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Wherever UHECRs are produced, they are expected to interact
with the background radiation of the Universe. If UHECRs are protons,
they interact with the photons of the cosmic microwave background by
photopion production; this would predict a break in the cosmic
ray
flux, the so-called GZK cut-off,
unless the sources of UHECR above the GZK are nearer than about 50 Mpc.
The GZK cut-off has not been observed by the experiments such as Fly's
Eye, Haverah
Park, Yakutsk, and AGASA. However,
currently there seems to be a disagreement specifically between
the
AGASA ground array which detected about 10 events above 1020
eV, as opposed to about 2 expected from the GZK cut-off, and the HiRes fluorescence
detector which seems consistent with a cut-off.
The resolution of this problem may have to await the completion of the Pierre Auger project which will combine the two existing complementary
detection techniques, I.e. ground arrays and fluorescence detectors.
Excellent introductions to this subject can be found here.
Our publications on cosmic rays can be found here.
References:
G.Bertone,
C.Isola, M.Lemoine, G.Sigl, Phys.Rev.D66:103003,2002
C.Isola, G.Sigl,
G.Bertone, astro-ph/0312374
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