Measurement of the Ultra High Energy Cosmic Ray Spectrum

The Pierre Auger Observatory has uploaded a new measurement of the energy spectrum of ultra high energy cosmic rays. They looks at events with energies greater than 1018 eV. Since most cosmic rays are protons, this is actually an incredibly large amount of energy for a single subatomic particle. In fact, these are particles and nuclei with macroscopic amounts of kinetic energy. The huge amount of energy means that these particles create huge air showers high in the atmosphere. Auger measures the light signal emitted as the shower progresses as well as signals from muons hitting a massive array of water Cherenkov tanks on the ground at the detector site in Argentina.

The spectrum of these cosmic rays is actually quite important in astrophysics. The cosmic ray spectrum generally follows a power law spectrum, but the shape is expected to change around 5×1019 eV. A knee-like feature should be seen around that energy as the spectrum changes from one power law to another.

The reason for this expected ship is the so-called GZK cutoff. The cosmic microwave background consists of a thermal distribution of microwaves (photons) with a temperature of 2.7 K, or an energy of around 225 μeV. It turns out that a proton can interact with a photon to create a short-lived Δ that then decays to a nucleon (proton or neutron) and a pion. The Δs decays via the strong force, so while they have a mass of around 1232 MeV, they also have widths of over 100 MeV. This means that the lifetime is very short and so we generally refer to a Δ “resonance” rather than a Δ “particle.” The GZK cutoff represents the approximate energy at which significant numbers of protons will interact with the CMB to create Δs.

The pion in the final state after the Δ decays carries a sizable fraction of the initial proton energy. So, the proton will continue to lose energy until the energy is too low for this process to continue. As a result, the proton spectrum at energies higher than the GZK cutoff should be suppressed compared to the spectrum at energies lower than the cutoff. There is a mean free path for this process that is still quite large, so some protons might be expected at energies above the cutoff, but they should originate in some region in the vicinity (on cosmological scales) of Earth.