Ultra high-energy cosmic rays—experimental status

Several experiments have accumulated over 100 events with energies of cosmic rays exceeding 4 × 10 19 eV. These events have macroscopic energies, unobserved sources, unknown chemical composition and the spectrum astonishingly shows no GZK cut-off. There are no easy explanations as to how known astrophysical objects can accelerate particles to these energies. Their observation thus remains a puzzle for conventional astrophysics mainly due to the lack of statistics. This paper reviews the present experimental status of the field. Special attention is devoted to the running experiments and to the Pierre Auger observatory. (Some figures in this article are in colour only in the electronic version; see www.iop.org)