[1] Univ Pavia, Dipartimento Fis Nucl & Teor, I-27100 Pavia, Italy
[2] Ist Nazl Fis Nucl, Sez Pavia, I-27100 Pavia, Italy
[3] CERN, Div Theory, CH-1211 Geneva 23, Switzerland
[4] Univ Ferrara, Dipartimento Fis, I-44100 Ferrara, Italy
[5] Ist Nazl Fis Nucl, Sez Ferrara, Ferrara, Italy