Astroparticle Physics

166 7 Secondary Cosmic Raysλ γγ ≈ 10 kpc . (7.27)photonic origin?The γ -ray sources have to be relatively near to explain thehigh-energy showers. This would mean that they must beof galactic origin, which appears rather unlikely, becauseof the limited possibility for their parent particles to be acceleratedin our Milky Way up to the highest energies required.High-energy photons, furthermore, would initiateair showers at high altitudes above sea level (≈ 3000 km)due to interactions with the Earth’s magnetic field. Thereforeone would theoretically expect that they would reacha shower maximum at ≈ 1075 g/cm 2 (calculated from sealevel). The event observed by the Fly’s Eye experiment hasa shower maximum at (815 ± 40) g/cm 2 , which is typicalfor a hadron-induced cascade. Photons as candidates for thehighest-energy events can therefore be firmly excluded.Recently, neutrinos were discussed as possible candi-dates for the high-energy events. But neutrinos also encountersevere problems in explaining such events. The ratioof the interaction cross section for neutrino–air and proton–air interactions at 10 20 eV isσ(ν–air)σ(p–air) ∣ ≈ 10 −6 . (7.28)E≈10 20 eVneutrinosQuite enormous neutrino fluxes are required to explain theevents with energies > 10 20 eV. It has been argued that themeasurements of the structure function of the protons atHERA 1 have shown that protons have a rich structure of partonsat low x(x= E parton /E proton ).Eveninviewoftheseresults showing evidence for a large number of gluons in theproton, one believes that the neutrino interaction cross sec-tion with nuclei of air cannot exceed 0.3 µb. This makes interactionsof extragalactic neutrinos in the atmosphere veryimprobable, compare (3.56):rising neutrino cross section?φ = σ(ν–air) N AA d≤ 0.3 µb 6 × 1023g −1 × 1000 g/cm 214≈ 1.3 × 10 −5 (7.29)(N A is the Avogadro number, d is a column density of theatmosphere).1 HERA – Hadron Elektron Ring Anlage at the Deutsches Elektronensynchrotron(DESY) in Hamburg