Continuum-coupling effects of final state interactions

Nuclear reactions that produce two or more light ejectiles are subject to modulation by open-quotes final stateclose quotes interactions among the ejectiles. The final state of the reaction is in the simplest cases a system of three interacting particles containing a residual nucleus and two ejectiles. Two common approaches to analyzing the wavefunction of this three-body system are (1) the exact Faddeev three-body theory and (2) the old Watson-Migdal (WM) theory. Since the Faddeev approach is too difficult for routine applications, experimentalists usually fall back on WM approximations. The present work is intended to test the WM approach by comparing WM results for a typical reaction with results from coupled channels (CC) models, i.e., a simplified Faddeev theory. The case treated is the neutron transfer reaction 28Si(3He,2p)29Si* at 33 MeV, studied experimentally at Manchester University. Since the protons are emitted with low relative energy, we expect the threshold 1S0 resonance in the two nucleon system to produce important final state interaction effects. One particular concern is to learn how the p-p resonance is modified by the simultaneous interaction of the protons with the residual nucleus B. The WM approximation assumes the p-B interactions do not affect the p-p resonance, as if the resonance were so narrow that it determines by itself the energy sharing between the relative motion of the two protons and the motion of their center of mass. Under the WM approximation the final state wavefunction factors, one factor for the relative motion of the proton and the other for the center of mass motion. A CC calculation treats the wavefunction as a linear combination of factors of the kind described above, allowing a variety of possible energy sharing in each final state. Our CC calculations show that coupling between the exit channels sharpens and enhances the resonance