1. Dipartimento di Fisica, Università di Catania, Via Santa Sofia 64, I-95123 Catania, Italy
2. INFN, Sezione di Catania, I-95123 Catania, Italy
3. Institute of Particle and Nuclear Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic