Energy spectrum and mass composition of high-energy cosmic rays

Primary cosmic rays above energies of about 100 TeV are investigated by observations of extensive air showers (EAS) using large area ground based detector installations for registering various components of the EAS cascade development. By such indirect studies of the primary cosmic rays a steepening of the power-law spectrum at around 3-5 PeV known as the knee, has been identified. At higher energies around 5 EeV there appears a further change of the spectral index towards a flattening of the spectrum, called the ankle. The energy region above ca 50 EeV, where a cut-off of the cosmic ray spectrum (Greisen-Zatsepin-Kuz'min (GZK) cut-off) is theoretically predicted, is of particular current interest and provides an astrophysical enigma, since obviously trans-GZK events have been observed. Any explanation of these features of the cosmic ray spectrum needs sufficiently detailed knowledge of the shape of the spectrum and of the variation of the mass composition of cosmic rays. In this paper different experimental approaches deducing mass and energy sensitive information from the EAS experiments and their results are discussed. The experiments involve measurements of secondary particle distributions at various observation levels and of muons by deep underground detectors, as well as measurements of air Cherenkov light and, in particular at higher energies, of air fluorescence light emitted during the EAS development. Recently, methods for analysing multi-dimensional EAS parameter distributions have been favoured. They take into account correlations of different EAS parameters and, in particular by non-parametric techniques, also the influence of the intrinsic fluctuation of the air shower development. This paper illustrates the application of such methods in a coherent view of recent results. The advanced analysing methods are corroborated by hybrid experimental set-ups registering a larger set of different EAS observables simultaneously in an event-by-event mode. In addition such approaches provide the possibility to test the consistency of the hadronic interaction models and Monte Carlo procedures used as reference for the analyses. The physical and astrophysical implications of the current findings in various energy regions are briefly discussed and prospects of future experiments are presented.