[1] CERN, DPNC, CH-1211 Geneva 4, Switzerland
[2] Univ Chicago, Chicago, IL 60637 USA
[3] Univ Pisa, Pisa, Italy
[4] Scuola Normale Super Pisa, Pisa, Italy
[5] Ist Nazl Fis Nucl, Pisa, Italy
[6] Ist Nazl Fis Nucl, Trieste, Italy
[7] Joint Inst Nucl Res, Dubna, Russia