High Level Trigger applications for the ALICE experiment

For the ALICE experiment at the LHC, a high level trigger system for on-line event selection and data compression has been developed and a computing cluster of several hundred dual-processor nodes is being installed. A major system integration test was carried out during the commissioning of the time projection chamber (TPC), where the HLT also provides a monitoring system. All major parts like a small computing cluster, hardware input devices, the on-line data transportation framework, and the HLT analysis could be tested successfully. A common interface for HLT processing components has been designed to run the components from either the on-line or off-line analysis framework without changes. The interface adapts the component to the needs of the on-line processing and allows the developer at the same time to use the off-line framework for easy development, debugging, and benchmarking. Results can be compared directly. For the upcoming commissioning of the whole detector, the HLT is currently prepared to run on-line data analysis for the main detectors, e.g. the inner tracking system (ITS), the TPC, and the transition radiation detector (TRD). The HLT processing capability is indispensable for the photon spectrometer (PHOS), where the on-line pulse shape analysis reduces the data volume by a factor 20. A common monitoring framework is in place and detector calibration algorithms have been ported to the HLT. The article describes briefly the architecture of the HLT system. It focuses on typical applications and component development.