Search

The NA60 experiment studied low-mass dimuon production in proton-nucleus and indium-indium collisions at the CERN SPS. While the p-A data can be well described by a superposition of light neutral meson decays, an excess is observed in the heavy-ion data, increasing with collision centrality, with respect to these expected sources. Thanks to the high statistics and good quality of our measurement, we can subtract the muon pairs due to h, h0, w and f decays from the signal dimuon mass distribution. The resulting spectra are compatible with a r spectral function broadening from peripheral to central collisions, with no change in the pole mass.

B. Alessandro, C. Alexa, R. Arnaldi, M. Atayan, C. Baglin, A. Baldit, M. Bedjidian, S. Beole, V. Boldea, P. Bordalo,* G. Borges, A. Bussiere, L. Capelli, C. Castanier, J. Castor, B. Chaurand, B. Cheynis, E. Chiavassa, C. Cicalo, M. P. Comets, S. Constantinescu, P. Cortese, G. Dellacasa, A. Devaux, A. De Falco, N. De Marco, S. Dita, B. Espagnon, J. Fargeix, A. Ferretti, P. Force, M. Gallio, C. Gerschel, P. Giubellino, M. B. Golubeva, A. A Grigoryan, S. Grigoryan, F. F. Guber, A. Guichard, H. Gulkanyan, M. Idzik, D. Jouan, T. L. Karavicheva, L. Kluberg, A. B. Kurepin, Y. Le Bornec, P. Macciotta, M. Mac Cormick, A. Marzari-Chiesa, M. Masera, A. Masoni, M. Monteno, A. Musso, C. Oppedisano, A. Piccotti, J. R. Pizzi, F. Prino, G. Puddu, C. Quintans, L. Ramello, S. Ramos,* P. Rato Mendes, L. Riccati, H. Santos, P. Saturnini, E. Scalas, E. Scomparin, S. Serci, R. Shahoian, F. Sigaudo, M. Sitta, P. Sonderegger,* X. Tarrago, N. S. Topilskaya, G. L. Usai, E. Vercellin, L. Villatte, and N. Willis (NA50 Collaboration) Dipartimento di Fisica Sperimentale, Universita di Torino and INFN-Torino, Torino, Italy IFA, Bucharest, Romania YerPhI, Yerevan, Armenia LAPP, CNRS-IN2P3, Annecy-le-Vieux, France LPC, Universite, Blaise Pascal and CNRS-IN2P3, Aubiere, France Institut de Physique Nucleaire de Lyon (IPNL), Universite, Claude Bernard, IN2P3-CNRS, Villeurbanne, France LIP, Lisbon, Portugal Laboratoire Leprince-Ringuet, Ecole Polytechnique and CNRS-IN2P3, Palaiseau, France Universita di Cagliari and INFN, Cagliari, Italy IPN, Universite, de Paris-Sud and CNRS-IN2P3, Orsay, France Universita del Piemonte Orientale, Alessandria and INFN, Alessandria, Italy INR, Moscow, Russia CERN, Geneva, Switzerland (Received 20 June 2003; published 12 March 2004)

Hybrid assemblies of LHC1 read-out chips and 300 μm thick silicon pixel detectors have been tested with a 120 GeV/ c pion beam at the CERN SPS. The equivalent noise charge at the input of the preamplifier is ∼200 e − . The particle detection efficiency is higher than 99.9%. A 30 ns wide efficiency plateau is reached for a 60 ns wide trigger pulse applied after a ∼2 μs delay. The 50 μm×50 μm pixels yield a spatial precision of σ =129.4±0.2 μm in the large pitch direction. In the other direction the precision is σ =12.2±0.2 μm for single-pixel clusters and σ =8.2±0.2 μm for double-pixel clusters, yielding an average of σ =11.4±0.2 μm. Three different detector layouts on the same pitch were characterized.