Elementary Particles and Quarks

Several decades ago the number of elementary particles known was limited, and the system of elementary particles seemed to be comprehensible. Electrons had been known since 1858 as cathode rays, although the name electron was not used until 1881. Protons had been known since 1886 in the form of channel rays and since 1914 as constituents of hydrogen atoms. The discovery of the neutron in 1932 by Chadwick initiated intensive development in the field of nuclear science. In the same year positrons were discovered, which have the same mass as electrons, but positive charge. All these particles are stable with the exception of the neutron, which decays in the free state with a half-life of 10.25 min into a proton and an electron. In the following years a series of very unstable particles were discovered the mesons, the muons, and the hyperons. Research in this field was stimulated by theoretical considerations, mainly by the theory of nuclear forces put forward by Yukawa in 1935. The half-lives of mesons and muons are in the range up to 10 s, the half-lives of hyperons in the order of up to 10 s. They are observed in reactions of high-energy particles. 

Open questions with respect to decay (e.g. conservation of energy and spin) led to Fermi s hypothesis of the existence of another elementary particle, the neutrino, in 1934. This particle should be neutral and have a mass of approximately zero. Due to these properties its detection was very difficult. The first successful experiment was 

Name Symbol Rest mass [u] Electric charge [units] Corresponding antiparticle  

Electric charge and quantum numbers are opposite to those of the corresponding particles. 

Another group of fundamental particles are the leptons (light particles), comprising also three families, electron and electron neutrino, muon and muon neutrino, tau particle and tau neutrino. Properties of the leptons are summarized in Table 3.3. The most important particles of this group are the electron and the electron neutrino, which are both stable. 

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