Mesonic Atom

an atom in which one of the electrons of the atomic shell is replaced by a negatively charged meson, more accurately, by a μ- muon or by a π- or K-meson. The existence of mesonic atoms was predicted by the American physicist J. Wheeler in 1949, soon after the discovery of π- mesons. The existence of atoms in which an electron is replaced by a Σ-hyperon, a Σ-hyperon, or an antiproton was proven in 1970. The radii of mesonic atoms in the unexcited state are r_μ = 5.3 X 10 -9/mZ cm, where Z is the atomic number of the nucleus and m is approximately equal to the ratio of the mass of a meson to the mass of an electron.

Mesonic hydrogen atoms are the simplest mesonic atoms. They consist of a hydrogen nucleus and a negatively charged meson. Their radii are equal to, respectively, r_π= 2.8 × 10^-11cm, r_π = 2.2 × 10^-11cm, and r_K = 0.8 × 10^-11 cm. Such neutral systems of small size, just like neutrons, freely penetrate the electron shells of atoms, approach the nuclei of the atoms, and may cause numerous mesonic atomic processes, including the formation of mesonic molecules, the catalysis of nuclear reactions, and the capture of a meson by the nuclei of other atoms. In mesonic atoms, the mesons are located hundreds of times closer to the nucleus than electrons. For example, the radius of the innermost μ^- orbit in a mesonic atom of lead is nearly 50 percent smaller than the radius of the lead nucleus; that is, in a mesonic atom of lead the μ^--muon spends most of the time within the nucleus. This fact enables us to use the properties of mesonic atoms with μ^--muons to study the shape and the dimensions of nuclei and the distribution of the electric charge over the volume of the nucleus. Mesonic atoms containing π^-- and K^--mesons are also used to study strong interactions of elementary particles and the distribution of neutrons in nuclei.

Mesonic atoms are formed when mesons produced in high-energy accelerators are decelerated and stopped in targets made of various substances. The capture of a meson into a mesonic atomic orbit is accompanied by the ejection of one of the atomic electrons, usually an outer electron. For example, if a beam of μ^--muons is directed into a chamber containing liquid hydrogen, the μ^--muons lose their energy in collisions with hydrogen atoms until their energy becomes < 1 kiloelectron volt. In the process, if these muons approach close to the nucleus of the hydrogen atom, they form with it an electric dipole, whose field is unable to hold the atomic electron. Consequently, the hydrogen atom loses its electron and the μ^--muon remains bound to the nucleus (proton, deuteron, or triton). All mesonic atoms are generally formed in highly excited states. A meson subsequently passes into a less excited state of a mesonic atom, releasing energy in the form of γ quanta (mesonic γ radiation) or Auger electrons.

The production of mesonic atoms is affected by the structure of the electron shell of the molecules within which the corresponding mesonic atom is found. This makes it possible to study the electronic structure of molecules by investigating the X-radiation of mesonic atoms and the products of nuclear reactions with the nucleus of a mesonic atom. This field of research has been called mesonic chemistry.