1. High Energy Accelerator Research Organization (KEK), Ibaraki 305-0801, Japan
2. Department of Physics, Boston University, Boston, Massachusetts 02215
3. European Organization for Nuclear Research (CERN), Geneva 23, CH-1211 Switzerland
4. INFN, Sezione di Roma, and Dipartimento di Fisica dell'Universitá Sapienza, Roma I-00185, Italy