Measurements of the Cosmic Ray Composition with Air Shower Experiments

TL;DR

This review synthesizes how extensive air-shower measurements constrain the mass composition of cosmic rays from around the knee to the highest energies. It explains how longitudinal development ($X_ ext{max}$) and ground-level observables (electron and muon numbers, LDF steepness, timing, rise-time) relate to primary mass, and emphasizes the dominant role of hadronic-interaction-model uncertainties in shaping interpretations. The paper surveys techniques across surface arrays, non-imaging Cherenkov detectors, and fluorescence telescopes, highlighting consistent qualitative trends (light composition near the ankle, heavier tendencies toward higher energies) but substantial quantitative spread due to model differences. It also discusses searches for neutral primaries (photons and neutrinos) as complementary tests of origin scenarios and outlines future prospects with LHC-era data and hybrid detectors to reduce systematic uncertainties.

Abstract

In this paper we review air shower data related to the mass composition of cosmic rays above 10$^{15}$ eV. After explaining the basic relations between air shower observables and the primary mass and energy of cosmic rays, we present different approaches and results of composition studies with surface detectors. Furthermore, we discuss measurements of the longitudinal development of air showers from non-imaging Cherenkov detectors and fluorescence telescopes. The interpretation of these experimental results in terms of primary mass is highly susceptible to the theoretical uncertainties of hadronic interactions in air showers. We nevertheless attempt to calculate the logarithmic mass from the data using different hadronic interaction models and to study its energy dependence from 10$^{15}$ to 10$^{20}$ eV.

Figures (18)

• Figure 1: Air shower simulation of the shower maximum vs. calorimetric energy. Contour lines illustrate the regions which include 90 % of the showers and the inset shows a detailed view at [$10^{20}$] e V.
• Figure 2: Composition sensitivity of a combined measurement of the shower maximum and its fluctuations. Dots denote different models of the extragalactic cosmic ray composition at Earth: Pure proton composition in the dip-model Berezinsky:2002nc and mixed composition with a large Allard:2005cx or small Allard-11 maximum energy of the sources and different spectral indices $\beta$ at the source. Energies range from [$10^{18.5}$] e V to [$10^{20}$] e V with a spacing of $\Delta\lg E=0.1$. Red lines are simulations with Sibyll at the same energies for various two-component transitions. The contour defined by these transition contains all other possible mixtures for $1\le A \le 56$.
• Figure 3: Air shower simulation of the number of muons vs. electrons at ground level for a vertical shower observed at 800 g/cm$^2$. Contour lines illustrate the regions which include 90 % of the showers and the inset shows a detailed view at [$10^{20}$] e V.
• Figure 4: Sensitivity of air shower observables on characteristics of hadronic interactions as a function of a change in model characteristics growing logarithmically from zero at Tevatron energies to the quoted values on the x-axis at [$10^{19}$] e V (proton showers at [$10^{19.5}$] e V, adapted from Ulrich:2010rg).
• Figure 5: Unfolded fluxes from GAMMA Garyaka-07 and KASCADE KASCADE-05 using two different interaction models. The spectra of elemental groups have been adjusted by a common factor (within 35%) to match the all-particle spectra of the two experiments. KASCADE points were slightly displaced in energy for better visibility.