The NA60 silicon vertex spectrometer

Abstract The NA60 experiment studies the production of open charm and prompt dimuons in proton–nucleus and nucleus–nucleus collisions at the CERN SPS. To access the kinematics of charged particles already at the vertex level, a radiation tolerant silicon pixel detector has been placed in a 2.5 T magnetic field near the target. This vertex spectrometer was built from 96 ALICE1LHCB pixel chips arranged in 12 tracking planes. The vertex spectrometer was successfully operated in a run with a 158 GeV/nucleon indium ion beam incident on indium targets in October–November 2003. During the five-week-long run it was exposed to high levels of radiation distributed inhomogeneously over the detector. The most exposed regions of the silicon sensors underwent type inversion. With only a fraction of the total statistics analysed, the vertex spectrometer can already be seen to have dramatically enhanced the physics performance of NA60 with respect to that of its predecessors.