Effects of Negative-Energy Propagations in Deuteron-Nucleus Scattering

Abstract

The effects of negative· energy propagations in the effective optical potentials for relativistic deuteron·nucleus elastic scattering are investigated utilizing the Breit approach. First, we consider the effects of the (+ -) and (- +) propagations. They represent that one of the nucleons in the deuteron propagates in its positive·energy state, while another propagates in negative·energy one. Second, the effects of the (- -) propagation, which represents that both the nucleons propagate in their negative· energy states, are studied. It has been found that the characteristic features of the optical potentials near 400 MeV - the wine·bottle·bottom·shape of the central part and the enhanced spin·orbit strength - are pure relativistic effects mainly due to the ( + -) and (- +) propagations, whereas the effects of the (- -) propagation are small. Recently, several efforts have been devoted to investigating the structures of the propagators and their effects in the relativistic nuclear scattering theories. 1 J-S) In the Dirac phenomenology for nucleon-nucleus (N-A) scattering, Cooper and Jennings have found that the negative-energy part of the Feynman propagator has short-ranged structure and plays an important role in producing the relativistic character of the effective optical potentials. 2 ) In the Breit 6 ) and Duffin-Kemmer-Petiau (DKP)1H) theories for relativistic deuteron-nucleus (d-A) scattering, the present author has found that the short-ranged structures of their propagators also contribute consider­ ably to the relativistic characters of d-A optical potentials. 5 ) However, in that work he has not been able to clarify another important aspect of the relativistic scattering theories, that is, the effect of negative-energy propagations. This is partially because that in the Breit or DKP approach short-ranged terms in the propagator are not identical with negative-energy terms, in contrast to the Feynman propagator. For the relativistic scattering of a composite particle, it becomes an important subject to study the effects of the negative-energy propagations of each constituent particle. 1OH2 ) Most of the recent studies of d-A elastic scattering at intermediate energies are inspired by the success of the Dirac phenomenology.9),13),14) Therefore, we want to clarify the effects of the negative-energy propagations of Dirac nucleons in the deuteron. For this purpose, in the present work we will treat d-A elastic scattering within a relativistic folding-model utilizing the Breit equation.