Implications of the UHECRs penetration depth measurements

TL;DR

This paper analyzes UHECRs' penetration depth data, revealing a transition from protons to heavier nuclei around 10^18 eV, and constrains the composition and source metallicity, suggesting potential new physics at high energies.

Contribution

It provides the first robust upper bounds on proton fractions in UHECRs and highlights the need for extreme source metallicity, challenging existing astrophysical models.

Findings

Proton fraction in UHECRs is strongly limited by data.

Observed composition requires near-equal metals to protons at sources.

Current astrophysical models struggle to explain the required source metallicity.

Abstract

The simple interpretation of PAO's UHECRs' penetration depth measurements suggests a transition at the energy range $1.1 - 35 \cdot 10^{18} $ eV from protons to heavier nuclei. A detailed comparison of this data with air shower simulations reveals strong restrictions on the amount of light nuclei (protons and He) in the observed flux. We find a robust upper bound on the observed proton fraction of the UHECRs flux and we rule out a composition dominated by protons and He. Acceleration and propagation effects lead to an observed composition that is different from the one at the source. Using a simple toy model that take into account these effects, we show that the observations requires an extreme metallicity at the sources with metals to protons mass ratio of 1:1, a ratio that is larger by a factor of a hundred than the solar abundance. This composition imposes an almost impossible constraint on all current astrophysical models for UHECRs accelerators. This may provide a first hint towards new physics that emerges at $\sim 100$ TeV and leads to a larger proton cross section at these energies.