Portions of this entry contributed by Neil A. Rubin

Cosmic rays are extremely energetic particles generated by high-energy astrophysical processes. There is a whole spectrum of cosmic rays, spanning energies from 10¹⁰ eV (electron volts) all the way up to "ultrahigh energy cosmic rays" (UHECR), having energies on the order 10²⁰ eV (which, on average strike once per square kilometer per hundred years).

Cosmic rays are often used to help calibrate detectors used in particle accelerator experiments and consist overall of 98% primary particles and 2% secondary particles. The primary particles are ionized atomic nuclei, and consist of 87% H⁺ and 13% He⁺². The secondary particles are mostly electrons and positrons.

The particular interest in UHECRs stems from the fact that the primary protons with energies above the so-called "GZK cutoff" of eV are able to scatter off photons of the cosmic background radiation, producing pions. As a result, any protons with energies of 10²⁰ eV should lose most of their energy within a few million light-years. The problem is that no one can seem to find any sources that close to us that could be producing these particles with such immense energies at the rates observed. The resolution of this puzzle is an important outstanding puzzle which will hopefully be resolved by the Pierre Auger Observatory (www.auger.org).

Centauro Event, Electron, Muon