Nuclear Physics A663&664 (2000) 857c-860c www.elsevier.nl/locate/npe
Probing Particle Physics and Astrophysics with Extragalactic Cosmic-Rays, Gamma-Rays and Neutrinos Giinter Sigl" aDARC, Observatoire de Paris-Meudon, F-9219S Meudon Cedex, France and Department of Astronomy & Astrophysics, Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637-1433, USA The highest energy cosmic rays observed posses macroscopic energies and their origin is likely to be associated with the most energetic processes in the Universe. Their existence triggered a flurry of theoretical explanations ranging from conventional shock acceleration to particle physics beyond the Standard Model and processes taking place at the earliest moments of our Universe. Furthermore, many new experimental activities promise a strong increase of statistics at the highest energies and a combination with gamma-ray and neutrino astrophysics will put strong constraints on these theoretical models. Detailed Monte Carlo simulations indicate that charged ultra-high energy cosmic rays can also be used as probes of large scale magnetic fields whose origin may open another window into the very early Universe. We give an overview over this quickly evolving research field. 1. Introduction
The highest energy cosmic ray (HECR) events observed above 100EeV (1 EeV= 1018 eV) [1-3] are difficult to explain within conventional models involving first order Fermi acceleration of charged particles at astrophysical shocks [4,5]. Also, the range of nucleons above ~ 70 EeV is limited to less than ~ 100Mpc  due to photo-pion production on the cosmic microwave background (CMB), the Greisen-Zatsepin-Kuzmin (GZK) effect . Heavy nuclei are photodisintegrated in the CMB within a few Mpc . There are no obvious astronomical sources within ~ 100 Mpc and a few degrees of observed HECR arrival directions, as would be expected if large scale magnetic fields are ;S 10-9 Gauss . In the following we discuss the main solution categories to this problem that have been suggested in the literature. Finally, we briefly summarize how the deflection and time delay of charged HECR events can be used to obtain information on the poorly known cosmological large scale magnetic fields. For a detailed review of this subject see Ref. .
2. New Primary Particles and New Interactions A possible way around the problem of missing counterparts within acceleration scenarios is to propose primary particles whose range is not limited by the GZK effect. Within the Standard Model the only candidate is the neutrino, whereas in supersymmetric extensions 0375-9474/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PH S0375-9474(99)00731-9
G. Sigl/Nuclear Physics A663&664 (2000) 857c-860c
of the Standard Model, new neutral hadronic bound states of light gluinos with quarks and gluons, so-called R-hadrons that are heavier than nucleons, and therefore have a higher GZK threshold, have been suggested . Neutrino primaries have the advantage of being well established particles, however, within the Standard Model their interaction cross section with nucleons is too small by about five orders of magnitude to produce ordinary air showers. Interestingly, in theories with n additional large compact dimensions and a quantum gravity scale M4+n rv TeV that recently received much attention in the literature because they provide a solution to the hierarchy problem in grand unifications of gauge interactions, the exchange of bulk gravitons (Kaluza-Klein modes) leads to an extra contribution to any two-particle cross section given by  41l"S
~ Mt+n ~ 10
(M4+n)-4 ( TeV
E ) 2 1020 eV cm,
where in the last expression we specified to a neutrino of energy E hitting a nucleon at rest. Note that a neutrino would typically start to interact in the atmosphere for 27 (JIIN ~ 10em", i.e, for E ~ 102oeV, assuming M4+n ~ 1 TeV. The neutrino therefore becomes a primary candidate for the observed HECR events. A specific signature of this scenario would be the absence of any events above the energy where (Jg grows beyond ~ 10- 27 cm2 in neutrino telescopes based on ice or water as detector medium , and a hardening of the spectrum above this energy in atmospheric detectors such as the Pierre Auger Project  and the Orbital Wide-angle Light Collector (OWL) . In both the neutrino and R-hadron scenario the primary would have to be produced as a secondary in interactions of a primary proton that is accelerated in a powerful active galactic nucleus that can now, however, be at high redshift. Consequently, these scenarios predict a correlation between primary arrival directions and high redshift sources. In fact, possible evidense for an angular correlation of the five highest energy events with compact radio quasars at redshifts between 0.3 and 2.2 was recently reported . Only a few more events could confirm or rule out the correlation hypothesis.
3. Top-Down Scenarios As opposed to "bottom-up" acceleration scenarios, in "top-down" (TD) scenarios HECR are created directly as decay or interaction products of particles with masses much higher than the observed energies. In current scenarios, predominantly ,-rays and neutrinos are initially produced at ultra-high energies (UHEs) by the decay of supermassive elementary "X" particles related to some grand unified theory (GUT). Such X particles could be released from topological defect relics of phase transitions which might have been caused by spontaneous breaking of GUT symmetries in the early Universe , possibly after inflation  . Alternatively, superheavy long lived particles with the required abundances can be produced non-thermally in the early Universe, either gravitationally through the effect of the expansion of the background metric on the vacuum quantum fluctuations of the X field, or during reheating at the end of inflation if the X field couples to the inflaton field . The X particles would decay into leptons and/or quarks of roughly comparable energy. The quarks interact strongly and hadronize into nucleons and pions,
G. Sigl/Nuclear Physics A663&664 (2000) 857c-860c
the latter decaying in turn into , -rays, electrons, and neutrinos . TD models typically predict injection spectra which are considerably harder than shock acceleration spectra and can extend up to GUT energies >- 1016 GeV, allowing good fits to the data beyond the GZK cutoff . Since the sources of the X particles need not be associated with any visible astrophysical sources, the absence of identifiable sources can also be explained. TD models are considerably constrained by observational data on ,-ray, nucleon, and neutrino fluxes at various energies . The absolute flux levels predicted by TD models are in general uncertain. While some processes involving cosmic strings seem to yield negligibly low fluxes, others such as those involving annihilation of magnetic monopoleantimonopole pairs, cosmic necklaces, and possible (but controversial) direct emission of X particles from cosmic strings can, for reasonable parameter values, yield X particles at rates sufficient to explain the observed HECR flux, see, e.g., Ref.  for details.
4. UHE Cosmic Rays and Cosmological Large Scale Magnetic Fields Angle-time-energy distributions of UHE cosmic rays contain important information both on cosmological large scale magnetic fields through angular deflection and the associated time delay, and the source mechanism characterized by source distribution, injection spectrum, activity time scale etc. , see, e.g., Ref. . In that respect it is interesting to note that a sub class of events above 4 x 1019 eV seems to cluster in arrival directions . If these clusters originated in discrete sources, some interesting qualitative consequences result already, such as a limit on the intercepted magnetic fields that is comparable to the Faraday rotation limit  . Scaling to the exposures expected, next generation experiments should in this case see clusters of several tens or even hundreds of events at these energies in case of the Pierre Auger Project  and the Orbital Wide-angle Light Collector (OWL) , respectively. Detailed feasibility studies for the potential of future experiments to reconstruct parameters characterizing the source mechanism and the largescale magnetic field both of which are poorly known at present, have been performed . Furthermore, it is possible that our local Supercluster inhabits a comparatively strong magnetic field of strength up to a micro Gauss. This would have important influences on both the spectral and the angular distributions of observed UHE cosmic rays. Discrete sources would be magnetically lensed with the possibility of multiple images, depending on the coherence properties of the magnetic field . Furthermore, if a magnetic field of strength ;<: 0.05 micro Gauss permeates the Supercluster, a more continuous source distribution associated with the Supergalactic Plane can accomodate both the large scale isotropy (due to diffusion) and the small scale clustering (due to amplification by magnetic lensing) of events above 10 EeV  that was reported by AGASA . Magnetic fields of such strength can also lead to HECR deflection angles larger than a few degrees which could reconcile acceleration scenarios with the data by mitigating the problem of missing counterparts.
Acknowledgments At the University of Chicago, this work was supported by the DoE, NSF, and NASA.
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