A Highly Selective Dilepton-Trigger System Based on Ring Recognition
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
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.