Quasi-α-particle mechanism of pion double charge exchange on light nuclei

A microscopic theory of π-meson double charge exchange (DCE) on light nuclei has been suggested and developed on the supposition that the corresponding elementary process proceeds by quasi-α-particle formation within the nucleus. Light nuclei consisting of both α-particles and α-particles and clusters of other kinds are considered. To describe the bound state of the quasi-α- particle and the continuum spectrum state of four identical nucleons, the four-body hyperspherical basis has been applied, while to obtain the wave functions of the centers of mass of the cluster relative motion we solve either the three-body Schrödinger equation (in the case of a three-body cluster configuration) or the two-body Schrödinger equation (in the case of a two-body cluster configuration). The reactions π^± + ¹²C → π + 4p(4n) + 2α, π⁻ + ⁷Li → π⁺ + 4n + ³H, π^± + ⁶Li → π + 4p(4n) + n + p, π^± + ⁶Li → π + 4p(4n) + d are investigated. It is shown that the effect of the final-state interaction between the four nucleons emitted by the nucleus in the process of π-meson DCE is rather important. The available experimental data on the ⁷Li nucleus can be explained quite satisfactorily on the supposition that this nucleus has a two-body cluster structure, and, hence, the π-meson DCE process occurs only on the α-particle.The differential and total cross sections of the reactions under investigation calculated as functions of the incident pion energy are essentially different for different nucleon-nucleon potentials. Experimental study of DCE on α-particle nuclei is shown to be a timely problem.