The helicity is measured by the relative direction of the spin and the momentum. The beta-decay electrons, which have a preferred negative helicity, thus have a spin with a sense opposite to the motion. The remarkable new property of "helicity", which is most clearly exhibited in neutrinos (which have no rest mass), was discovered in 1956-1957 by the Chinese physicists Lee, Yang, and Wu. The characteristic polarization phenomena resulting from helicity have been studied in detail by Soviet, American, and other physicists (Alikhanov, Sokolov, Kerimov, Touschek, Tolhoek, and others).
Of course, the process of the production of an electron-positron pair may be reversed; the electron and positron are then annihilated and are transformed into photons: (for particles with antiparallel spin), e_± 3y (for particles with parallel spin).
These remarkable processes of the production and annihilation of particles possessing rest mass from and into field quanta that have no rest mass were predicted by Dirac and then observed in practice, in full agreement with the theoretical result, first in cosmic rays (Blackett and Occhialini, 1933) and later by inducing them in the laboratory. It was subsequently found that all the particles may be produced under suitably given conditions, and on the other hand, that all the particles may transform into others, either spontaneously (like the neutron and it meson) or by colliding with another particle of the appropriate kind. Consequently, there is no such thing as a perfectly stable particle.
In addition, it is possible for an electron and a positron to form a metastable system, positronium, in which the electron and the positron move around their common center of gravity. The positronium atom must eventually transform, either into two photons after about sec (in the case of parapositronium), or into three photons after about sec (in the case of orthopositronium).
During the time it exists, the positronium may combine into molecules with other atoms or diffuse through matter to a considerable distance.
Let us now return to the products of a-meson decay, the mesons; the latter decay spontaneously, and their decay reactions are: Thus, it would have also been possible to come to the discovery of positrons by studying the decay of positive muons. Until recently it was quite clear that the decay of muons gives rise to neutrinos and antineutrinos. However, assumptions have been entertained for some time now that there actually exist two types of neutrinos and antineutrinos; the neutrino v and the antineutrino are associated with positron or electron beta-decay or K capture, i. e. , with processes of the type e_ + v, while the other type of neutrino, v', and antineutrino, 7 (sometimes called neutrettos for the sake of distinction) are associated with muon decay. In the summer of 1962 the American physicists Lederman and Steinberger conclusively proved the existence of neutrinos of the other, 4-meson type. Their experiment was based on the fact that the absorption of the ordinary, beta-decay anti neutrinos by protons leads to the production of positrons (as was shown by Reines and Cowan): whereas the neutrinos produced together with µ mesons, on being absorbed give rise to it mesons again, and not to electrons or positrons! The reactions are thus as follows: An important advance was made in the fall of 1955 at Berkeley by Segre and Chamberlain, who, after searching for a long time in cosmic rays and in the laboratory, proved the existence of the antiproton and later of the antineutron. The antiproton is the antiparticle of the proton; it has the
same mass and the same charge, except that its charge is opposite (negative). It has a negative magnetic moment and is designated by the baryon (heavy) number B =-1, while for the proton B = +1. Like the electrons and positrons, when protons and antiprotons collide they may transform into gamma photons, though their transformation into a mesons has a much higher probability. In that case, at moderate energies, about 5 pions are produced on the average: .
These most interesting particles—the beta-decay and the µ-meson neutrinos ( + and their corresponding antiparticles, the antineutrinos ( +vz) —have no rest mass, like the photons, but, unlike the photons which have integral spin (S= 1 2 2c) they possess half-integral spin (S = 2 2 lc), like the electrons. It has been shown with great precision that beta-decay neutrinos lack any rest mass; the mass of the µ-meson neutrinos is either exactly zero, or at the most extremely small. Being devoid of charge or magnetic moment, the neutrinos interact very weakly with other particles and have a high penetrating power. The neutrino differs from the anti neutrino, its antiparticle, by its helicity, which may be represented as the direction of the spin with reference to the direction of motion, as well as by the sign of its lepton charge. Like the photons, the massless neutrinos move at the speed of light. The existence of the neutrino was predicted at the beginning of the thirties by Pauli on the basis of an analysis of the energy balance in beta-decay, and was later indirectly confirmed by the success achieved by the theory of beta-decayformulatedbyF. Perrin and particularly developed by E. Fermi in 1934. Direct experimental evidence was secured much later, in the middle of the fifties, when Cowan and Reines with their colleagues detected a stream of antineutrinos shooting out of a nuclear reactor. The neutrinos were produced in the beta-decay of excited nuclear fragments resulting from the fission of uranium. It proved possible to observe the reaction of antineutrino capture by protons in a bubble chamber, with the transformation of these particles into a neutron and positron: It is interesting to note that an appreciable part, about 10%, of the energy of the Sun and other stars is carried off into space in the form of neutrinos, which are emitted in the thermonuclear reactions occurring in stellar interiors.