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

Abstract

The origin of ultra high energy cosmic rays (UHECR) is a long-standing mystery. The aim of the Telescope Array (TA) experiment is to reveal this by observing the spectrum, anisotropy, and mass composition by utilizing an array of surface detectors (SD) and fluorescence detectors. 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 the ground from UHECR induced air showers is expected to depend upon the composition of primary cosmic rays. The MC prediction depends also on hadronic interaction models. One may test the hadronic interaction models by comparing the measured number of muons with the MC prediction. We studied muon densities in the UHE extensive air showers by analyzing the signal of surface detector 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. The condition implies that the muons dominate the signal, i.e. contribute about $\sim$65% of the total signal. The number of particles from air showers observed on the condition of muon purity is typically 1.88 $\pm$ 0.08 (stat.) $\pm$ 0.43 (syst.) times larger than the MC prediction value using the QGSJET I$\hspace{-.1em}$I-03 model for protons. The lateral distribution of the muons also falls more slowly in the data than the MC, causing larger discrepancy at larger lateral distance. The same feature was also obtained for other hadronic models, such as QGSJET I$\hspace{-.1em}$I-04.