A highly selective dilepton trigger based on ring recognition

The dilepton spectrometer HADES at GSI Darmstadt investigates lepton pair production in elementary and heavy ion reactions. Since the branching ratio of these decays is on the order of 10−5, a highly selective real time second level trigger (LVL2) is needed. This LVL2 consists of Image Processing Units which perform pattern recognition to detect lepton signatures in different sub detectors and a Matching Unit which combines the position and momentum information of these signatures into tracks to select events with dilepton pairs. Since the recognition of Cherenkov rings is the most selective algorithm of this LVL2, its behavior has been characterized with the help of simulations and by a comparison with the offline analysis algorithm in a dedicated experiment, which allows a better understanding of the main background sources present in the experiment.