Multiple alpha-transfer and dynamic Regge poles in anomalous large angle scattering

We investigate the effects on the elastic S matrix of the coupling to ..cap alpha..-transfer channels. On the basis of a separable approximation for the propagator in the transfer channel, we obtain simple expressions for the elastic S matrix which include effects of the coupling to all orders. These expressions are used to study the elastic scattering of /sup 16/O+/sup 28/Si at 180/sup 0/. It is shown that a standard two-step distorted-wave Born approximation treatment of the coupling is unable to explain the order of magnitude of the data. On the other hand, the inclusion of high-order processes leads to a qualitative agreement with the data. This modification in the results was traced down to the appearance of a pole in the elastic S matrix near the real axis. The S matrix calculated here was shown to be consistent with others obtained phenomenologically from fits to the available data. We suggest that the apparent importance of ..cap alpha..-transfer effects in elastic scattering implies that such processes should be relevant in other nuclear reactions involving n..cap alpha.. nuclei