[1] Univ Sci & Technol China, Interdisciplinary Ctr Theoret Study, Hefei 230026, Anhui, Peoples R China
[2] Univ Nottingham, Sch Phys & Astron, Univ Pk, Nottingham NG7 2RD, England
[3] Univ Stavanger, Fac Sci & Technol, N-4036 Stavanger, Norway
[4] Peng Huanwu Ctr Fundamental Theory, Hefei 230026, Anhui, Peoples R China
[5] Imperial Coll, Blackett Lab, Theoret Phys, London SW7 2AZ, England