基金Supported by the National Basic Research Programme of China under Grant No 2006CB705500, the National Natural Science Foundation of China under Grant Nos 60744003, 10635040, 10532060 and 10472116, the Special Research Funds for Theoretical Physics Frontier Problems under Grant Nos 10547004 and A0524701, the President Funding of Chinese Academy of Sciences, and the Specialized Research Fund for the Doctoral Programme of Higher Education of China. We thank ZHAO Ming and LIU Jian-Guo for useful discussion.