A Kaon Trigger for FOPI : Development and Evaluation of a Trigger System for Strange Particles
During collisions of heavy ions at high energies at SIS/GSI, nuclear matter can be exposed to high densities and temperatures. The FOPI experiment detects the traces of the charged particles produced in the nuclear reaction. Their analysis, in particular with respect to the strange K mesons, can extend our knowledge about the structure of nuclear matter and the processes during the evolution of neutron stars and black holes. However, kaons are rarely observed, hence the derived physics results suffer from large uncertainties. In order to significantly enhance the kaon yield, a trigger, which is able to prevent the time consuming readout of the complete detector data for uninteresting events, was developed within the scope of this work. For that purpose, a Hough transform based algorithm was created. It reconstructs particle tracks from a small fraction of the data of the drift chamber CDC. A geometrical matching to the information from the new high-precision time-of-flight detector GRPC allows the determination of the found particles' species. In order to fulfill the requirements from the data bandwidth and the computing intensity, special-purpose processors are required. About 5 to 6 of the FPGA based MPRACE boards developed at Mannheim University can provide this performance cost-effectively. The consistent parallelization of the individual program steps makes the exploitation of the full power of MPRACE and thus a processing time of less than 100 us per event possible. Detailed simulations of the trigger systems show, that in experiments with light nuclei like nickel at beam energies of 1.93 GeV/u the K^+ yield can be enhanced by a factor of 6, and the K^- yield by a factor of 11.
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