Isospin-symmetry violation. Collective treatment of the I.A.S. Original Research Article

We study the effects of isospin symmetry violation in the description of the isobaric analogue state (I.A.S.). The sources of such violation are either spurious (like isospin violations induced by the choice of the independent-particle basis), or have a physical origin (as those arising from the presence of isospin violating terms in the residual interactions). We perform a treatment based on the use of collective variables. The restoration of the symmetry is enforced at the collective level, in order to calculate physical isospin violating terms within an isospin conserving basis. The method is illustrated for a schematic model and for realistic single-particle model spaces. In the last situation, we obtain an excellent agreement with data around A=208, both for the energy and width of the I.A.S.