Abstract
One of the uncertainties in ultrahigh energy cosmic ray (UHECR) observation derives from the hadronic interaction model used for air shower Monte-Carlo (MC) simulations. One may test the hadronic interaction models by comparing the measured number of muons observed at the ground from UHECR induced air showers 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 UHECRs 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. 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 proton-induced showers. The same feature was also obtained for other hadronic interaction models, such as QGSJET II-04.
Highlights
The Telescope Array (TA) experiment [1] observes ultrahigh energy cosmic rays (UHECRs) in Utah, USA
The TA surface detectors (SDs) event reconstruction consists of the following steps [10]: First, air shower SD signals are selected by determining a cluster which is contiguous in space and time
The data becomes closer to the Monte Carlo (MC) at R ≳ 4000 m, since the atmospheric muon background dominates the SD signals at the distance
Summary
The Telescope Array (TA) experiment [1] observes ultrahigh energy cosmic rays (UHECRs) in Utah, USA. The Auger group reported that the observed hadronic signal in UHECR air showers is 1.61 ± 0.21 (1.33 ± 0.16) times larger than the post-LHC MC prediction values for QGSJET II-04 [5] (EPOS-LHC [6]), including statistical and systematic errors [7]. These reports suggest that present hadronic interaction models do not fully reproduce air showers.
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