LABORATORI NAZIONALI DI LEGNARO
The spontaneous symmetry breaking mechanism of the mean field approximation together with the characteristics of the nuclear interaction lead to an easy understanding of many collective properties of the nuclear excitation spectrum. The best known example is that of the breaking of rotational invariance leading to deformed shapes for the ground state and low lying excitations and the concept of rotational bands built upon those band heads. On the other hand, breaking of reflection symmetry leads to octupole deformed shapes and the non-conservation of the parity quantum number. The analogous of rotational bands is the parity doublets observed experimentally, for instance in the 224Ra nucleus. In the same way as the rotational invariance breaking leads to strong electromagnetic transitions among the members of the rotational band, the strong breaking of parity leads to large E3 transition strengths (and enhanced E1) between the members of the parity doublet. In order to compute transition probabilities in an adequate framework the restoration of quantum numbers is necessary. This is done using symmetry restoration techniques based on taking linear combinations of the deformed states after being acted upon by the symmetry operator. All these concepts will be discussed using both results obtained with realistic calculations and results coming from the solution of the simple and analytical LMG model.

DATA: 09-05-2014