The Notion of Age Emerged from the Longitudinal Evolution of Purely Electromagnetic Showers: A Review

Abstract

The state of the longitudinal development of high-energy cosmic-ray air showers has been characterized by the notion of shower age. Moreover, at a given atmospheric level, the shape of the energy spectrum of electrons in a cosmic-ray shower with ultra-high-energy depends only on the shower age at this level. This work is an in-depth review of the conceptual underpinnings of the electromagnetic cascade theory, which extends to Rossi and Greisen’s viewpoint as well as to the recently acknowledged quests by others. The structure of this work consists of two parts. In the first part, we review how basic arguments can be used to grasp the properties linked to the shower age. In the second part, we look at how the structures of the electron and photon spectra, as well as the relative normalization corresponding to a particular age, can be known more rationally. As we end our review, we refer to several new concepts derived from recent Monte Carlo shower analyses.