Consider the process γ + p → p + π0 (π0 photoproduction) with the proton at rest.
(a) Find the minimum energy of the photon Eγ. The Universe is filled by ‘background electromagnetic radiation’ at the temperature of T = 3 K, and photons with energy Eγ,3K ≈ 1 meV.
(b) Find the minimum energy Ep of the cosmic-ray protons needed to induce π0 photoproduction
(c) If the cross-section, just above threshold, is σ = 0.6 mb and the background photon density is ρ ≈ 108 m-3, find the attenuation length. Is it small or large on the cosmological scale?
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