Alignment of the ALICE Inner Tracking System with cosmic-ray tracks

The ALICE Inner Tracking System (ITS) consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two pixel, two drift and two strip layers. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 μm in some cases (pixels). The sources of alignment information are the survey measurements and the reconstructed tracks from cosmic rays and from proton–proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10 5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnetic field switched off.