From PeV to GZK: Recent Results from Cosmic Rays

From the time of Enrico Fermi’s seminal paper on statistical acceleration, it was thought that the cosmic ray spectrum followed a power law spectrum with at most one or two breaks. Recent data from a variety of experiments have shown that this picture is greatly oversimplified. A surprising richness of structure in the spectrum above (and below) 1 PeV ( 1015 eV) is now established. Accumulating data also indicates corresponding changes in the cosmic ray composition. We will review these experimental results which challenge theoretical understanding, with particular emphasis on the Telescope Array data; which, uniquely, covers the whole range between ~1 Pev and 100 EeV (1020 eV) with a single technique.