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 Nuclear Interactions at Ultra-high Energy in Light of Recent Results from Auger
 (INT workshop Feb 20-22, 2008)

  Reported by Larry McLerran
  Date posted May 12, 2008

The Auger cosmic ray experiment is designed to measure the properties of the highest energy cosmic rays, up to 1021 eV, corresponding to a center of mass energy of 103 TeV. This is two orders of magnitude higher than the energy of pp collisions which will soon be measured in the CERN LHC experiments.

There should be a strong suppression in the spectrum of cosmic rays above 6 × 1019 eV. This Greisen-Zatsepin-Kuzmin (GZK) cutoff is from the interaction of high energy cosmic rays with the 3 degree blackbody radiation. In these collisions, the center of mass energy is sufficiently high that pions can be inelastically produced. Cosmic rays from distant sources with energy greater than this lose energy until their energy is below the nominal cutoff.

Nuclei, which form a substantial component of cosmic rays at lower energies, are also expected to dissociate from interactions with the black body photons. Nuclei of an energy E have an energy per nucleon of E/A, where A is the baryon number of the nucleus. Although nuclei have a lower energy per nucleon at fixed total energy, they can dissociate in inelastic scattering. By 1019 eV, it is expected that a substantial fraction of the nuclei will have suffered dissociation.

The Auger experiment was motivated in part by results which indicated that there were substantial events above the GZK cutoff. Meanwhile, the HiRes fluorescence experiment recently reported evidence for the GZK suppression. The Auger experiment was an improvement over previous experiments since it uses a combination of techniques which allow measurement of both the energy of the shower as it hits the ground, late in its development, but also the deposition of energy as it penetrates the atmosphere. The latter measurement of the longitudinal development of the shower involves detecting ultraviolet light from the fluorescence of nitrogen nuclei. The Auger experiment did not confirm the early results which had suggested a significant excess of events above the GZK cutoff energy.

Those events with energy close to and above the GZK cutoff may allow for the unique opportunity to do cosmic ray astronomy. Such high energy cosmic rays have a finite mean free path due to the GZK effect. The galactic and intergalactic magnetic fields also do not bend such cosmic rays much if they originate in nearby galaxies.

The Auger experiment measured the distribution of arrival directions of their highest energy events and found a substantial correlation with the directions to nearby active galactic nuclei. Active galactic nuclei are believed to be gigantic black holes in the centers of galaxies. The Auger result remains to be confirmed.

If one assumes that these high energy cosmic rays originate from active galactic nuclei, then the computations of bending of cosmic rays and the effects of scattering, and models of the production of cosmic rays near the black hole seem to rule out nuclei as their source. They should be primarily protons.

Nevertheless, measurements of the longitudinal development of the showers suggests that the interactions begin earlier in the air, and the muon content of the showers is significantly larger than would be expected for proton interactions. This suggests that something funny has happened to the interactions of the high energy protons with the air nuclei, or that, contrary to the arguments above, the cosmic rays contain a significant component of nuclei.

Because the equivalent center of mass energy of the cosmic ray interactions is significantly larger than that observed in laboratory experiments, it is not unreasonable to expect that the high energy interactions of cosmic rays is somewhat different than expected from extrapolations from low energy. The produced particles which control the nature of cascade development for cosmic rays have energies close to the particle which initiates the interactions. This is because such particles carry most of the energy of the showering particles. The surprise is that at accelerator energies, these distributions are to a good approximation independent of energy. This property of hadronic interactions, Feynman scaling or limiting fragmentation was in fact first discovered in cosmic ray interactions. The Auger results suggest a significant violation of limiting fragmentation.

Recently, there has been an increased understanding of high energy strong interactions. This came about from theoretical developments which argued that high energy strong interactions were controlled by the properties of universal forms of matter, the Color Glass Condensate which is the high density gluonic matter which composes the part of a hadron responsible for high energy interactions,, and the matter made during the collision, the Glasma. These developments arose from attempts to understand Hera data on deep inelastic scattering, and high energy nucleus-nucleus collisions at RHIC. There is now widespread agreement about the general features of the high energy limit of QCD (Quantum Chromodynamics). The theory of the Color Glass Condensate predicts approximate limiting fragmentation, but also the deviations from this scaling. This arises in part because there is a scale, the saturation momentum which grows with increasing energy. In scattering off of air nuclei, the incident proton will see an air nucleus with a saturation momentum estimated to be of order Q2sat ~ 10 − 50 GeV 2. This has increased quite significantly from collisions at accelerator energies, where Q2sat ≤ 1 GeV 2. Issues such as limiting fragmentation or a detailed description of particle production in collisions at the energy of the Auger experiment remain to be worked out but such computations are feasible since the typical momentum scales are large, and weak coupling methods may be used in the QCD computations.

The purpose of this meeting was to bring people together from the cosmic ray community, the high energy nuclear physics community and the astrophysics community, to outline the problems associated with the Auger experiment and to begin discussion about how to proceed towards better understanding of the highest energy cosmic rays.