Astroparticle Physics

212 The Standard Modelof Elementary Particles“Most basic ideas of science are essentiallysimple and can usually be expressed in alanguage that everyone understands.”Albert EinsteinOver the last years a coherent picture of elementary particleshas emerged. Since the development of the atomic model,improvements in experimental resolution have allowed scientiststo investigate smaller and smaller structures. Even theatomic nucleus, which contains practically the total mass ofthe atom, is a composite object. Protons and neutrons, thebuilding blocks of the nucleus, have a granular structure thatbecame obvious in electron–nucleon scattering experiments.In the naïve quark parton model, a nucleon consists of threequarks. The onion-type phenomenon of ever smaller constituentsof particles that were initially considered to be fundamentaland elementary, may have come to an end withthe discovery of quarks and their dynamics. While atoms,atomic nuclei, protons, and neutrons can be observed as freeparticles in experiments, quarks can never escape from theirhadronic prison. In spite of an intensive search by numerousexperiments, nobody has ever been able to find free quarks.Quantum chromodynamics, which describes the interactionof quarks, only allows the asymptotic freedom of quarks athigh momenta. Bound quarks that are inside nucleons typicallyhave low momenta and are subject to ‘infrared slavery’.This confinement does not allow the quarks to separatefrom each other.Quarks are constituents of strongly interacting hadronicmatter. The size of quarks is below 10 −17 m. In additionto quarks, there are leptons that interact weakly and electromagnetically.With the resolution of the strongest microscopes(accelerators and storage rings), quarks and leptonsappear to be pointlike particles, having no internal structure.Three different types of leptons are known: electrons,muons, and taus. Each charged lepton has a separate neutrino:ν e , ν µ , ν τ . Due to the precise investigations of the Zparticle, which is the neutral carrier of weak interactions, itis known that there are exactly three particle families withcomposition of atomic nucleicomposition of nucleonsquark confinementquarks and leptons