Prague Relativistic Astrophysics

Jan Ebr

With the advent of large ground-based cosmic-ray observatories, from which the Pierre Auger Observatory is currently the largest, a new window into the universe has opened. Particles with laboratory energies as large as 10²⁰ eV are observed and the statistic of the data in the 10¹⁹ eV range increases rapidly. Nevertheless, the interpretation of the data is not easy: not only do we observe only the extensive air showers of secondary particles in the atmosphere instead of "seeing" the incoming particle directly - the crucial difficulty lies in the fact that the sensitivty range of ground-based detectors does not overlap with the energies currently accessible in accelerator experiments. Thus some way of extrapolation is necessary - a genuinely tricky task considering the lack of fundamental understading even for the existing data in the accelerator energy range.

This talk aims to clarify these difficulties to a non-particle physicist. A brief introduction to strong interaction phenomenology is given. The Quantum Chromodynamics (QCD) is introduced, showing its strengths and weaknesses alike. A small digression into the field-theoretical aspects is made, explaining the important differences between different coupling regimes of QCD, gently touching the crucial idea of a running coupling constant. Then the somehwat mystical pomeron is introduced and the general ideas of current highest-energy hadronic Monte Carlo event generators are presented. At the end, the listener shall understand why, even with the amazing amount of data and theories at hand, the utility of ground-based cosmic ray observatories as "particle telescopes" for astrophysics is still sohewhat limited.