[1] Univ Roma Tor Vergata, Dept Phys, Rome, Italy
[2] Ist Nazl Fis Nucl, Sezione Roma 2, Rome, Italy
[3] INRIA, IRISA, Rennes, France
[4] Univ Paris 11, LPT, Orsay, France
[5] Ist Nazl Fis Nucl, Sezione Roma, Rome, Italy
[6] Univ Parma, Dept Phys, I-43100 Parma, Italy
[7] Grp Collegato Parma, Ist Nazl Fis Nucl, Parma, Italy
[8] Ist Nazl Fis Nucl, Sezione Pisa, Pisa, Italy
[9] CERN, Geneva, Switzerland
[10] DESY, NIC, Zeuthen, Germany
[11] Univ Clermont Ferrand, LPC, Clermont Ferrand, France
[12] IN2P3, Clermont Ferrand, France
[13] Univ Ferrara, Dept Phys, I-44100 Ferrara, Italy