Highest Energy Cosmic Rays

It is proposed that the highest energy ∼ 10 20 eV cosmic ray primaries are protons, decay products of a long-lived progenitor whose high kinetic energy arises from decay of a distant (cosmological) superheavy particle, G. Such a scenario can occur in e.g. SU (15) grand unification and in some preon models, but is more generic; if true, these unusual cosmic rays provide a window into new physics. Important discoveries in particle physics were at one time dominated by the study of cosmic rays 1. Examples are the discovery of the positron 2 , the muon 3 , the pion 4 , and the first strange particles including the kaon 5. In the last several decades, most of the important discoveries have been made under the more controlled situation of accelerators. Nevertheless, the distinguished history for cosmic rays may be about to repeat, if the highest-energy cosmic rays reflect new physics. The cosmic rays which exceed the GKZ cutoff 6 at a few times 10 19 eV are of particular interest, because for protons this cutoff which is based on pion photoproduction from the cosmic microwave background (CMB) seems very well founded. The derivation is as follows: using for Boltzmann's constant k B = 8.62 × 10 −5 eV/ o K and taking the temperature of the CMB as 3 o K gives an average photon energy ǫ = 8 × 10 −4 eV. In the CMB frame a collision p(p 1) + γ(p 2) → ∆ → N π has p 1 = (E, 0, 0, k), p 2 = (ǫ, 0, 0, −ǫ) and squared center of mass energy s = (p 1 +p 2) 2 = m 2 p +2(E+k)ǫ = m 2 ∆. For the relativistic case E p ≃ k and E (resonance) p = (m 2 ∆ − m 2 p)/4ǫ = 2 × 10 20 eV. This is for the average energy: considering the more energetic CMB photons, the limit falls to a few ×10 19 eV. For protons above this energy, the pion photoproduction from CMB will dominate beyond the mean free path. In the nonrelativistic case, when E p is not equal to k one must keep all terms and find then E X + (E 2 X − m 2 X) 1/2 = (s threshold − m 2 X)/2ǫ where X is the primary and s threshold is the appropriate …