作者机构:[1]Institute of High Energy Physics, CAS, P.O.Box 918(4), Beijing 100049, China, [2]Department of Physics, Peking University, Beijing 100871, China
相关基金:Supported by National Natural Science Foundation of China (10625521, 10721063, 10775146), China Postdoctoral Science Foundation (20070420526) and Ministry of Education of China. We thank Prof. B. Desplanques for helpful discussions.