Very High Energy Cosmic Rays and Their Interactions

The investigation of very high energy cosmic rays and their interactions are inherently connected subjects of astroparticle physics. On one hand the understanding of cosmic ray interactions is needed to study the flux, acceleration and propagation of cosmic rays. For example, the high energy cosmic ray flux can only be measured by linking secondary particle cascades observed in detectors or the Earth’s atmosphere to primary particles of certain energy, mass number and arrival direction. Furthermore the knowledge of particle production is needed for the interpretation of secondary particle fluxes due to cosmic ray interactions in various astrophysical environments. On the other hand cosmic rays provide us with a continuous beam of high energy particles that can be exploited for studies of interaction physics at energies and phase space regions not accessible at man-made accelerators. Cosmic ray research of the last years is characterized by substantial progress in measuring primary and secondary particle fluxes. Examples for new results on the primary cosmic ray flux are the measurements below the knee by AMS, BESS and ATIC, in the knee energy region by KASCADE and TIBET, and at the highest energies by the High Resolution Fly’s Eye (HiRes) experiment. Still some of the experimental results appear contradictory and are subject of controversial discussions. For example, the results of the composition analyses of the KASCADE and EAS-TOP data seem to be in variance with a first, preliminary analysis of the TIBET data. Similarly, there appears to be a discrepancy between the AGASA measurements of the cosmic ray flux above 10 eV and the new HiRes data. In addition to the measurement of the primary cosmic ray flux the most powerful method of improving our understanding of cosmic ray physics is the study of secondary particle fluxes. New instruments measuring gamma-rays (CANGAROO, HESS, MAGIC, VERITAS, and Milagro), muons and neutrinos (AMANDA, BAIKAL, NESTOR, and ANTARES) have begun taking data or successfully performed prototype runs. Regarding cosmic ray physics, they are expected not only to test models of cosmic ray acceleration and interaction in supernova remnants and other astrophysical objects but also to provide valuable clues on cosmic ray composition and the characteristics of high energy particle production. There are many efforts to develop better models for cosmic ray interactions or to derive information on hadronic multiparticle production. The progress in this field is closely linked to measurements of forward multiparticle production in fixed-target and collider experiments. One of the central problems is the consistent implementation of the consequences of the steeply rising parton densities measured in deep inelastic e-p collisions at HERA and the indications of parton density saturation seen at the Relativistic Heavy Ion Collider RHIC. RHIC data clearly demonstrate the difficulties of extrapolating models tuned to accelerator data to higher energy or other projectile/target combinations. Many models predicted a secondary particle multiplicity exceeding that measured in central Au-Au collisions by ∼ 30% or more. The impact of the RHIC