Report of the Working Group on the Composition of Ultra High Energy Cosmic Rays

TL;DR

This paper compares the average depth of shower maximum ($X_{ m max}$) measurements from Pierre Auger and Telescope Array observatories, finding consistent results within uncertainties and enabling direct comparison of cosmic ray composition data.

Contribution

It presents the first direct comparison of $X_{ m max}$ distributions from two major cosmic ray observatories using a unified simulation and analysis approach.

Findings

TA-MD $X_{ m max}$ is consistent with a simulated Auger dataset.

The average difference between the two datasets is 2.9 g/cm² with large uncertainties.

The analysis enables direct comparison of cosmic ray composition measurements.

Abstract

For the first time a proper comparison of the average depth of shower maximum ($X_{\rm max}$) published by the Pierre Auger and Telescope Array Observatories is presented. The $X_{\rm max}$ distributions measured by the Pierre Auger Observatory were fit using simulated events initiated by four primaries (proton, helium, nitrogen and iron). The primary abundances which best describe the Auger data were simulated through the Telescope Array (TA) Middle Drum (MD) fluorescence and surface detector array. The simulated events were analyzed by the TA Collaboration using the same procedure as applied to their data. The result is a simulated version of the Auger data as it would be observed by TA. This analysis allows a direct comparison of the evolution of $\langle X_{\rm max} \rangle$ with energy of both data sets. The $\langle X_{\rm max} \rangle$ measured by TA-MD is consistent with a preliminary simulation of the Auger data through the TA detector and the average difference between the two data sets was found to be $(2.9 \pm 2.7\;(\text{stat.}) \pm 18\;(\text{syst.}))~\text{g/cm}^2$.