Influence of Breakup Channels in Elastic Scattering of Composite Particles

We treat $d-\alpha$ elastic scattering, using a simple, physically motivated approximation to include the effects of the ${\mathrm{He}}^4(d, np){\mathrm{He}}^4$ (breakup) channel. This article extends and amplifies preliminary results published earlier. A treatment of the breakup amplitude is given, in addition to a more complete derivation of the dispersion relation for the elastic amplitude. We are able to obtain a closed-form solution for the elastic particle-wave amplitudes in the one-pole approximation. We compare the elastic phase shifts parametrized in this way with experiment, obtaining good agreement. We thus estimate with some confidence the contribution arising from the inclusion of the breakup channel. Since we treat the (tightly bound) ${\mathrm{He}}^4$ nucleus as an elementary particle, our model may be regarded as representing scattering on a polarizable targer. From this viewpoint we discuss the implications our findings may have for several nuclear structure and reaction theories.