Chinese Acad Sci, Inst Modern Phys, Lanzhou 730000, Peoples R China.^Univ Sci & Technol China, Hefei 230026, Peoples R China.^Cent China Normal Univ, Inst Particle Phys, Wuhan 430079, Peoples R China.^Univ Calif Berkeley, Lawrence Berkeley Lab, Div Nucl Sci, Berkeley, CA 94720 USA.