Particle-Number Reprojection in the Shell Model Monte Carlo Method: Application to Nuclear Level Densities

We introduce a particle-number reprojection method in the shell model Monte Carlo that enables the calculation of observables for a series of nuclei using a Monte Carlo sampling for a single nucleus. The method is applied to calculate nuclear level densities in the complete (pf+g9/2)-shell using a good-sign Hamiltonian. Level densities of odd- A and odd-odd nuclei are reliably extracted despite an additional sign problem. Both the mass and the Tz dependence of the experimental level densities are well described without any adjustable parameters. The odd-even staggering observed in the calculated backshift parameter follows the experimental data more closely than do empirical formulas.