"Signature" neutrinos from photon sources at high redshift
The temperature of the cosmic microwave background increases with redshift; at sufficiently high redshift it becomes possible for ultrahigh-energy photons and electrons to produce muons and pions through interactions with background photons. At the same time, energy losses due to interactions with radio background and intergalactic magnetic fields are negligible. The energetic muons and pions decay, yielding a flux of ``signature'' neutrinos with energies $E_ν\sim 10^{17}$eV. Detection of these neutrinos can help understand the origin of ultrahigh-energy cosmic rays.
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