One focus of the HADES experiment (High Acceptance Di-Electron Spectrometer; Gesellschaft für Schwerionenforschung (GSI) mbH (Germany)) is the investigation of the decay of light vector mesons inside a dense medium into lepton pairs. These decays provide a conceptually ideal tool to study the invariant mass of the vector meson in-medium, since the lepton pairs of these meson decays leave the reaction without further strong interaction. Thus, no final state interaction affects the measurement. Unfortunately, the branching ratios of vector mesons into lepton pairs are very small (ca. 0.00001). This calls for a high rate, high acceptance experiment. In addition, a sophisticated real time trigger system is used in HADES to enrich the interesting events in the recorded data.
The focus of this thesis is the development of a next generation real time trigger method to improve the enrichment of lepton events in the HADES trigger. In addition, a flexible hardware platform (GE-MN (Gigabit-Ethernet-Multi-Node)) was developed to implement and test the trigger method.
The GE-MN features two Gigabit-Ethernet interfaces for data transport, a VMEbus for slow control and configuration, and a TigerSHARC DSP for data processing. It provides the experience to discuss the challenges and benefits of using a commercial standard network technology based system in an experiment.
The developed and tested trigger method correlates the ring information of the HADES RICH (RICH detector: Ring Imaging Cherenkov detector) with the fired wires (cells) of the HADES MDC detector (MDC: Mini Drift Chambers; drift chamber based detector system for the tracking of charged particles). This correlation method operates by calculating for each event the cells which should have seen the signal of a traversing lepton, and compares these calculated cells to all the cells that did see a signal. The cells which should have fired are calculated from the polar and azimuthal angle information of the RICH rings by assuming a straight line in space, which is starting at the target and extending into a direction given by the ring angles. The line extends through the inner MDC chambers and the traversed cells are those that should have been hit. To compensate different sources for inaccuracies not the exact cell numbers are required, but instead the cells in a matching window are investigated, and only a fraction of the MDC layers that should be hit are required to have seen a hit.
It is demonstrated that the use of the correlation method can enhance the number of events which contain leptons by an experiment dependent factor. The possible enhancements of different reactions are approx. 30 for p+p, approx. 50 for C+C and approx. 5.5 for Ar+KCl. The efficiency at these enhancements is above 80%, and it is constant in relation to the parameters polar angle, azimuthal angle and momentum. It is demonstrated how to obtain the correlation parameters from the online available raw data only.
The performance of the correlation method in terms of the events analyzed per second has been studied with the GE-MN. The performance depends on the input data and ranges from 6600 events/s for p+p data to 1400 events/s for Ar+KCl data for a single GE-MN system.
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