We now turn to another very important fact. In addition to the electro magnetic field, there are in atomic nuclei also other fields, responsible for nuclear forces. The fundamental forces that hold the proton and neutron together in the nucleus are neither gravitational nor electromagnetic. The field of nuclear forces, which are tremendously strong, thus displays distinctive characteristics, such as the fact that its particles possess rest mass. This was established in the early theoretical studies made of nuclear forces, which were supposed to be due to electron-antineutrino (or positron neutrino) pairs (Tamm, Ivanenko, and later Heisenberg, A subsequent signal achievement was that of the Japanese physicist Yukawa, who predicted the existence of a new, mesic field that is responsible for the nuclear forces and whose quanta in the free state are the particles called mesons, having an intermediate mass between that of the electron and the nucleon (1935). It proved very difficult to cause the emission of mesons, i.e. , to separate the nuclear field from the nucleons, for the reason that the mesons represent a "strong" coupling, in fact the strongest known as yet between any kind of particles. Quanta of the nuclear field, emitted by atomic nuclei in the collisions between protons or neutrons, were discovered in 1947 by Powell and indeed turned out to be particles with a mass of 274 m (electron masses). As theoretically predicted, they possess integral spin, or, more precisely, zero spin. These particles were named It mesons, or "pions" (the Greek letter a was used to stand for the words "primary" and "Powell").
It is thus fundamentally the pionic field that gives rise to the nuclear forces, in the same way as the electric field between protons and electrons is caused by their charges. The nuclear forces between nucleons are produced by one nucleon emitting a pion and another absorbing it. There are also neutral pions which are slightly lighter, with a mass of 264 m. Great advances have been made in the theory of interaction of nucleons and pions and in the experimental work associated with it, and it is now possible to explain many things about the scattering of pions, their production, and absorption. The theory of nuclear forces, which must be essentially due to n mesons, also elucidated many important points on the interaction between nucleons, in particular the fact that the forces are short-range, charge-independent, and central, and also on the form of spin dependence.
The calculation of the nuclear forces at extremely small ranges is actually quite difficult, owing to the fact that many factors have to be taken into account: the recoil of the nucleon in meson exchange, the exchange of two or more pions, vacuum corrections, relativistic corrections for time differences between individual nucleons, K meson exchange, and, lastly, it is also very important to take into consideration the inner structure of the nucleons themselves.
According to modern views the nucleon (a proton or a neutron) consists of some kind of "kernel", surrounded by a cloud or "fuzz" of pions and K mesons. At very short ranges, therefore, the forces between the pions themselves should come into play, which has only very recently began to be taken into account. Finally, it is possible, as was recently proposed by Sakurai, that the primary field directly associated with the nucleons is some still hypothetical vector field of "intermediate mesons", and that the 7 mesons are only the decay product of the latter. All this shows that the theory of nuclear forces has not yet reached any final form.
As we now understand it then, the atom with its nucleus is constituted of electrons, protons, and neutrons (in actual form), together with the electromagnetic and mesic fields (in latent form). Thus it might seem that the investigation of the structure of matter has now been completed. As it turns outs, however, the structure of matter cannot be explained by these particles alone, because a whole new set of elementary particles, closely associated with the former, has been lately discovered.
First of all, it was found that the neutrons and pions are unstable particles, i. e. , they spontaneously decay, giving birth to new particles that are rat directly involved in the structure of matter. The charged TC mesons invariably decay after an average lifetime of about 2 • sec, producing two new particles: a neutrino or an antineutrino, together with a new meson-type particle, known as the µ meson or "muon" (discovered in cosmic rays in 1937): Neutral muons have never been found. The neutral pion decays within an extremely short time, after about sec, giving rise to two gamma photons: Ito Approximately one ten-thousandth of the pions decay according to the scheme: The free neutron invariably decays after a lifetime of about 12 minutes into a proton, an electron, and an antineutrino: The decay of a neutron within the nucleus depends on the stability of the whole nucleus; the electrons produced in this case are termed beta particles. The reverse process, the absorption of an electron by a nuclear proton, is known as K capture: e_ +p n+ v. The free protons are stable, while an excited nuclear proton decays according to the scheme: giving birth to a new particle, the positron, which is essentially similar to the electron; the positron has the same mass and spin, only its charge and magnetic moment are positive. Further, the positron has the lepton number 1=-1 (in the electron 1=+1) and a preferred positive "helicity", in opposition to the electron. The positron is thus said to be the antiparticle of the electron. Electron-positron pairs may be produced when gamma photons hit atomic nuclei. The µ+ and µ- mesons are also "mirror" antiparticles and have lepton charges of opposite sign.