Resolution of (Heavy) Primaries in Ultra High Energy Cosmic Rays

TL;DR

This paper investigates how the choice of primary nuclei affects the inferred composition of ultra-high energy cosmic rays, introducing a new distance measure to reduce bias and providing bounds on heavy nuclei contributions.

Contribution

The study introduces a novel distance measure in $X_{max}$ distribution space and offers a systematic method to select primaries for unbiased UHECR composition analysis.

Findings

Limiting primaries can bias composition inference.

New measure ensures unbiased results at given confidence.

First observational bounds on heavy nuclei contributions.

Abstract

Measurements of Ultra-High Energy Cosmic Rays (UHECR) suggest a complex composition with significant contributions from heavy nuclei at the highest energies. We systematically explore how the selection and number of primary nuclei included in the analysis impact the inferred UHECR mass composition. Introducing a distance measure in the space of $X_{\rm max}$ distribution moments, we demonstrate that limiting the analysis to a few primaries can introduce significant biases, particularly as observational data improves. We provide lists of primaries approximately equidistant in the new measure, which guaranty unbiased results at given statistical confidence. Additionally, we explore consistent inclusion of nuclei heavier than iron and up to plutonium, deriving first observational upper bounds on their contributions to UHECR with the Pierre Auger Open Data.