[1] Univ Zaragoza, Lab Fis Nucl & Altas Energias, Zaragoza 50009, Spain
[2] Univ Milan, Dipartimento Fis, I-20136 Milan, Italy
[3] Ist Nazl Fis Nucl, Sezione Milano, I-20133 Milan, Italy
[4] Univ S Carolina, Dept Phys & Astron, Columbia, SC 29208 USA
[5] Univ Calif Berkeley, Lawrence Berkeley Lab, Berkeley, CA 94720 USA
[6] Univ Calif Berkeley, Dept Mat Sci & Mineral Engn, Berkeley, CA 94720 USA
[7] Univ Florence, Dipartimento Fis, I-50125 Florence, Italy
[8] Ist Nazl Fis Nucl, Sezione Firenze, I-50125 Florence, Italy
[9] Lab Nazl Gran Sasso, I-67010 Assergi, Laquila, Italy
[10] Leiden Univ, Kamerlingh Onnes Lab, NL-2300 RA Leiden, Netherlands
[11] Univ Insubria, Dipartimento Sci Chim Fis & Matemat, I-22100 Como, Italy
[12] Ist Nazl Fis Nucl, Sezione Milano, I-22100 Como, Italy
[13] Lab Nazl Legnaro, I-35020 Legnaro, Padova, Italy