Dynamics of quantum equilibration in low-energy collisions

Abstract:  A complete description of the interplay between equilibration, dissipation, and fluctuation mechanisms in interacting quantum many-body systems is an open problem in physics at every scale [1]. In nuclear physics, equilibration of mass and neutron-to-proton asymmetries is studied in collisions of atomic nuclei in which nucleons can be transferred between initially isolated nuclei [2]. Nucleon transfer is also expected to be a source of dissipation by transforming kinetic energy into internal excitation [3]. However, since we are dealing with quantum systems there are quantal effects that influence the equilibration process. Among these are the shell effects and deformations. For low-energy collisions time-dependent Hartree-Fock (TDHF) [4] theory and its extensions provide a microscopic approach to study these effects and processes. In this presentation, we shall first discuss the collision dynamics to demonstrate some of these effects. After that we shall propose a universal measure to investigate the timescales of equilibration, dissipation, and fluctuation mechanisms [5]. Despite direct comparisons between wildly different systems spanning light and heavy nuclei at a range of collision energies, common timescales were found for each of the studied quantities. The longest process by far is mass equilibration, with a general equilibration time of 2x10-20s. Timescales for neutron-to-proton equilibration, mass fluctuation build up, kinetic energy dissipation, and angular momentum dissipation however are found to be on the order of 10-21s, an order of magnitude faster than that of mass equilibration. This vast separation implies the relative independence of dissipation mechanisms on mass equilibration, and that the primary generator of dissipative effects is fast nucleon exchange between interacting fragments. *This work has been supported by the U.S. DOE under grant No. DE SC0013847 with Vanderbilt University. [1] Eisert, M. Friesdorf, and C. Gogolin, Quantum many-body systems out of equilibrium, Nat. Phys. 11, 124 (2015). [2] A. Jedele, A. B. McIntosh, K. Hagel et al., Characterizing Neutron-Proton Equilibration in Nuclear Reactions with Subzeptosecond Resolution, Phys. Rev. Lett. 118, 062501 (2017). [3] Randrup, Mass transport in nuclear collisions, Nucl. Phys. A 307, 319?348 (1978). [4] C. Simenel and A. S. Umar, Heavy-ion collisions and fission dynamics with the time-dependent Hartree-Fock theory and its extensions, Prog. Part. Nucl. Phys. 103, 19?66 (2018). [5] C. Simenel, K. Godbey, and A. S. Umar, Phys. Rev. Lett. 124, 212504 (2020).