1. Dip. di Fisica, Università di Roma “Tor Vergata”, I-00133 Rome, Italy
2. INFN, sez. Roma “Tor Vergata”, I-00133 Rome, Italy
3. Dip. di Fisica, Università di Roma “La Sapienza”, I-00185 Rome, Italy
4. INFN, sez. Roma, I-00185 Rome, Italy
5. Laboratori Nazionali del Gran Sasso, INFN, Assergi, Italy
6. Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, P.O. Box 918/3, Beijing 100049, China
7. University of Jinggangshan, Jiangxi, China