Density of GeV muons in air showers measured with IceTop

TL;DR

This study measures GeV muon densities in near-vertical air showers at the South Pole using IceTop data, comparing results with various hadronic interaction models to evaluate their accuracy across different energy ranges.

Contribution

First measurement of muon densities at these energies and distances, providing critical data to test and refine hadronic interaction models in cosmic ray physics.

Findings

Measured muon densities are consistent with baseline models within uncertainties.

Post-LHC models predict higher muon densities than observed between 2.5 PeV and 100 PeV.

Models overestimate muon densities at energies below 1 EeV, contrasting with higher energy observations.

Abstract

We present a measurement of the density of GeV muons in near-vertical air showers using three years of data recorded by the IceTop array at the South Pole. Depending on the shower size, the muon densities have been measured at lateral distances between 200 m and 1000 m. From these lateral distributions, we derive the muon densities as functions of energy at reference distances of 600 m and 800 m for primary energies between 2.5 PeV and 40 PeV and between 9 PeV and 120 PeV, respectively. The muon densities are determined using, as a baseline, the hadronic interaction model Sibyll 2.1 together with various composition models. The measurements are consistent with the predicted muon densities within these baseline interaction and composition models. The measured muon densities have also been compared to simulations using the post-LHC models EPOS-LHC and QGSJet-II.04. The result of this comparison is that the post-LHC models together with any given composition model yield higher muon densities than observed. This is in contrast to the observations above 1 EeV where all model simulations yield for any mass composition lower muon densities than the measured ones. The post-LHC models in general feature higher muon densities so that the agreement with experimental data at the highest energies is improved but the muon densities are not correct in the energy range between 2.5 PeV and about 100 PeV.