Within the framework of an exactly solvable model of three particles interacting through zero-range forces it is shown that the high-energy scattering with high momentum transfer is due to collective phenomena. The spectrum of particles scattered through 180° is compared with the results of impulse approximation.
The exact solution is presented of the scattering problem of three equal particles interacting in one-dimension via two- and/or three-body inverse-square potentials. Both the classical and the quantal problems are treated. It is shown that the outcome of this scattering problem is an extremely simple relation between initial and final momenta, the latter being univocally determined by the former even in the quantal case. The solvability of the problem, and the simple results just mentioned, are peculiar to the equal particle case.
Within the framework of scattering integral equations in momentum space, we present numerical results of scattering of three identical bosons at low energies in two dimensions for short-range separable potentials. An analysis of the present numerical results reveals the three-particle scattering observables to be independent of potential shape provided the low-energy two-particle binding energy and scattering length are held fixed throughout the investigation. We think that the present conclusion of model independence will be valid for any potential, local or nonlocal, whose range is much smaller than the size of the two-particle bound state.
The scattering and bound states of a system of three resonantly interacting particles are analyzed. The characteristics of the scattering and the bound states are shown to be independent of the particular nature of the two-particle forces; instead, these characteristics are governed simply by the fact that resonances occur. The resonances lead to an effective long-range interaction among the three particles. The physical characteristics of the system obey a scaling law. Explicit equations are derived for the particle pair scattering length, for the particle pair scattering cross section at and above the threshold for disintegration of the three-particle system, and for the ternary-collision amplitude at zero energy. The properties of the system are compared with those of a system of two resonantly interacting particles.
The exactly solvable Luttinger model for a system of one-dimensional, massless, two-component fermions interacting via general two-body forces is used as a test case for a systematically generated sequence of decoupling approximations commonly referred to as random-phase approximations (RPA) of which the simple RPA is the lowest. It is found, irrespective of both the nature and strength of the interaction, that the exact (collective) spectrum, as well as the exact correlation energy, can already be obtained in the simple RPA, and furthermore, that the next higher RPA gives a vanishing correction to these quantities. It is also shown, quite in contradistinction to the exact result, that the free-gas momentum distribution nk persists in the second RPA as well as in the first, thereby eliminating the higher RPA as a possible approach to the calculation of nk. Thus, in the context of this particular model, although the first few orders of RPA are highly effective for calculating the excitation spectrum and correlation energy, they are sensibly powerless in their ability to predict any variation in structure beyond that of the free-gas distribution, a result which most probably holds as well in the realistic three-dimensional case.