Abstract

Measurements of proton and nuclear collisions at the Large Hadron Collider at nucleon-nucleon c.m. energies up to $ \sqrt {S_{NN} } = 13\,{\rm{TeV}} $ have improved our understanding of hadronic interactions at the highest energies reached in collisions of cosmic rays with nuclei in the earth atmosphere, up to $ \sqrt {S_{NN} } $ ≈ 450 TeV. The Monte Carlo event generators (epos, qgsjet, and sibyll) commonly used to describe the air showers generated by ultrahigh-energy cosmic rays (UHECR, with ECR ≈ 1017-1020 eV) feature now, after parameter retuning based on LHC Run-I data, more consistent predictions on the nature of the cosmic rays at the tail of the measured spectrum. However, anomalies persist in the data that cannot be accommodated by the models. Among others, the total number of muons (as well as their maximum production depth) remains significantly underestimated (overestimated) by all models. Comparisons of epos, qgsjet, and sibyll predictions to the latest LHC data, and to collider MC generators such as pythia, indicate that improved description of hard multiple minijet production and nuclear effects may help reduce part of the data-model discrepancies, shed light on the UHECR composition approaching the observed ECR ≈ 1020 eV cutoff, and uncover any potential new physics responsible for the observed anomalies.

Highlights

  • Ultrahigh-energy cosmic rays (UHECR), with energies ECR ≈ 1017–1020 eV, are produced and accelerated in extreme astrophysical environments

  • Part of the muon excess observed at large radii can be solved adding harder minijet activity in the Reggeon Field Theory (RFT) models, for real UHECR collisions on air, and at variance with the results found in the proton-hydrogen setup, epos-lhc produces more μ± than qgsjet-ii-04

  • The determination of the identity of the highestenergy cosmic rays reaching earth relies heavily on Monte Carlo (MC) hadronic generators with parameters tuned to collider data

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Summary

Introduction

Ultrahigh-energy cosmic rays (UHECR), with energies ECR ≈ 1017–1020 eV, are produced and accelerated in (poorly-known) extreme astrophysical environments. More speculative explanations have been suggested based on changes in the physics of the strong interaction at energies beyond those tested at the LHC [28, 29], or on the production of electroweak sphalerons leading to the final production of many energetic muons [30] This writeup compares the RFT Monte Carlo predictions to a basic set of QCD observables measured recently in proton and nuclear collisions at the LHC (Sec. 2), and summarizes the results of a recent study [31] that, for the first time, interfaced conex with the standard p-p collider MC pythia 6 [32, 33] to assess the impact of heavy-quark and hard jet production on the EAS muon production (Sec. 3)

LHC data versus UHECR Monte Carlo generators
Muon anomalies
Findings
Summary
Full Text
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