Diabatic and adiabatic collective motion in a model pairing system

Large amplitude collective motion is investigated for a model pairing Hamiltonian containing an avoided level crossing. A classical theory of collective motion for the adiabatic limit is applied utilizing either a time-dependent mean-field theory or a direct parametrization of the time-dependent Schrödinger equation. A modified local harmonic equation is formulated to take account of the Nambu-Goldstone mode. It turns out that in some cases the system selects a diabatic path. Requantizing the collective Hamiltonian, a reasonable agreement with an exact calculation for the low-lying levels is obtained for both weak and strong pairing force. This improves on results of the conventional Born-Oppenheimer approximation.