1. INFN, Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy
2. Physics Department, University of Pavia, Via Bassi 6, 27100 Pavia, Italy
3. Institute for Astroparticle Physics, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany
4. INFN, Sezione di Milano, Via Celoria 16, 20133 Milano, Italy