1. Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, New York 11794-3800;
2. Institute of Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwanoha Kashiwa-shi, Chiba 277-8582, Japan;
3. Department of Physics, Tufts University, Medford, Massachusetts 02155;