The ALICE Inner Tracking System: Performance with Proton and Lead Beams

The Inner Tracking System (ITS) of the ALICE experiment consists of six cylindrical layers of silicon detectors. Each layer has hermetic structure and it is coaxial with the beam pipe. The ITS covers the pseudorapidity range |η|≤ 0.9 and the distance from the nominal beam line ranges from 3.9 cm for the innermost layer up to 43 cm for the outermost. The two innermost layers are made of Silicon Pixel Detectors (SPD), the two central layers of Silicon Drift Detectors (SDD) and the two outermost layers of double sided Silicon Strip Detectors (SSD). The ITS has the main purposes of providing both primary and secondary vertices reconstruction and of improving the ALICE barrel tracking capabilities in the vicinity of the interaction point. Furthermore, as a standalone tracker, the ITS recovers particles which do not reach or are missed by the external barrel detector, due to acceptance limitations and momentum cutoff. After a short summary on the status of spatial alignment and detector calibration, this paper will focus on the ITS performances with with proton and lead beams in 2010 for what concerns vertexing and tracking.