[1] Ist Nazl Fis Nucl, Sez Pavia, I-27100 Pavia, Italy
[2] SSC ITEP, Moscow, Russia
[3] Ist Nazl Fis Nucl, Sez Padova, Padua, Italy
[4] Univ Pavia, Dipartimento Fis Nucl & Teor, I-27100 Pavia, Italy
[5] Univ Padua, Dipartimento Fis G Galilei, Padua, Italy