Greisen—Zatsepin—Kuzmin effect: top—down and bottom—up view
Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, Leninskie Gory 1 build. 2, Moscow, 119991, Russian Federation
In 1966, K. Greisen, and independently G.T. Zatsepin and V.A. Kuzmin, published evidence for the existence of a relic (GZK) cutoff in the proton energy spectrum of >5× 1019 eV ultra-high energy cosmic rays (UHECRs). Half a century of experimental ground-based UHECR research has resulted in a large amount of data on energy spectra, anisotropy and mass composition. The first space experiment to measure UHECRs was launched in 2016. This paper discusses the results of and prospects for experimental UHECR research in the light of the proposed theoretical model of GZK cutoff.