Nuclear Physics A 732 (2004) 49–70 www.elsevier.com/locate/npe Isospin-symmetry violation. Collective treatment of the I.A.S. D.R. Bes a,b , O. Civitarese c,∗ a Departamento de Física, CNEA, Av. Libertador 8250, (1429) Buenos Aires, Argentina b Departamento de Física, Universidad Favaloro, Belgrano 1723, (1093) Buenos Aires, Argentina c Departamento de Física, UNLP, C.C. 67, (1900) La Plata, Argentina Received 14 October 2003; received in revised form 8 December 2003; accepted 9 December 2003 Abstract 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.  2003 Elsevier B.V. All rights reserved. PACS: 21.60.Jz; 22.40.Hc; 21.60.Fw 1. Introduction In nuclear structure calculations, the treatment of the isospin degree of freedom is fre- quently hampered by use of single-particle bases which violate the isospin symmetry. This fact has produced ambiguities both in the description of isobaric analogue states (I.A.S.) and in the treatment of Fermi transitions in single- and double-beta decay processes. A common source of violations is to only include terms of the type T 0 .τ 0 in the con- * Corresponding author. E-mail address: civitare@fisica.unlp.edu.ar (O. Civitarese). 0375-9474/$ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2003.12.008