ABSTRACT
In 1936 muons µ±, discovered by K. Anderson and S. Niedermayer in cosmic rays, were added to the list of elementary particles. Before 1953, when the first accelerator was constructed (Brookhaven proton synchrotron with maximum energy 3 Gev), elementary particle investigation were intimately connected with cosmic ray investigation. In 1947 the group by S. Powell discovered pi+ and pi−-mesons. The situation with elementary particles already became not entirely simple. The electrons and the nucleons are necessary to build atoms. The photons and the pions play the role of the carriers of the electromagnetic and nuclear forces, respectively. An electron antiparticle, a positron, can be viewed as a delicate hint (for the experienced mind) on the existence of antimatter searching, which should be continued until antinucleons are found. However, what does one do with muons, which do not find their place in this world scheme? In the late 1940s-early 1950s a real demographic explosion had occurred in the elementary
particle world. A whole zoo of new particles, called “strange” particles, had been discovered. A main peculiarity of those particles and the ones, discovered later, is that they are not the component of matter observed. They live for a very short time and decay into stable particles (protons, electrons, photons, and neutrinos). First particles from this group, K+- and K−-mesons, Λ-hyperons were discovered in cosmic rays, the next ones-in accelerators. From the early 1950s accelerators become the main tool to investigate matter microparticles. Accelerators’s energy is growing and the tendency for increasing the number of fundamental particles becomes more and more apparent. Nowadays the list of elementary particles has become tremendously large ∼ 400. Properties of discovered elementary particles prove to be unusual in many respects. To describe them, characteristics taken from classical physics, such as electric charge, mass, momentum, angular moment, and magnetic moment, proved to be insufficient. It was necessary to introduce many new quantum numbers, having no classical analogs, which we call internal quantum numbers. The first reason for their introduction deals with additional degrees of freedom of elementary particles. The second reason is caused by striving to explain the nonobservation of some “acceptable” reactions∗
Particles, Fields, and Quantum
by means of the existence of an internal symmetry that leads to the conservation of a corresponding charge. The latter circumstance carries out the generous dispensation of the conserved charges to some groups of particles, according to the following principle: “dynamic symmetry corresponds to a closed channel of an acceptable reaction.” It is possible, that a reader, being experienced by any sort of direct and inverse theorems about the existence and uniqueness of the solutions, will feel no deep satisfaction. As quieting reason, one may call attention to aesthetic attractiveness of the symmetric approach in all spheres of our life (it is hardly probable that the statue of Venus from Milos being deprived of symmetry could find a place within the walls of the Louvre). Let us proceed to the classification of known particles. Elementary particles are divided
into three categories:
(i) Hadrons, which participate in the strong, weak, and gravitation interactions. Being electrically charged they participate in electromagnetic interaction too. (ii) Leptons, which do not participate in the strong interaction. (iii) Field quanta, which carry the strong, electromagnetic, and weak interactions.
