Monitoring the data quality of the real-time event reconstruction in the ALICE High Level Trigger

ALICE is a dedicated heavy ion experiment at the CERN LHC. The ALICE High Level Trigger was designed to select events with desirable physics properties. Data from several of the major subdetectors in ALICE are processed by the HLT for real-time event reconstruction, for instance the Inner Tracking System,the Time Projection Chamber,the electromagnetc calorimeters, the Transition Radiation Detector and the muon spectrometer. The HLT reconstructs events in real-time and thus provides input for trigger algorithms. It is necessary to monitor the quality of the reconstruction where one focuses on trackand event properties.Also,HLT implemented data compression for the TPC during the heavyion data taking in 2011 to reduce the data rate from the ALICE detector.The key for the data compression is to store clusters(spacepoints)calculated by HLT rather than storing raw data. It is thus very important to monitor the cluster finder performanceas away to monitor the data compression. The data monitoring is divided into two stages.The first stage is performed during data taking.Apart of the HLT production chain is dedicated toper forming on line monitoring and facilities are available in the HLT production cluster to have real-time access to the reconstructed events in the ALICE controlroom. This includes track and event properties,and in addition, this facility gives away to display as mall fraction of the reconstructed events in an online display. These cond part of the monitoring is performe dafter the data has been transferred to permanent storage. After a post-process of thereal-time reconstructed data,one can look in more detail at the cluster finder performance,the quality of the reconstruction of tracks,vertices and vert exposition.The monitoring solution is presented in detail,with special attention to the heavy ion data taking of 2011.