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 ∼65% of the total signal. The number of particles from air showers observed on the condition of muon purity is typically 1.88 ± 0.08 (stat.) ± 0.43 (syst.) times larger than the MC prediction value using the QGSJET II-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 II-04.

Past, Present and Future of UHECR Observations

“…The improved sensitivity to muons is provided by making very selective cuts that maximise the absorption of electrons by the atmosphere. Results indicate that the discrepancy increases with core distance, which may imply problems with our understanding of the early part of EAS development, since the muons at large core distances would originate there [133].…”

“…In the case of TA's plastic scintillation detectors, electrons and muons have similar detector response 26/42 and the mismatch can only be partly attributed to a muon excess. Studies are proceeding to investigate whether scintillation detectors at large distances from the core which should have mainly muon initiated signals are consistent with the Auger results [133]. In any case, it is clear that the hadronic models that are used to simulate showers are not adequate.…”

Potential for measuring the longitudinal and lateral profile of muons in TeV air showers with IACTs

“…Discrepancies between hadronic interaction models have been largely reduced in the light of LHC data [46]. Extensive checking with data from UHE CR experiments such as the Pierre Auger Observatory and Telescope Array has shown that the two experiments are mostly compatible within systematic uncertainties, with both showing a discrepancy to the latest hadronic interaction models [20,38,48].…”