Online drift velocity calibration with the laser system of the ALICE-TPC

The ALICE Time Projection Chamber (TPC) is the main tracking detector of ALICE which was designed to perform well at multiplicities of up to 20000 charged primary and secondary tracks emerging from Pb-Pb collisions. For a precise reconstruction of particle tracks in the TPC, the calibration of the drift velocity, which provides time information and thus the z position of the traversing particles, is essential. In this presentation an online method for the calibration of the drift velocity is presented, using the TPC laser system. The resulting time dependent drift velocity correction parameters are entered into a database and provide start values for the offline reconstruction process of ALICE. Even though no tracking information is used, the online drift velocity calibration is in agreement with the full offline calibration including tracking on the level of 2 x 10-4.