Langevin dynamics in 4-dimensional model of nucleus-nucleus collisions

Description of a realistic dynamic model of nucleus-nucleus collisions and fusion reactions, combined with stochastic effects (fluctuations) is given. We solve Langevin equations of motion in which stochastic white noise term is added to deterministic conservative and dissipative forces. The equations of motion are solved in 4-dimensional configuration space including three geometrical variables defining shape of the system and one variable defining charge asymmetry. Dissipative forces are calculated assuming one-body dissipation mechanism. Two sources of fluctuations are considered: thermal fluctuations determined by the fluctuation-dissipation theorem (Einstein relation) and fluctuations associated with exchange of nucleons. The width of the thermal fluctuations turns out to dominate stochastic effects in fusion reactions. It is shown that in near-threshold fusion reactions, the fluctuations remove flux from reseparation processes and direct it to fusion (and vice versa). Consequently, fluctuations enhance sub-barrier fusion reactions and lower the effective fusion threshold.