The Observation of the GZK Suppression in Ultra-high Energy Cosmic Rays

Pierre Sokolsky
University of Utah

Forty years after the discovery of the 2.7 deg. black body radiation
we can finally report on the definitive observations by the HiRes and
Auger experiments of the inelastic interaction of ultra-high energy
particles with the microwave photons. This interaction was proposed by
Greisen, Zatsepin and Kuzmin within a year of the microwave background
discovery. The effect severely restricts the horizon for the highest
energy charged particles. Recent claims of correlation of arrival
directions of these particles with nearby AGN's will also be examined.