Effects of angular momentum projection on the nuclear partition function and the observation of the giant dipole resonance in hot nuclei

Abstract

Procedures for projecting angular momentum in a model describing a hot nucleus that takes into account large-amplitude quadrupole fluctuations are discussed. Particular attention is paid to the effect angular-momentum projection has on the observables associated with the $\gamma$-decay of the giant-dipole resonance (GDR). We also elaborate on which of the different projection methods provides the best overall description of the GDR, including angular distributions. The main consequence of angular-momentum projection is the appearance of an effective volume element in the integrals associated with the thermal average of the physical observables. This effective volume element is controlled by the value of the moments of inertia of the system. In the limit of rigid-body moments of inertia, the effective volume element is found to differ only slightly from the volume element associated with the normalization of the five-dimensional quadrupole oscillator wavefunction in the $\beta$, $\gamma$, and $\Omega$ space, namely $D[\alpha]=\beta^4d\beta sin(3\gamma)d\gamma d\Omega$. In the limit of irrotational flow moments of inertia, the leading behavior in the $\beta$ degree of freedom is $D[\alpha]\propto \beta d\beta$.