Tracking with the Transition Radiation Detector in the High Level Trigger of ALICE

During the LHC Run 2, which started in 2015, unexpected localized cm-scale distortions caused by space-charge were observed in the ALICE Time Projection Chamber (TPC). The distortions are corrected for employing the detectors inside (Inner Tracking System) and outside (Transition Radiation Detector - TRD) the TPC. The correction is currently done within the offline reconstruction procedure. In order to speed up the reconstruction, it is planned to move parts of the calibration procedure online in the High Level Trigger (HLT). As a first step the TRD tracking is implemented in the HLT. For this, TPC tracks are extrapolated towards the TRD and used as seeds for tracking. In contrast to the offline correction procedure, the new tracking algorithm is based on online TRD tracklets instead of clusters. The implementation is also a preparation for LHC Run 3 (beyond 2020) where the whole reconstruction and calibration will be moved online. First results of this tracking algorithm are presented, and the next steps necessary to correct the space charge distortions online in the HLT are discussed.