Unveiling the nature of the knee in the cosmic ray energy spectrum

TL;DR

This paper introduces a new way to analyze the cosmic ray energy spectrum's knee feature, revealing a rigidity-dependent knee and an iron excess at 9.7 PeV, advancing understanding of cosmic ray origins.

Contribution

It proposes the total logarithmic mass energy spectrum and demonstrates the dominance of protons and a rigidity-dependent knee, challenging previous mass-dependent models.

Findings

Proton dominance in the knee formation

Rigidity-dependent knee feature identified

An iron excess at 9.7 PeV with high significance

Abstract

The knee-like structure around 4 PeV is the most striking feature in the cosmic ray energy spectrum, whose origin remains enigmatic. We propose a novel concept of the total logarithmic mass energy spectrum to characterize the knee, taking into account LHAASO measurements of the all-particle energy spectrum and the mean logarithmic mass. The predominant role of proton in the knee formation is unearthed. The case of a mass-dependent knee is ruled out with a significance of 22.9${\sigma}$ and the rigidity-dependent knee feature is revealed. An ankle-like structure stemming from the excess of iron is discovered at 9.7${\pm}$ 0.2 PeV with a significance of 25.9${\sigma}$. Our findings pierce the mist of the puzzling knee for the first time since its discovery.