schol@mit.edu

¹ University of Bologna and INFN Bologna, Via Irnerio 46, 40126 Bologna, Italy
² Sky & Telescope, 49 Bay State Road, Cambridge, MA 02138-1200, USA
³ Department of Physics and Astronomy, Laurentian University, Sudbury, Ont. P3E 2C6, Canada
⁴ Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
⁵ IFSI-CNR Torino, University of Torino and INFN Torino, Via P Giuria 1, 10125 Torino, Italy
⁶ Department of Physics, University of Minnesota, Duluth, MN 55812-2496, USA
⁷ Los Alamos National Laboratory, Los Alamos, NM 87545, USA
⁸ Department of Physics, Queen's University, Kingston, Ont. K7L 3N6, Canada
⁹ Department of Physics, Boston University, Boston, MA 02215, USA
¹⁰ Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
¹¹ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹² Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000, USA
¹³ Current address: Department of Physics, University of California, Santa Barbara, CA 93106, USA.
¹⁴ Author to whom all correspondence should be addressed.
¹⁵ Current address: Department of Physics, University of Toronto, Toronto, Ont. M5S 1A7, Canada.