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
with their energy thresholds around 50 GeV and the planned satellite
whereas existing instruments such as the ground
may have too high an energy threshold,
may not have looked in the right direction.
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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