[1] Univ Leipzig, Inst Theoret Phys, D-04109 Leipzig, Germany
[2] Univ Regensburg, Inst Theoret Phys, D-93040 Regensburg, Germany
[3] Vrije Univ Amsterdam, Dept Phys & Astron, NL-1081 HV Amsterdam, Netherlands
[4] Univ Edinburgh, Sch Phys, Edinburgh EH9 3JZ, Midlothian, Scotland
[5] DESY, John von Neumann Inst Comp NIC, D-15738 Zeuthen, Germany
[6] Univ Liverpool, Dept Math Sci, Div Theoret Phys, Liverpool L69 3BX, Merseyside, England
[7] DESY, D-22603 Hamburg, Germany