The GZK limit (for Greisen-Zatsepin-Kuzmin limit or Greisen-Zatsepin-Kuzmin effect or GZK effect) is a theoretical limit on the kinetic energy of cosmic-ray protons, which is 5×1019 eV or about 8 joules. The limit was calculated independently by its three developers based upon the expected interaction of the protons with CMB photons if they travel through the CMB for a substantial distance, specifically, distances on the order of 50 kpc (termed the GZK horizon) and above. The expected products of the interaction (including its follow-on interactions) are less-energetic protons, electrons and positrons and very high energy neutrinos termed GZK neutrinos or cosmogenic neutrinos.
Cosmic rays above this limit have been observed and are called extreme-energy cosmic rays or EECRs. If the observations are accurate, it is assumed they are from within 50 kpc or that they are more massive particles such as alpha particles. There is no consensus regarding the mechanism that creates them.