1. The School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, P. R. China
2. School of Physics, Nankai University, Tianjin 300071, P. R. China
3. Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
4. School of Physics, Peking University, Beijing 100871, P. R. China