Astroparticle Physics

84 6 Primary Cosmic Raysantiparticlesin primary cosmic rays¯p generationastronomical sources in the immediate neighbourhood ofour galaxy, one might resort to the assumption that new andso far unknown elementary particles might be responsiblefor the events exceeding 6 × 10 19 eV, if the AGASA resultsare confirmed, e.g., by the Auger experiment.Antiparticles are extremely rare in primary cosmic rays.The measured primary antiprotons are presumably generatedin interactions of primary charged cosmic rays withthe interstellar gas. Antiprotons can be readily producedaccording top + p → p + p + p +¯p (6.7)e + generationprimary electronsstars of antimatter?(compare Example 1, Chap. 3), while positrons are mosteasily formed in pair production by energetic photons (compareExample 4, Chap. 3). The flux of primary antiprotonsfor energies > 10 GeV has been measured to beN( ¯p)N(p) ∣ ≈ 10 −4 . (6.8)>10 GeVThe fraction of primary electrons in relation to primaryprotons is only 1%. Primary positrons constitute only 10%of the electrons at energies around 10 GeV. They are presumablyalso consistent with secondary origin.One might wonder whether the continuous bombardmentof the Earth with predominantly positively chargedparticles (only 1% are negatively charged) would lead to apositive charge-up of our planet. This, however, is not true.When the rates of primary protons and electrons are compared,one normally considers only energetic particles. Thespectra of protons and electrons are very different with electronspopulating mainly low-energy regions. If all energiesare considered, there are equal numbers of protons and electronsso that there is no charge-up of our planet.To find out whether there are stars of antimatter in theuniverse, the existence of primary antinuclei (antihelium,anticarbon) must be established because secondary productionof antinuclei with Z ≥ 2 by cosmic rays is practicallyexcluded. The non-observation of primary antimatter withZ ≥ 2 is a strong hint that our universe is matter dominated.The chemical composition of high-energy primary cos-mic rays (> 10 15 eV) is to large extent an unknown territory.If the current models of nucleon–nucleon interactions areextrapolated into the range beyond 10 15 eV (correspondingto a center-of-mass energy of > ∼ 1.4 TeV in proton–protonchemical compositionof high-energy cosmic rays