1. Department of Physics, University of Siena and INFN, V. Roma 56, 53100 Siena (I), Italy
2. Department of Physics, University of Florence and INFN, V. Sansone 1, 50019 Sesto F.no, Florence (I), Italy
3. INFN sez. di Pisa, Edificio C - Largo B. Pontecorvo 3, 56127 Pisa (I), Italy
4. Department of Physics, University of Pisa and INFN, Largo B. Pontecorvo 3, 56127 Pisa (I), Italy
5. INFN sez. di Firenze, V. Sansone 1, 50019 Sesto F.no, Florence (I), Italy