Further evidence for an increase of the mean mass of the highest-energy cosmic-rays with energy

Abstract

Sokolsky and D'Avignon have recently reported an examination of a variety of measurements that relate to the question as to how the mass composition of the highest-energy cosmic-rays evolves with energy. They assert that cosmic rays arriving from the Northern Hemisphere have a different mass composition from those arriving from the Southern Hemisphere, implying a diversity of sources of high-energy cosmic-rays in the two hemispheres. Were this conclusion to be correct, it would have profound implications for theories of cosmic-ray origin and would influence planning of future projects. Their claim thus merits careful scrutiny. In this paper their analysis is examined in detail with the verdict being that evidence for a North/South difference is not proven, a conclusion supported by other data from the Northern and Southern Hemispheres. However, what is of major importance is that the study of Sokolsky and D'Avignon provides long-awaited confirmation of the claim that the mean mass of cosmic rays increases with energy above ∼3 EeV made by the Pierre Auger Collaboration in 2014.