Dissipative dynamics of interacting quantal degrees of freedom in spherical nuclei

A model of quantal Brownian motion in fermionic reservoirs is applied to study the time evolution of a giant isovector dipole mode in 208Pb. The coupled equations of irreversible motion for the excited collective mode and for the nucleon intrinsic degrees of freedom are simultaneously solved and their energies, entropies, and relative populations are observed over a lengthy time interval. The resonant decay can be seen together with the excitation of the Fermi sea towards a non-Fermi asymptotic distribution. It is found that the effective decay rate is smaller than the downwards transition rate, in contrast to the widespread assumption in microscopic models of resonance damping. Diffusion, as well as dissipation, appears as a feature of mutual equilibration