Depth of Maximum of Air-Shower Profiles at the Pierre Auger Observatory: Composition Implications

TL;DR

This study analyzes the depth of maximum of air-shower profiles at the Pierre Auger Observatory to infer cosmic ray composition, revealing a preference for intermediate mass nuclei and highlighting model discrepancies.

Contribution

It provides the first comprehensive comparison of Xmax distributions with multiple hadronic interaction models across a broad energy range.

Findings

Data are inconsistent with pure proton or iron compositions.

Intermediate nuclei improve fit quality across models.

Proton fraction increases with energy, no significant iron contribution.

Abstract

Using the data taken at the Pierre Auger Observatory between December 2004 and December 2012, we have examined the implications of the distributions of depths of atmospheric shower maximum (Xmax), using a hybrid technique, for composition and hadronic interaction models. We do this by fitting the distributions with predictions from a variety of hadronic interaction models for variations in the composition of the primary cosmic rays and examining the quality of the fit. Regardless of what interaction model is assumed, we find that our data are not well described by a mix of protons and iron nuclei over most of the energy range. Acceptable fits can be obtained when intermediate masses are included, and when this is done consistent results for the proton and iron-nuclei contributions can be found using the available models. We observe a strong energy dependence of the resulting proton fractions, and find no support from any of the models for a significant contribution from iron nuclei. However, we also observe a significant disagreement between the models with respect to the relative contributions of the intermediate components.