Testing Hadronic Interactions at Ultrahigh Energies with Air Showers Measured by the Pierre Auger Observatory


Ultra-high energy cosmic-ray air showers probe particle physics at energies beyond the reach of accelerators. We introduce a new method to test hadronic interaction models without relying on the absolute energy calibration, and apply it to events with primary energy 6-16 EeV, whose longitudinal development and lateral distribution were simultaneously measured by the Pierre Auger Observatory. The average hadronic shower is 1.33 ± 0.16 (1.61 ± 0.21) times larger than predicted using the leading LHC-tuned models EPOS-LHC (QGSJet II-04), with a corresponding excess of muons.

Rescaling of hadronic component (Rhad) and energy calibration rescaling (RE) that maximize the ground signal measured by the Pierre Auger Observatory are plotted for model QGSJet II-04 (QII-04) and EPOS-LHC. Mixed composition of primaries with the best description of distributions of longitudinal shower profiles and single protons are considered in the plot. The ellipses and gray boxes show the 1-σ statistical and systematic uncertainties, respectively.