News from Cosmic Ray Air Showers (ICRC 2019 -- Cosmic Ray Indirect Rapport)

TL;DR

This paper summarizes recent advances in cosmic-ray air-shower measurements, revealing new spectral features, composition variations, and anisotropy signals, while highlighting ongoing efforts to understand cosmic-ray origins and improve experimental techniques.

Contribution

It reports updated measurements from multiple observatories, identifies new spectral features and composition trends, and discusses future experimental strategies for cosmic-ray research.

Findings

Discovery of a softening in the light component around 10^13.5 eV

Observation of a smooth second knee extending over 100-200 PeV

Confirmation of a changing mixture of protons and nuclei up to several 10 EeV

Abstract

This rapport summarizes the cosmic-ray indirect (CRI) session of the 36th ICRC conference. Updated measurements from several air-shower arrays with higher precision lead to the discovery of new features in the energy spectrum: HAWC measures a softening of the light component (p+He) around $10^{13.5}\,$eV; measurements of the Pierre Auger Observatory show that the second knee is a smooth feature extending at least over the range of $100-200\,$PeV and that the energy spectrum between the ankle and the cut-off cannot be described by a simple broken power law. Measurements of the mass composition confirm that the composition is a varying mixture of protons and nuclei at least up to several $10\,$EeV. A joint effort of several collaborations helps to better assess deficiencies of hadronic interaction models, e.g., by quantifying the muon deficit in the models over the shower energy. Anisotropy measurements with higher precision generally confirm earlier results. A change of the amplitude and phase of the equatorial dipole indicates that in the energy range between the second knee and the ankle there likely is a transition from Galactic to extragalactic sources. However, neither the most energetic Galatic nor the extragalactic sources have been discovered, which remains a primary science goal. Next to more exposure, an increase of measurement accuracy and decrease of systematic uncertainties will provide future progress. It is particularly exciting that new experiments are built and existing experiments upgraded to increase the measurement accuracy of the energy and mass composition, e.g., by combining radio antennas with particle detectors. Last but not least, there is a trend that experiments are designed such that they can target cosmic rays, photons, and neutrinos at the same time, which will facilitate multi-messenger astrophysics at the highest energies.