Damping of the continuum response from 2p-2h excitations

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Abstract
The nuclear response function in the random-phase-approximation theory may be substantially altered at high excitation energy due to coupling of 1p-1h to 2p-2h or higher excitations. In the continuum region of quasi-elastic scattering, this coupling can be included by inserting into the random-phase-approximation Green’s function a complex self-energy for the p-h states, which depends only on their energy, spin (S), and isospin (T). This allows the full response to be expressed directly as an integral over the random-phase-approximation response. The self-energies, which can be determined from empirical single-particle spreading widths, are large in the spin and isospin channels, but small in the T,S=0,0 channel, giving rise to substantial damping in all channels except 0,0. We apply this method to calculate damping of the random-phase-approximation surface response, and discuss the consequences for cross sections and spin observables in quasi-elastic (p,p’) and (n,p) reactions.