Measurement of the Depth of Maximum of Extensive Air Showers above 10^18 eV

TL;DR

This paper reports on the measurement of the depth of maximum (Xmax) of extensive air showers caused by cosmic rays above 10^18 eV, revealing energy-dependent evolution and fluctuations that inform cosmic ray composition.

Contribution

It presents the first detailed measurement of Xmax for cosmic rays above 10^18 eV using the Pierre Auger Observatory, highlighting energy dependence and shower fluctuations.

Findings

Xmax increases with energy below 10^18.24 eV at 106 g/cm^2/decade

Xmax increases more slowly above 10^18.24 eV at 24 g/cm^2/decade

Shower-to-shower fluctuations decrease from 55 to 26 g/cm^2

Abstract

We describe the measurement of the depth of maximum, Xmax, of the longitudinal development of air showers induced by cosmic rays. Almost four thousand events above 10^18 eV observed by the fluorescence detector of the Pierre Auger Observatory in coincidence with at least one surface detector station are selected for the analysis. The average shower maximum was found to evolve with energy at a rate of (106 +35/-21) g/cm^2/decade below 10^(18.24 +/- 0.05) eV and (24 +/- 3) g/cm^2/decade above this energy. The measured shower-to-shower fluctuations decrease from about 55 to 26 g/cm^2. The interpretation of these results in terms of the cosmic ray mass composition is briefly discussed.