Astroparticle Physics

776 Primary Cosmic Rays“It will be found that everything dependson the composition of the forces with whichthe particles of matter act upon one another,and from these forces, as a matter of fact, allphenomena of nature take their origin.”R. J. BoscovichCosmic rays provide important information about highenergyprocesses occurring in our galaxy and beyond. Cosmicradiation produced in the sources is usually called primordialcosmic rays. This radiation is modified during itspropagation in galactic and extragalactic space. Particles ofgalactic origin pass on average through a column density of6g/cm 2 before reaching the top of the Earth’s atmosphere.Of course, the atmosphere does not really have a ‘top’ butit rather exhibits an exponential density distribution. It hasbecome common practice to understand under the top of theatmosphere an altitude of approximately 40 km. This heightcorresponds to a residual column density of 5 g/cm 2 correspondingto a pressure of 5 mbar due to the residual atmosphereabove altitudes of 40 km. Cosmic rays arriving unperturbedat the Earth’s atmosphere are usually called primarycosmic rays.Sources of cosmic rays accelerate predominantly chargedparticles such as protons and electrons. Since all elementsof the periodic table are produced during element formation,nuclei as helium, lithium, and so on can be also accelerated.Cosmic rays represent an extraterrestrial or even extragalacticmatter sample whose chemical composition exhibits certainfeatures similar to the elemental abundance in our solarsystem.Charged cosmic rays accelerated in sources can producea number of secondary particles by interactions in thesources themselves.These mostly unstable secondary particles, i.e., pionsand kaons, produce stable particles in their decay, i.e., photonsfrom π 0 → γγ and neutrinos from π + → µ + + ν µdecays. Secondary particles also emerge from the sourcesand can reach Earth. Let us first discuss the originally acceleratedcharged component of primary cosmic rays.primordial radiationtop of the atmosphereproductionof secondary particles