Optical-Model Description of Low-Energy Excitations in Nuclear Matter

The decay of single-particle excitations in nuclear matter is calculated in the low-density-gas approximation (independent-pair approximation) using a separable nucleon-nucleon interaction. For low-energy excitations this decay is describable in terms of an optical model during a time interval which is longer than 5×{}10^-23 sec but shorter than 5×{}10^-21 sec. In contrast to earlier calculations, the applicability of the optical model is verified and the mean energy of an excitation is computed prior to the evaluation of the optical-model parameters. Previous calculations of these parameters are shown to lead either to a circumstance in which an optical-model description of the decay is inapplicable or to an excitation whose mean energy is far off the energy shell.