Cosmic ray energy and composition measurements with GRAPES-3 and other experiments

Cosmic rays (CRs) are charged nuclei accelerated by natural astrophysical processes to extremely high energies. Despite significant progress in CR research over the past century, their origin and the astrophysical processes responsible for accelerating them to such high energies have yet to be fully understood. A comprehensive understanding requires precise measurement of their energy spectrum and mass composition. This article reviews the CR energy spectrum and mass composition measurements above 100 GeV. The CR energy spectrum is generally described by a power law, with notable spectral features commonly referred to as the knee, ankle, and flux suppression. Direct experiments offer high-precision measurements in the GeV–TeV energy range and have revealed spectral hardening around a few hundred GeV and softening near several tens of TeV in the proton and helium spectra. At energies above hundreds of TeV, CRs are observed by indirect experiments using extensive air showers (EASs). Indirect measurements have also observed further spectral features, such as the hardening in light primaries near hundreds of TeV, the second knee, and the instep. ... mehrThis article highlights the GRAPES-3 experiment, in Ooty, India, focusing on the methodology of using muon multiplicity to estimate mass composition and the observation of spectral hardening at 166 TeV in the CR proton spectrum. These observed spectral features provide important constraints for refining theoretical models of CR origin, acceleration, and propagation. Indirect measurements, particularly of mass composition, are subject to large systematic uncertainties because the interpretation of observables relies on simulations based on hadronic interaction models. This article briefly discusses significant discrepancies in mass composition results arising from the limited theoretical understanding of these models, emphasizing the need for improved hadronic interaction modeling.