giant air shower

Ultra-High Energy Cosmic Rays:
Overview: Production Scenarios, Signatures, Experiments

The detection of extremely high energy cosmic rays with energies above 10**(20) eV (about 20 Joules in a single elementary particle!) by several cosmic ray experiments including the Fly's Eye experiment at Utah and the Akeno Giant Air Shower Array (AGASA) experiment in Japan have stirred a lot of interest among physicists and astrophysicists. Particles around 10**(20) eV were already detected between the 60s and 80s by the Volcano Ranch (New Mexico, USA), Haverah Park (Leeds, UK), Yakutsk (Russia) and the Sydney University Giant Air Shower Recorder (SUGAR) (Australia) ground arrays. It is difficult to accelerate protons and heavy nuclei up to such energies even in the most powerful astrophysical objects such as radio galaxies and active galactic nuclei. In addition, nucleons above about 7 x 10**(19) eV undergo photopion production on the cosmic microwave background (CMB), which is known as the Greisen-Zatsepin-Kuzmin (GZK) effect and limits the distance to possible sources to less than 100Mpc. Heavy nuclei are photodisintegrated in the CMB within a few Mpc.

We study modifications of the conventional first order Fermi acceleration mechanism and we are searching for alternative physical processes which can produce such particles. Together with David Schramm (U of C) and Pijush Bhattacharjee (Indian Institute for Astrophysics, Bangalore), I worked on "top down" mechanisms where particles above 10**(20) eV are produced by decay from energies up to grand unification scales. Karsten Jedamzik (Lawrence Livermore National Laboratory), David Schramm, Venya Berezinsky (Istituto Nucleare Fisica Nationale, Gran Sasso), and I showed that top down mechanisms involving topological defects left over from cosmological phase transitions can produce observable cosmic ray fluxes without violating observational constraints on primordial element abundances and microwave background distortions.

In such particle physics motivated top down scenarios gauge bosons, Higgs bosons or superheavy fermions with a mass near the grand unification scale are released from topological defects and instantaneously decay typically into a lepton and a quark. The subsequent hadronization of the quark into nucleons and pions is governed by quantum chromodynamics (QCD). Observable spectra significantly depend on the total hadronic fragmentation function for which I use solutions of the QCD evolution equations in modified leading logarithmic approximation. These provide good fits to up to date particle accelerator data at LEP energies.

Diagnostic tools such as composition and expected anisotropy of cosmic ray events could discriminate between different scenarios in the near future when better statistics will become available from the next-generation experiments now under construction. A specific signature for grand unification scale physics, namely a "gap" feature above the Greisen-Zatsepin-Kuzmin cutoff, has been discussed in detail in an article published in Science (for a popular account of this work see also the 26 Aug 1995 issue of the New Scientist, p.17, and the March 1996 issue of the Scientific American).

A very important ingredient in developing such diagnostic tools is the inclusion of modifications of the cosmic ray spectrum during propagation in extragalactic space. In case of the topological defect models, a comparatively large ultra-high energy neutrino flux would be an additional detectable signature. The interface to gamma-ray and neutrino astronomy is important in all scenarios.

Additional information on ultra-high energy cosmic rays is encoded in the distributions of energies and arrival times as well as in angular images in a strongly magnetized local environment.

Another possibility consists of somehow tying the observed primaries to neutrinos or new weakly interacting particles which can propagate unattenuated over large distances. Neutrinos could have new interactions beyond the electroweak scale which could be probed even if they are not sufficiently strong to make neutrino primary candidates. New neutral particles could be signatures of supersymmetry.

Much experimental activity is underway to explore the ultrahigh energy end of the cosmic ray spectrum, most notably the Giant Air Shower Detectors proposed by the Pierre Auger Project (see also the French Auger site), the Japanese Telescope Array. and the High Resolution Fly's Eye, the latter currently being under construction. For the farther future, concept studies are being performed for observing the giant atmospheric air showers triggered by ultrahigh energy cosmic rays from space with dedicated satellite experiments. The main projects concern the Orbiting Wide-angle Light collectors (OWL) in the US and the Extreme Universe Observatory (EUSO) within the AirWatch program in Europe. Furthermore, old ideas on the detection of radio pulses from showers produced by ultra high energy cosmic ray primaries are being revived. The relatively cheap radio detection method could increase the detection rates and complement the more accurate optical and ground array techniques, see for example the recent conference on radio detection.

Given these excellent prospects that the cosmic ray spectrum at the ultra-high energy end will be measured with much higher statistics, I expect that some of the tools we are developing will be helpful in the near future to unveal the nature and origin of these elusive particles. I further hope to use cosmic ray data also to gain knowledge on particle interactions beyond accelerator energies.

For reviews on ultra-high energy cosmic rays I have been involved in see here.


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