1. Physics Department, University of Bologna, CNISM and INFN, Via Irnerio 46, I-40126, Bologna, Italy
2. Depto. de Matemáticas, Univ. Carlos III de Madrid, 28911 Leganés, Madrid, Spain
3. Dipartimento di Scienze Fisiche, University of Napoli and INFN, Via Cinzia, I-80126 Napoli, Italy
4. Physics Department, University of Bologna, CNISM and INFN, V.le B. Pichat 6/2, I-40127, Bologna, Italy