On the chemical composition of cosmic rays of highest energy

TL;DR

This paper explores the chemical composition of ultra-high-energy cosmic rays, proposing that observed variations may be due to interaction point distributions rather than composition changes, and introduces a new observable to clarify this.

Contribution

It introduces a new observable that minimizes the influence of interaction point distribution, aiding in more accurate determination of cosmic ray composition at high energies.

Findings

The new observable reduces the impact of interaction point fluctuations.

Reconciliation of Auger and HiRes results is possible without assuming composition change.

Proposes a method to better interpret shower maximum data.

Abstract

We present arguments aiming at reconciling apparently contradictory results concerning the chemical composition of cosmic rays of highest energy, coming recently from the Auger and HiRes collaborations. In particular, we argue that the energy dependence of the mean value and root mean square fluctuation of shower maxima distributions observed by the Auger experiment are not necessarily caused by the change of nuclear composition of primary cosmic rays. They could also be caused by the change of distribution of the first interaction point in the cascade. A new observable, in which this influence is strongly suppressed, is proposed and tested.