A Highly Selective Dilepton-Trigger System Based on Ring Recognition

Alberica Toia

II. Physikalisches Institut, Justus-Liebig-Universitat GieBen 
for the HADES collaboration

    The HADES second level trigger is used to reduce the total number of events by selecting events with a dilepton candidate. Electron signatures from several detectors, i.e. a Time Of Flight wall, an electromagnetic preshower detector and a hadron blind RICH are required to form single lepton candidates. Those are combined to lepton pairs within a given invariant mass range in a second step.
    A software emulation of the complete HADES second level trigger system was developed in order to verity the functionality of the single trigger components and to investigate their performance.
    Special emphasis was put on the impact of the hardware ring recognition algorithm on the trigger performance. This influence was investigated for different background scenarios in terms of photon and pad distributions.
    Moreover the efficiency and the event reduction of ring recognition were studied with data from C+C reactions at 1.5 AGeV and with simulations Comparing these figures to the corresponding ones for the complete second level trigger shows that the RICH is the most selective component of the trigger.