1. Department of Physics; University of Bari and Sezione INFN; 70126 Bari Italy
2. CERN; PH-UAI; 581/R-015 1211 Geneva 23 Switzerland
3. Polytechnic Institute of Coimbra; Coimbra Institute of Engineering; Rua Pedro Nunes 3030-199 Coimbra Portugal
4. Laboratory of Instrumentation and Experimental Particle Physics (LIP); Physics Department University of Coimbra; 3004-516 Coimbra Portugal