The deformation dependence of level densities in the configuration-interaction shell model

The auxiliary-field quantum Monte Carlo (AFMC) method has enabled the microscopic calculation of nuclear level densities from the underlying Hamil- tonian. However, AFMC is applied within the rotationally invariant framework of the configuration-interaction (CI) shell model, while deformation arises in the framework of a mean-field approximation that breaks rotational invariance. We review a recent method to study deformation in the CI shell model without invoking a mean-field approximation. Using a Landau-like expansion of the logarithm of the distribution of the quadrupole deformation tensor in the so- called quadrupole invariants, we determine the dependence of this distribution on intrinsic deformation. We can then calculate the dependence of nuclear state densities on intrinsic deformation. The method is demonstrated for a chain of even-mass samarium nuclei.