Session J3: Invited Session: Cosmic Rays and Hadron Physics

Cosmic Ray Muons in QCD

Despite 100 years of effort, many aspects of cosmic rays are still not well understood. Studies of high-energy muons produced in cosmic ray air showers are an important part of this effort. These muons are produced by the decays of pions and kaons at lower energies, and by charm quarks above about 1 TeV. Measurement of these muons can be used to determine the composition of cosmic rays. Moreover, the muons are produced at far-forward rapidities and are an excellent complement to data collected by accelerators at lower rapidity. Underground detectors have studied cosmic ray muons and measured lateral separations ranging from the meter scale to hundreds of meters. For larger separations, the muon transverse momentum is in excess of 6 GeV/c and muon interactions can be characterized using perturbative QCD. These measurements can be used to extend accelerator measurements to higher center of mass energies and rapidities, and have important implications for charm production as well as cosmic ray composition.   

Collision modelling in air showers and accelerators

A high energy cosmic particle entering the atmosphere interacts with air, initiating a so-called hadronic cascade of mostly pions, but also kaons, protons, and others. Some hadrons (like the neutral pion) decay into photons, which leads to an electromagnetic cascade of electrons, positrons, and photons. A good theoretical understanding of the dynamics of these two types of cascade is crucial for a realistic simulation of a complete air shower, which is the basis of a correct interpretation of experimental results. In particular, having measured properties of air showers, one wants to conclude about the energies and the chemical compositions of the incident particles. Can we perform realistic simulations? The electromagnetic cascade is theoretically well under control, whereas the hadronic cascade relies on phenomenological models, which have to be tested against data from accelerator experiments in an energy range from few GeV up to the highest available energies. We will discuss the current status of collision modelling and in particular in which way certain results from accelerator experiments affect the interpretation of cosmic ray data, and vice versa.   

Hadronic Physics and Air Shower Detectors

The Earth is continuously hit by cosmic particles of very high energy. When entering the Earth's atmosphere, these particles interact hadronically with nuclei in air and produce gigantic cascades of secondary particles. The cascades, called extensive air showers, contain up to several 100 billion particles reaching the ground at almost the same time. Operating many particle detectors in coincidence that are distributed over a large area, these air showers can be detected and moreover their features can be studied. Not only the energy, direction and most probable mass of the primary cosmic particle can be estimated but also conclusions on the characteristics of hadronic interactions in the early stages of the cascades can be drawn. Air shower based studies of hadronic multiparticle production are complementary to those carried out at accelerators, extending the reach in energy well beyond that of man-made accelerators. After giving a brief introduction to the physics of extensive air showers an overview of current shower detectors and their data will be given. Recent results on properties of hadronic interactions at different energies will be discussed, emphasizing the relation between cosmic ray and accelerator data.