\nresults of studies of muon densities in UHE extensive air showers obtained by\nanalyzing the signal of surface detector stations which should have high\n$\\it{muon \\, purity}$. The muon purity of a station will depend on both the\ninclination of the shower and the relative position of the station. In 7 years'\ndata from the Telescope Array experiment, we find that the number of particles\nobserved for signals with an expected muon purity of $\\sim$65% at a lateral\ndistance of 2000 m from the shower core is $1.72 \\pm 0.10{\\rm (stat.)} \\pm 0.37\n{\\rm (syst.)}$ times larger than the MC prediction value using the QGSJET II-03\nmodel for proton-induced showers. A similar effect is also seen in comparisons\nwith other hadronic models such as QGSJET II-04, which shows a $1.67 \\pm 0.10\n\\pm 0.36$ excess. We also studied the dependence of these excesses on lateral\ndistances and found a slower decrease of the lateral distribution of muons in\nthe data as compared to the MC, causing larger discrepancy at larger lateral\ndistances.\n