giant air shower

Ultra-High Energy Cosmic Rays:
A Probe of Supersymmetry ?

Five of the highest energy cosmic ray events have been found by Peter Biermann and Glennys Farrar to be aligned with compact radio sources at redshifts larger than 0.5. Since the chance likelihood for this is smaller than 1 %, this will be easy to confirm or disprove with only modestly more data from future experiments. If confirmed, the primary could not have been any of the known particles due to their interaction cross sections at these energies. A recent study perfomed by Diego Torres, Gustavo Romero (Argentina), Luis Anchordoqui (U Norteastern, USA), and myself, of the newest data set does, however, not confirm a significant correlation.

A possible new massive neutral particle which could evade photo pionproduction because, among other reasons, its higher rest mass, is provided in certain implementations of supersymmetry (SUSY). This particle could be produced as secondaries of accelerated protons in the source and would arrive at the observer almost unattenuated and undeflected. However, at the same time, ordinary pions should be produced by the very same proton primaries which will give rise to gamma rays and neutrinos. At least the gamma rays below a TeV should be easily observable by next generation detectors such as MAGIC, VERITAS, and HESS with their energy thresholds around 50 GeV and the planned satellite experiment GLAST, whereas existing instruments such as the ground based HEGRA and WHIPPLE may have too high an energy threshold, and EGRET may not have looked in the right direction.

spectra from an active galaxy 
at redshift 0.5 Future instruments will therefore lead to a stringent lower limit on the branching ratio for production of the heavy neutral, as schematically illustrated by the point fluxes shown in the figure on the left for a source at z=1 injecting a E**(-2.2) proton spectrum up to at least 10**22 eV, a branching ratio for production of the new heavy neutral in nucleon interactions of 0.01, and a beaming factor of 10 for neutrinos and the heavy neutrals. The 1 sigma error bar at 3*10**(20) eV represents the point flux corresponding to the highest energy Fly's Eye event. The predicted fluxes were normalized such that this highest energy event is explained as a new heavy particle. The points with arrows represent point flux sensitivities, as in figures on previous pages.

The SUSY parameter space for which this may work is already rather constrained by accelerator experiments such as E761 and KTeV: A light gluino is required in the 10 GeV range; much heavier particles would be inconsistent with the observed shower development. This already strongly constrains the possible SUSY breaking mechanism which is of great interest theoretically. We plan to perform more detailed calculations of the branching ratios and resulting fluxes as functions of the SUSY parameters.

New interactions beyond the electroweak scale may also be tested by ultra-high energy neutrinos.


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