Positron theory

Most particles of interest to physicists and chemists are found to be antisymmetric under permutation. They include electrons, protons and neutrons, as well as positrons and other antiparticles These particles, which are known as Fermions, all have spins of one-half. The relation between the permutation symmetry and the value of the spin has been established by experiment and, in the case of the electron, by application of relativistic quantum theory. 

The actual curve, however, is somewhat modified by Coulomb interaction between the electron or positron and the nucleus. This is allowed for by multiplication with a dimensionless function F(Z p), which leads to a correction factor / for the total decay rate, and it is the product ft that is used for purposes of comparing measured lifetimes with theory. The most rapid decays, with ft = 103 to 104 s, are known as super alio wed . These include 0+ to 0+ decays having A//, 2 = 2 and ft is found experimentally to be close to 3000 s, giving the coupling constant for the Fermi interaction... 

The Bohr criterion k I depends on the projectile speed rather than its kinetic energy. This, together with the fact that Zi — l, implies that for electrons or positrons the validity of semiclassical collision theory becomes... 

When Dirac completed work on his theory in 1928, it was a notable success. Among other things, it explained electron spin, which turned out to be a relativistic effect, rather than something analogous to the spin of a macroscopic object like a top. But the theory also made what seemed to be a very strange prediction. If Dirac s theory was correct, then there had to exist particles that had properties like the electron, but that carried a positive rather than a negative charge. At the time, such particles, called positrons, had never been observed. 

Meanwhile, the electron was found to have a positively charged counterpart called the positron the electron and positron could annihilate each other, with the emission of light quanta. The theory of the electron did in fact predict the existence of such a particle. It was later found that the existence of such opposite particles (antiparticles) was a much more general phenomenon than once surmised. 

In the quantum field theories that describe the physics of elementary particles, the vacuum becomes somewhat more complex than previously defined. Even in empty space, matter can appear spontaneously as a result of fluctuations of Ihe vacuum. It may be pointed, out, for example, that an electron and a positron, or antielectron, can be created out of the void, Particles created in this way have only a fleeting existence they are annihilated almost as soon as they appear, and their pressure can never be detected directly. They are called virtual particles in order to distinguish them from real particles. Thus, the traditional definition of vacuum (space with no real particles in it) holds. In their excellent paper, the aforementioned authors discuss how, near a superheavy atomic nucleus, empty space may become unstable, with the result that matter and antimatter can be created without any input of energy. The process may soon be observed experimentally. 

The next paper was by Dirac on the Theory of the Positron. In the following discussion Niels Bohr made a long intervention on the correspondence principle in connection with the relation between the classical theory of the electron and the new theory of Dirac. 

The hole theory was perceived as a Active mathematical construction and was initially rejected by prominent contemporary physicists such as Pauli and Bohr. The physical reality of antiparticles was not taken seriously even by Dirac himself. In 1931 he wrote about his anti-electron we should not expect to find it in Nature [2]. Surprisingly, the first anti-electrons were discovered already in 1932 by Anderson, who studied cosmic rays in Caltech s magnet cloud chamber. Anderson noticed abnormally bending trajectories indicating the presence of light positively charged particles and, as related by Fowler [3], "could not resist the devastating conclusion that they are caused by positive electrons The first piece of antimatter, a positron, made its physical appearance. 

The prediction, and subsequent discovery, of the existence of the positron, e+, constitutes one of the great successes of the theory of relativistic quantum mechanics and of twentieth century physics. When Dirac (1930) developed his theory of the electron, he realized that the negative energy solutions of the relativistically invariant wave equation, in which the total energy E of a particle with rest mass m is related to its linear momentum V by... 

The positron was subsequently discovered by Anderson (1933) in a cloud chamber study of cosmic radiation, and this was soon confirmed by Blackett and Occhialini (1933), who also observed the phenomenon of pair production. There followed some activity devoted to understanding the various annihilation modes available to a positron in the presence of electrons radiationless, single-gamma-ray and the dominant two-gamma-ray processes were considered (see section 1.2). The theory of pair production was also developed at this time (see e.g. Heitler, 1954). 

Current theories of particle physics predict that, in a vacuum, the positron is a stable particle, and laboratory evidence in support of this comes from experiments in which a single positron has been trapped for periods of the order of three months (Van Dyck, Schwinberg and Dehmelt, 1987). If the CPT theorem is invoked then the intrinsic positron lifetime must be > 4 x 1023 yr, the experimental limit on the stability of the electron (Aharonov et al., 1995). 

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