baryons

a group of heavy elementary particles with a half-integer spin and a mass not less than the mass of a proton. Protons and neutrons (the particles forming an atomic nucleus), hyperons, and baryon resonances all come under the heading of baryons. The name “baryon” is derived from the fact that the lightest of them, the proton, is 1,836 times as heavy as an electron.

The only stable baryon is the proton; all other baryons are unstable, and by sequential decay are converted into a proton and lighter particles. (The neutron in a free state is an unstable particle; however, in the bound state within atomic nuclei, it is stable.)

Baryons take part in all known elementary interactions: strong, electromagnetic, weak, and gravitational. When a baryon participates in a strong interaction, the result is that the baryon interacts with atomic nuclei.

In any nuclear reaction, if interactions of baryons are involved (for energies below the threshold of production of antibaryons), their total number remains unchanged. Thus, in processes of beta-decay, neutrons and protons in nuclei can be converted into each other (with the emission of electrons and neutrinos or their antiparticles), but their total number is always preserved. Baryon decay inevitably results in the formation of another baryon. No process may be observed in which baryons are converted into lighter particles without the emission of baryons. For example, the processes of decay of a proton into a positron and a photon, of the capture of an atomic electron by the proton of a nucleus with the emission of two photons, or of the conversion of a neutron into an electron and a positively charged pi-meson are not observed—although all of these processes would seem permissible from the standpoint of the laws of conservation of electrical charge, energy, impulse, and angular momentum. But the existence of such processes would result in the instability of matter.

The rules that were deduced were formulated as the law of conservation of baryon number. This law may be stated in a form resembling the law of conservation of electrical charge. If the baryon is assigned a specific charge, called baryon charge (B), while taking into account the fact that the charge is absent in the other particles (photons, neutrinos, electrons, and mu-mesons, for all of which B = 0), then the law of conservation of baryon number takes the form of a law of conservation of baryon charge.

In extremely high-energy interactions of the baryon, the production of antibaryons is possible. The law of conservation of baryon number, or baryon charge, is extended to processes involving antibaryons, if it is assumed that the baryon charges of the antibaryon and baryon are of opposite signs (as follows from the general principles of quantum field theory). If the baryon charge of the baryon is set equal to one (B = 1), then for the antibaryon B equals -1, and the baryon charge of a system of particles is simply equal to the difference between the number of baryons and antibaryons in this system. One of the manifestations of the law of conservation of baryon charge is the fact that the formation of antibaryons is always accompanied by the production of additional baryons, a process called annihilation and pair production.

There is a hypothesis concerning the existence of a fundamental analogy between electric and baryon charges. Just as an electric charge is the source of an electromagnetic field, a baryon charge can be considered the source of a field of strong interaction. The electromagnetic interaction of charged particles is realized owing to their exchange of uncharged particles, called photons; analogously, the strong interaction of baryons, for example of protons and neutrons, is due to their exchange of mesons, which are particles having no baryon charge.