The ALICE Dimuon Spectrometer High Level Trigger

The ALICE Dimuon Spectrometer High Level Trigger (dHLT) is an on-line processing stage whose primary function is to select interesting events that contain distinct physics signals from heavy resonance decays such as J/ψ and ϒ particles, amidst unwanted background events. It forms part of the High Level Trigger of the ALICE experiment, whose goal is to reduce the large data rate of about 25 GB/s from the ALICE detectors by an order of magnitude, without loosing interesting physics events. The dHLT has been implemented as a software trigger within a high performance and fault tolerant data transportation framework, which is run on a large cluster of commodity compute nodes. To reach the required processing speeds, the system is built as a concurrent system with a hierarchy of processing steps. The main algorithms perform partial event reconstruction, starting with hit reconstruction on the level of the raw data received from the spectrometer. Then a tracking algorithm finds track candidates from the reconstructed hit points. Physical parameters such as momentum are extracted from the track candidates and finally a dHLT decision is made to readout the event based on certain trigger criteria. Various simulations and commissioning tests have shown that the dHLT can expect a background rejection factor of at least 5 compared to hardware triggering alone, with little impact on the signal detection efficiency.