1. Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
2. School of Physics, Peking University, Beijing 100871, P. R. China
3. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
4. Department of Physics, Tsinghua University, Beijing 100084, P. R. China
5. Joint Institute for Nuclear Research, RU-141980 Dubna, Russian Federation