[1] CERN, EP Dept, CH-1211 Geneva 23, Switzerland
[2] UCL, Math & Phys Sci Fac Off, Gower St, London WC1E 6BT, England
[3] Ecole Polytech Fed Lausanne, High Energy Phys Lab, CH-1015 Lausanne, Switzerland
[4] Photek Ltd, 26 Castleham Rd, St Leonards On Sea TN38 9NS, England
[5] Univ Bristol, HH Wills Phys Lab, Tyndall Ave, Bristol BS8 1TL, Avon, England
[6] Univ Oxford, Dept Phys, Keble Rd, Oxford OX1 3RH, England