Spectrum features
The spectrum has two main features: a steepening of the slope near 1015 and 1016 eV the "knee", and a flattening near 1018 and 1019 eV the "ankle". The interpretation of EAS results relies on the comparison of the measured effects with simulations of shower development in the atmosphere. The most crucial ingredient is the modelling of hadronic interactions, requiring bold extrapolations to regions where no accelerator data exist and theoretical guidelines are only vague.
In recent years these EAS simulations have nevertheless reached a high level of sophistication and employ many of the standard hadronic interaction models from accelerator particle physics. The status of EAS simulation was reviewed by J Knapp of Leeds who underlined the need for a common reference program as a standard tool.
Several explanations could account for the slope changes in the energy spectrum. The knee could be associated with features of propagation of galactic cosmic rays, and the ankle to a transition from galactic to extra-galactic cosmic rays. A reliable determination of the contribution of different nuclear species to the energy spectrum is needed to understand the implications of these features, and to discriminate between the various models.
Results presented at the conference, such as those from the KASCADE experiment in Karlsruhe, confirmed the existence and position of the knee, with evidence for a light composition below it and for a clear trend towards heavier composition above. Such behaviour ties in with models which interpret the knee as the onset of the leakage of cosmic rays outside the magnetic field of the galaxy for cosmic ray particles of the same energy, the lighter, less charged ones are less rigidly bound and would be the first to leave.
A different intriguing phenomenon in the study of EAS was reported by G B Zhdanov (Lebedev) who presented results from an 18 m2 neutron monitor at 3330 m used in conjunction with an EAS array. Considerable doses of neutrons (and of electrons) have been recorded in the wake of EAS of about 1016 eV, the delay relative to the EAS front being about 500 microseconds. This could be due to very massive primary particles.
The status of ultra-high-energy cosmic rays was reviewed by A M Hillas (Leeds) and V Berezinski (Gran Sasso). The energy spectrum of cosmic rays suggests a transition to an extra-galactic source at energies above the ankle and the composition at the highest energies is consistent with protons.
It is very difficult to understand how particles or nuclei can be accelerated to such multi-Joule energies. One scenario associates the highest energy cosmic rays with gamma-ray bursts (also awaiting a generally-accepted explanation). The two could be generated simultaneously, but the transit times of charged particles would be smeared over many years by intergalactic magnetic fields.
Another scenario avoids the problem of acceleration by attributing the origin of the highest energy cosmic rays to decays of particles with masses at the grand unification scale (1015-1016 GeV) created by topological defects (monopoles, cosmic strings and domain walls).
One of the most challenging issues in astroparticle physics is to understand the origin of cosmic rays with energies above 1020 eV when there are no obvious nearby sources. In 1966 Greisen, Zatsepin and Kuzmin pointed out that the universe is not transparent to protons above about 4x1019 eV as they would interact with the 2.7 K microwave background radiation. This led to the argument that if the sources of cosmic-ray particles of such energies are extra-galactic, they must be relatively near (within 300 million light-years) otherwise no such particles will be observed.
With such a long list of intriguing phenomena and challenging problems, this area of astroparticle physics will remain an active, exciting field for many years.