Benchmarking mean-field approximations to level densities
Citations Over TimeTop 10% of 2016 papers
Abstract
We assess the accuracy of finite-temperature mean-field theory using as a standard the Hamiltonian and model space of the shell model Monte Carlo calculations. Two examples are considered: the nucleus $^{162}\mathrm{Dy}$, representing a heavy deformed nucleus, and $^{148}\mathrm{Sm}$, representing a nearby heavy spherical nucleus with strong pairing correlations. The errors inherent in the finite-temperature Hartree-Fock and Hartree-Fock-Bogoliubov approximations are analyzed by comparing the entropies of the grand canonical and canonical ensembles, as well as the level density at the neutron resonance threshold, with shell model Monte Carlo calculations, which are accurate up to well-controlled statistical errors. The main weak points in the mean-field treatments are found to be: (i) the extraction of number-projected densities from the grand canonical ensembles, and (ii) the symmetry breaking by deformation or by the pairing condensate. In the absence of a pairing condensate, we confirm that the usual saddle-point approximation to extract the number-projected densities is not a significant source of error compared to other errors inherent to the mean-field theory. We also present an alternative formulation of the saddle-point approximation that makes direct use of an approximate particle-number projection and avoids computing the usual three-dimensional Jacobian of the saddle-point integration. We find that the pairing condensate is less amenable to approximate particle-number projection methods because of the explicit violation of particle-number conservation in the pairing condensate. Nevertheless, the Hartree-Fock-Bogoliubov theory is accurate to less than one unit of entropy for $^{148}\mathrm{Sm}$ at the neutron threshold energy, which is above the pairing phase transition. This result provides support for the commonly used ``back-shift'' approximation, treating pairing as only affecting the excitation energy scale. When the ground state is strongly deformed, the Hartree-Fock entropy is significantly lower than the shell model Monte Carlo entropy at low temperatures because of the missing contribution of rotational degrees of freedom. However, treating the rotational bands in a simple model, we find that the entropy at moderate excitation energies is reproduced to within two units, corresponding to an error in the level density of less than an order of magnitude. We conclude with a discussion of methods that have been advocated as beyond the mean-field approximation, and their prospects to address the issues we have identified.