Ultrahigh-Energy Neutrinos

Ultrahigh-energy neutrinos might be made in a variety of ways (in the warped space near black holes, say, or in the decay of exotic massive particles)and might be detected on Earth in a variety of ways. This issue is explored in three upcoming articles in Physical Review Letters.

One reason for the new interest in vigorous neutrinos is that it could help solve the mystery of why there are more than expected cosmic rays at energies above 10²⁰ eV. At these energies one would expect particles to lose much energy through interactions with the cosmic microwave background, a presumed limitation referred to as the Greisen-Zatsepin-Kuzmin (or GZK) cutoff.

The way around this cutoff might be to argue that ultrahigh-energy (UHE) neutrinos, which don't interact with photons, might be ferrying huge energies through the universe and that the surplus of highest-energy cosmic rays comes about from chance encounters between the UHE nu's and particles such as protons or other neutrinos. The nu's themselves are too ephemeral to see in terrestrial detectors but their presence and flux can be inferred indirectly. Hence the need for experimental ventures such as the Pierre Auger detector array now in preparation.

Here are the three reports:

Alexander Kusenko (UCLA and BNL) and Thomas Weiler (Vanderbilt) propose to compare UHE-nu-initiated showers of particles moving horizontally through the atmosphere ("Earth-skimming" events) with up-going showers in which the nu has interacted in the body of the Earth. (Phys. Rev. Lett. 22 April 2002.)

In a second paper, MIT scientists concentrate on the Earth-skimming nu events and propose a novel detection scheme in which the nu's planetary encounter causes it to convert into a lepton (electron, muon, or tauon, depending on what kind of neutrino was involved). This lepton, although it generally escapes, will leave a detectable fluorescent trace (Feng, Fisher, Wilczek, Yu, Physical Review Letters, 22 April 2002.)

In the third paper, Fodor, Katz, and Ringwald (DESY, Hamburg, and Eotvos University, Budapest; contact Andreas Ringwald, andreas.ringwald@desy.de) concentrate on the class of events in which a UHE nu scatters from a big bang relic neutrino (cosmic neutrino background in analogy to the cosmic microwave background mentioned above) and creates a Z boson (typically a carrier of the weak nuclear force) which in turn decays into a spray of particles (a "Z burst") among which there are protons and photons.

The latter can be detected on Earth as extended air showers and identified as originating from Z bursts rather than from astrophysical production sites ("ordinary" cosmic rays) such as active galaxies by the shape of their energy spectrum.

Working with existing cosmic-ray spectra, and using their models, these researchers actually produce an estimate for the mass of the relic neutrinos. If the background of ordinary cosmic rays is coming from our galactic halo, then the mass estimate is 2.75 eV. If the ordinary cosmic-rays are extragalactic in origin, then the mass estimate is 0.26 eV (Fodor, Katz, Ringwald, Physical Review Letters, 29 April 2002; for more background see preprint hep-ph/0203198.)