[1] CEA Saclay, Serv Phys Theor, F-91191 Gif Sur Yvette, France
[2] Univ Roma La Sapienza, INFM, Unita Roma 1, Dipartimento Fis, I-00185 Rome, Italy
[3] SMC, I-00185 Rome, Italy
[4] Ist Nazl Fis Nucl, Sez Roma 1, I-00185 Rome, Italy
[5] Univ Barcelona, Fac Phys, Dept Phys, E-08028 Barcelona, Spain