Ultrahigh Energy Cosmic Rays: Facts, Myths, and Legends

TL;DR

This paper reviews recent progress in understanding ultrahigh energy cosmic rays, covering experimental data, acceleration mechanisms, neutrino production, air shower phenomenology, and potential for new physics insights.

Contribution

It provides a comprehensive overview of current knowledge, experimental techniques, and theoretical models related to ultrahigh energy cosmic rays and their associated phenomena.

Findings

Progress in understanding cosmic ray energy spectrum and composition

Development of models for air shower physics and hadronic interactions

Potential to explore new physics beyond the electroweak scale

Abstract

This is a written version of a series of lectures aimed at graduate students in astrophysics/particle theory/particle experiment. In the first part, we explain the important progress made in recent years towards understanding the experimental data on cosmic rays with energies > 10^8 GeV. We begin with a brief survey of the available data, including a description of the energy spectrum, mass composition, and arrival directions. At this point we also give a short overview of experimental techniques. After that, we introduce the fundamentals of acceleration and propagation in order to discuss the conjectured nearby cosmic ray sources, and emphasize some of the prospects for a new (multi-particle) astronomy. Next, we survey the state of the art regarding the ultrahigh energy cosmic neutrinos which should be produced in association with the observed cosmic rays. In the second part, we summarize the phenomenology of cosmic ray air showers. We explain the hadronic interaction models used to extrapolate results from collider data to ultrahigh energies, and describe the prospects for insights into forward physics at the Large Hadron Collider (LHC). We also explain the main electromagnetic processes that govern the longitudinal shower evolution. Armed with these two principal shower ingredients and motivation from the underlying physics, we describe the different methods proposed to distinguish primary species. In the last part, we outline how ultrahigh energy cosmic ray interactions can be used to probe new physics beyond the electroweak scale.