Experience with the NA60 silicon pixel vertex tracker in a harsh radiation environment

The NA60 experiment at CERN's Super Proton Synchrotron studies the production of open charm and prompt dimuons in collisions induced by heavy-ion and proton beams on nuclear targets. The experimental setup includes a dimuon spectrometer previously used in the NA50 experiment, and a new silicon vertex tracker: a telescope built from 12 stations of radiation tolerant pixel detectors placed close to the target inside of a 2.5 T dipole field. For the first time under such harsh radiation conditions, the pixel telescope tracks the large number of charged particles produced at the target. Its good spatial and momentum resolution enables the reconstruction of primary and secondary vertices, and the matching of two of the tracks with the two muons that gave the trigger in the muon spectrometer, placed behind the hadron absorber. In Fall 2003, the NA60 experiment performed a five week long physics run with a beam of 158 GeV/nucleon Indium ions colliding with a segmented Indium target. The run yielded data from several hundred million dimuon and minimum bias triggers. The pixel detectors were exposed to high levels of radiation inhomogeneously distributed over its acceptance. The silicon sensors closest to the beam axis underwent type inversion. This article discusses selected aspects of the silicon pixel detector's design and operation. Its performance is illustrated, in particular, by the vastly improved dimuon mass resolution. The last section discusses the re-utilization of the irradiated pixel detectors in a vertex tracker optimized for NA60's proton-nucleus run in 2004.