Astroparticle Physics

4.6 Propagation and Interactions of Astroparticles in Galactic and Extragalactic Space 59tergalactic space does not attenuate the neutrino flux, andmagnetic fields do not affect their direction; therefore, neutrinospoint directly back to their sources.The matter density in our galaxy, and particularly in intergalacticspace, is very low. This signifies that the ionizationenergy loss of primary protons traveling from theirsources to Earth is extremely small. Protons can, however,interact with cosmic photons. Blackbody photons, in particular,represent a very-high-density target (≈ 400 photons/cm 3 ). The energy of these photons is very low, typically250 µeV, and they follow a Planck distribution (Fig. 4.8), seealso Chap. 11 on ‘The Cosmic Microwave Background’.The process of pion production by blackbody photonsinteracting with high-energy protons requires the protonenergy to exceed a certain threshold (Greisen–Zatsepin–Kuzmin cutoff). This threshold is reached if photo–pionproduction via the ∆ resonance is kinematically possible inthe photon–proton center-of-mass system (p+γ → p+π 0 ).If protons exceed this threshold energy, they quickly losetheir energy and fall below the threshold. The GZK cutofflimits the mean free path of the highest-energy cosmic rays(energy > 6 × 10 19 eV) to less than a few tens of megaparsecs,quite a small distance in comparison to typicalextragalactic scales. Of course, energetic protons lose alsoenergy by inverse Compton scattering on blackbody photons.In contrast to π 0 production via the ∆ resonance thisprocess has no threshold. Moreover, the cross section varieslike 1/s, i.e., with the inverse square of the available centerof-massenergy. Compared to the resonant π 0 productionthe cross section for inverse Compton scattering of protonson blackbody photons is small and therefore has no significantinfluence on the shape of the primary proton spectrum.A further possible process, p + γ → p + e + + e − ,eventhough it has a lower threshold than p + γ → p + π 0 , doesnot proceed through a resonance, and therefore its influenceon the propagation of energetic protons in the dense photonfield is of little importance. In addition, primary protons(charged particles) naturally interact with the galactic andextragalactic magnetic fields as well as the Earth’s magneticfield. Only the most energetic protons (energy ≫ 10 18 eV),which experience a sufficiently small magnetic deflection,can be used for particle astronomy.Photons are not influenced by magnetic fields. They do,like protons, however, interact with blackbody photons tocreate electron–positron pairs via the γγ → e + e − process.protons and nucleiFig. 4.8Blackbody spectrum of cosmicmicrowave background photonsGZK cutoffmagnetic deflection