Astroparticle Physics

6.2 Neutrino Astronomy 87is only conserved for very energetic protons (> 10 19 eV)because otherwise the irregular and partly not well-knowngalactic magnetic fields will randomize their original direction.For these high energies the Greisen–Zatsepin–Kuzmincutoff also comes into play whereby protons lose their energyvia the photoproduction of pions off blackbody photons.For protons with energies exceeding 6 × 10 19 eVthe universe is no longer transparent (attenuation lengthλ ≈ 10 Mpc). As a consequence of these facts, the requirementfor an optimal astronomy can be defined in the followingway:1. The optimal astroparticles or radiation should not be influencedby magnetic fields.2. The particles should not decay from source to Earth.This practically excludes neutrons as carriers unless neutronshave extremely high energy (τneutron 0 = 885.7s;atE = 10 19 eV one has γcτneutron 0 ≈ 300 000 light-years).3. Particles and antiparticles should be different. Thiswould in principle allow to find out whether particlesoriginate from a matter or antimatter source. This requirementexcludes photons because a photon is its ownantiparticle, γ =¯γ .4. The particles must be penetrating so that one can lookinto the central part of the sources.5. Particles should not be absorbed by interstellar or intergalacticdust or by infrared or blackbody photons.These five requirements are fulfilled by neutrinos in an idealway! One could ask oneself why neutrino astronomy hasnot been a major branch of astronomy all along. The factthat neutrinos can escape from the center of the sources isrelated to their low interaction cross section. This, unfortunately,goes along with an enormous difficulty to detectthese neutrinos on Earth.For solar neutrinos in the range of several 100 keV thecross section for neutrino–nucleon scattering isrequirementfor an optimal astronomyneutrino astronomyσ(ν e N) ≈ 10 −45 cm 2 /nucleon . (6.9)The interaction probability of these neutrinos with our planetEarth at central incidence isφ = σN A dϱ≈ 4 × 10 −12 (6.10)(N A is the Avogadro number, d the diameter of the Earth, ϱthe average density of the Earth). Out of the 7 × 10 10 neutri-