Online Drift Velocity Calibration with the Laser System of the ALICE-TPC

The online drift velocity calibration method provides calibration parameters required at the very beginning of the offline reconstruction process in order to determine the z position of the particle tracks with a sufficient accuracy before the combined track finding [2]. It uses the TPC Laser System which generates 336 straight laser tracks, similar to ionizing particle tracks, at known positions in the drift volume of the TPC. A subset of these tracks, showing sufficiently small distortion effects, is used in the analysis. The resulting time dependent drift velocity correction parameters are stored in a database and provide start values for the offline reconstruction process of the ALICE TPC.