UHECR: Signatures and Models

TL;DR

This paper reviews signatures and models of ultra-high-energy cosmic rays, comparing observational data from different detectors with theoretical predictions, and discusses implications for cosmic ray origins and composition.

Contribution

It provides a comprehensive analysis of spectral features, composition measurements, and models, highlighting discrepancies and constraints from recent experimental data.

Findings

HiRes and TA data support proton-dominated composition at 1-3 EeV.

Auger data indicate a transition to heavier nuclei above 4 EeV.

Ankle models with E_a > 3 EeV are disfavored by anisotropy and elongation data.

Abstract

The signatures of Ultra High Energy (E >1 EeV) proton propagation through CMB radiation are pair-production dip and GZK cutoff. The visible characteristics of these two spectral features are ankle, which is intrinsic part of the dip, beginning of GZK cutoff in the differential spectrum and E_{1/2} in integral spectrum. Measured by HiRes and Telescope Array (TA) these characteristics agree with theoretical predictions. However, directly measured mass composition remains a puzzle. While HiRes and TA detectors observe the proton dominated mass composition, the data of Auger detector strongly evidence for nuclei mass composition becoming progressively heavier at energy higher than 4 EeV and reaching Iron at energy about 35 EeV. The models based on the Auger and HiRes/TA data are considered independently and classified using the transition from galactic to extragalactic cosmic rays. The ankle cannot provide this transition. since data of all three detector at energy (1 - 3) EeV agree with pure proton composition (or at least not heavier than Helium). If produced in Galaxy these particles result in too high anisotropy. This argument excludes or strongly disfavours all ankle models with ankle energy E_a > 3 EeV. The calculation of elongation curves, X_{\max}(E), for different ankle models strengthens further this conclusion. Status of other models, the dip, mixed composition and Auger based models are discussed.