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MOST IMPORTANT SCIENTIFIC DISCOVERIES
Classification of elementary particles. History and essence of scientific discovery
Directory / The most important scientific discoveries “How many elementary particles have been discovered so far?” Regge asks in his book on physics. “Judging by the thickness of brief reference books describing their properties and which are in circulation among physicists, then several hundred. Many of these particles are collected in families similar to families of nucleons or pions. These families play a role comparable to that of the periodic system of Mendeleev, so useful in chemistry. But it is precisely this similarity that suggests that we are engaged in the classification of objects similar to atoms, and not at all elementary. One way or another , but the search for truly elementary constituents of matter had already begun again, and by 1963 it had become clear that particles should be grouped into larger families. Ancient Greek philosophers attributed exceptionally regular and symmetrical forms to atoms. Although real atoms are very far from this, the idea that the concept of symmetry should play an important role in physics remains. The classification of particles by families just reflects the existence of some kind of symmetry in nature ... " The physics of elementary particles in the fifties was at the stage of formation. The main means of experimental research in this branch of physics were accelerators, "shooting" a beam of particles into a stationary target: when the incident particles collide with the target, new particles were born. With the help of accelerators, experimenters managed to obtain several new types of elementary particles, in addition to the already known protons, neutrons and electrons. Theoretical physicists tried to find some scheme that would allow classifying all new particles. Scientists have discovered particles with unusual (strange) behavior. The rate of birth of such particles as a result of certain collisions indicated that their behavior is determined by the strong interaction, which is characterized by speed. Strong, weak, electromagnetic and gravitational interactions form four fundamental interactions that underlie all phenomena. At the same time, strange particles decayed for an unusually long time, which would be impossible if their behavior were determined by the strong interaction. The decay rate of the strange particles seemed to indicate that this process was determined by a much weaker interaction. On the solution of this most difficult task, and focused his attention Gell-Mann. Murray Gell-Mann was born on September 15, 1929 in New York and was the youngest son of emigrants from Austria Arthur and Pauline (Reichstein) Gell-Mann. At the age of fifteen, Murry entered Yale University. He graduated in 1948 with a Bachelor of Science degree. He spent the following years as a graduate student at the Massachusetts Institute of Technology. Here, in 1951, Gell-Mann received his Ph.D. in physics. After a year's stay at the Princeton Institute for Basic Research (New Jersey), Gell-Mann began working at the University of Chicago with Enrico Fermi, first as a lecturer (1952–1953), then as an assistant professor (1953–1954) and as an associate professor (1954–1955). In 1955, Gell-Mann became an associate professor on the faculty at Caltech. He chose the concept known as charge independence as the starting point of his constructions. Its essence lies in a certain grouping of particles, emphasizing their similarity. For example, despite the fact that the proton and neutron differ in electrical charge (the proton has a charge of + 1, the neutron - 0), in all other respects they are identical. Therefore, they can be considered as two varieties of the same type of particles, called nucleons, having an average charge, or center of charge, equal to 1/2. It is customary to say that a proton and a neutron form a doublet. Other particles can also be included in similar doublets or in groups of three particles called triplets, or in "groups" consisting of only one particle, called singlets. The general name for a group consisting of any number of particles is a multiplet. All attempts to group strange particles in a similar way have failed. Developing his scheme for their grouping, Gell-Mann discovered that the average charge of their multiplets differs from the average charge of nucleons. He came to the conclusion that this difference could be a fundamental property of strange particles and proposed introducing a new quantum property called strangeness. For algebraic reasons, the strangeness of a particle is equal to twice the difference between the average multiplet charge and the average nucleon charge + 1/2. Gell-Mann showed that strangeness is conserved in all reactions involving the strong force. In other words, the total strangeness of all particles before the strong interaction must be absolutely equal to the total strangeness of all particles after the interaction. Strangeness conservation explains why the decay of such particles cannot be determined by the strong interaction. When some other non-strange particles collide, strange particles are produced in pairs. In this case, the strangeness of one particle compensates for the strangeness of the other. For example, if one particle in a pair has strangeness +1, then the strangeness of the other is -1. That is why the total strangeness of non-strange particles, both before and after the collision, is equal to 0. After the birth, strange particles fly apart. An isolated strange particle cannot decay due to the strong interaction if its decay products must be particles with zero strangeness, since such a decay would violate the conservation of strangeness. Gell-Mann showed that the electromagnetic force (whose characteristic time lies between the times of the strong and weak interactions) also retains strangeness. Thus, strange particles, having been born, survive until decay, determined by the weak interaction, which does not preserve strangeness. The scientist published his ideas in 1953. In 1961, Gell-Mann discovered that the system of multiplets he proposed to describe strange particles could be included in a much more general theoretical scheme, which allowed him to group all strongly interacting particles into "families." The scientist called his scheme the eightfold path (by analogy with the eight attributes of a righteous life in Buddhism), since some particles were grouped into families with eight members each. The particle classification scheme he proposed is also known as octal symmetry. Soon, independently of Gell-Man, a similar classification of particles was proposed by the Israeli physicist Yuval Neeman. The American scientist's eightfold path is often compared to Mendeleev's periodic system of chemical elements, in which chemical elements with similar properties are grouped into families. Like Mendeleev, who left some empty cells in the periodic table, predicting the properties of yet unknown elements, Gell-Mann left vacant places in some families of particles, suggesting which particles with the right set of properties should fill the "voids". His theory received partial confirmation in 1964, after the discovery of one of these particles. In 1963, while a visiting professor at the Massachusetts Institute of Technology, Gell-Mann discovered that the detailed structure of the eightfold path could be explained by assuming that each particle involved in the strong interaction consisted of a triplet of particles with a fractional charge the electric charge of the proton. The same discovery was made by the American physicist George Zweig, who worked at the European Center for Nuclear Research. Gell-Mann called fractionally charged particles quarks, borrowing the word from James Joyce's Finnegans Wake ("Three quarks for Mr. Mark!"). Quarks can have a charge of +2/3 or -1/3. There are also antiquarks with charges of -2/3 or +1/3. A neutron with no electric charge consists of one quark with a charge of +2/3 and two quarks with a charge of -1/3 A proton with a charge of +1 consists of two quarks with charges of +2/3 and one quark with a charge of -1 /3. Quarks with the same charge may differ in other properties, which means that there are several types of quarks with the same charge. Thus, various combinations of quarks make it possible to describe all strongly interacting particles. Gell-Mann was awarded the Nobel Prize in Physics in 1969 "for discoveries related to the classification of elementary particles and their interactions." Ivar Waller of the Royal Swedish Academy of Sciences, speaking at the award ceremony, noted that Gell-Mann "has been regarded as a leading scientist in the field of elementary particle theory for more than a decade." According to Waller, the methods proposed by him "are among the most powerful means of further research in elementary particle physics." Author: Samin D.K.
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