[1] Tech Univ Berlin, Inst Theoret Phys, Hardenbergstr 36, D-10623 Berlin, Germany
[2] Imperial Coll London, Dept Phys, London SW7 2AZ, England
[3] Helmholtz Zentrum Dresden Rossendorf, Bautzner Landstr 400, D-01328 Dresden, Germany
[4] Univ Parma, Dipartimento SMFI, Parco Area Sci 7-A, I-43124 Parma, Italy
[5] Ist Nazl Fis Nucl INFN, Sez Milano Bicocca, Grp Collegato Parma, Parma, Italy