Constraining ultra-high-energy cosmic ray composition through cross-correlations

TL;DR

This paper proposes a method to infer the composition of ultra-high-energy cosmic rays by analyzing their magnetic deflections through cross-correlation with galaxy distributions, enabling indirect composition measurement.

Contribution

It introduces a novel approach using angular harmonic cross-correlation to distinguish cosmic ray atomic numbers with existing data.

Findings

Can differentiate atomic numbers using cross-correlation

Excludes 39% Fe56 nuclei fraction at 2σ for Z=1

Applicable to current ultra-high-energy cosmic ray data

Abstract

The chemical composition of the highest end of the ultra-high-energy cosmic ray spectrum is very hard to measure experimentally, and to this day it remains mostly unknown. Since the trajectories of ultra-high-energy cosmic rays are deflected in the magnetic field of the Galaxy by an angle that depends on their atomic number $Z$, it could be possible to indirectly measure $Z$ by quantifying the amount of such magnetic deflections. In this paper we show that, using the angular harmonic cross-correlation between ultra-high-energy cosmic rays and galaxies, we could effectively distinguish different atomic numbers with current data. As an example, we show how, if $Z=1$, the cross-correlation can exclude a $39\%$ fraction of Fe56 nuclei at $2\sigma$ for rays above $100\text{EeV}$.