Measurement of Energy Spectrum and Elemental Composition of PeV Cosmic Rays: Open Problems and Prospects

TL;DR

This paper reviews the challenges and open questions in measuring the energy spectrum and composition of PeV cosmic rays, highlighting discrepancies among experiments and discussing methods to improve understanding of their origins.

Contribution

It analyzes conflicting experimental results on cosmic ray measurements around the knee energy and discusses prospects for clarifying their origin and composition.

Findings

Significant discrepancies exist among ground-based cosmic ray experiments.

Understanding the knee in the spectrum is crucial for cosmic ray origin models.

Indirect measurement techniques face fundamental challenges and uncertainties.

Abstract

Cosmic rays represent one of the most important energy transformation processes of the universe. They bring information about the surrounding universe, our galaxy, and very probably also the extragalactic space, at least at the highest observed energies. More than one century after their discovery, we have no definitive models yet about the origin, acceleration and propagation processes of the radiation. The main reason is that there are still significant discrepancies among the results obtained by different experiments located at ground level, probably due to unknown systematic uncertainties affecting the measurements. In this document, we will focus on the detection of galactic cosmic rays from ground with air shower arrays up to 10$^{18}$ eV. The aim of this paper is to discuss the conflicting results in the 10$^{15}$ eV energy range and the perspectives to clarify the origin of the so-called `knee' in the all-particle energy spectrum, crucial to give a solid basis for models up to the end of the cosmic ray spectrum. We will provide elements useful to understand the basic techniques used in reconstructing primary particle characteristics (energy, mass, and arrival direction) from the ground, and to show why indirect measurements are difficult and results are still conflicting.