[1] Univ Bern, Albert Einstein Ctr Fundamental Phys AEC, Lab High Energy Phys LHEP, Sidlerstr 5, CH-3012 Bern, Switzerland
[2] Univ Napoli Federico II, Dept Phys Ettore Pancini, I-80126 Naples, Italy
[3] Natl Inst Nucl Phys INFN, IT-80125 Naples, Italy