[1] Univ Birmingham, Sch Phys & Astron, Birmingham B15 2TT, W Midlands, England
[2] Univ Sussex, Dept Phys & Astron, Brighton BN1 9QH, E Sussex, England
[3] Natl Phys Lab, Hampton Rd, Teddington TW11 0LW, Middx, England
[4] Univ Oxford, Dept Phys, Clarendon Lab, Parks Rd, Oxford OX1 3PU, England
[5] Max Planck Inst Kernphys, Saupfercheckweg 1, D-69117 Heidelberg, Germany
[6] Univ Sorbonne Paris Nord, CNRS, Lab Phys Lasers, 99 Ave Jean Baptiste Clement, F-93430 Villetaneuse, France
[7] Imperial Coll London, Blackett Lab, Prince Consort Rd, London SW7 2AZ, England
[8] Humboldt Univ, Inst Phys, Newtonstr 15, D-12489 Berlin, Germany
[9] Deutsch Elektronen Synchrotron DESY, Platanenallee 6, D-15738 Zeuthen, Germany
[10] Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA
[11] Univ Tokyo, Inst Adv Study, Kavli Inst Phys & Math Universe WPI, Kashiwa, Chiba 2778583, Japan
[12] Univ Sydney, Sch Phys, Sydney, NSW 2006, Australia