1. Department of Physics, Columbia University, 538 W. 120th Street, New York, NY 10027, USA
2. Department of Physics and Astronomy, Iowa State University, Ames, IA 50010, USA
3. Nuclear Science Division, MS 70R0319, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
4. Institute of Particle Physics, Huazhong Normal University, Wuhan 430079, China