H.Riege, J.Schuett, R.van Staa, II Institute fi~r Experimentalphysik Universitat Hamburg
V. Popov, Institute for Theoretical and Experimental Physics, Moscow

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The high-pt level-O trigger (pretrigger) system searches candidates for particles with the large transverse momentum and initiate the First Level Trigger(FLT) tracks finding process[l]. This task is being carried out in real-time with small delay. Information from chambers is being accepted synchronously with the HERA clock at a typical rate of 40 MHz.

The main components of the high-pt trigger system are three superlayers of gaseous pad chambers located in a magnet, readout electronics placed in the detector area and trigger logic placed 50m far from the detector.

Every bunch crossing all hit information together with timing information are being transfered from the chambers to the trigger logic in serial form via high-speed optical links with data rate of 800 Mbit/s.

The trigger logic defines track candidates by looking for triple coincidences of logical signals coming from pads of three superlayers with required topology. The result is being transformed into a data stream of initial track parameters and transmitted to the FLT.

The high-pt trigger logic electronics has sectional structure and is composed of two types of boards - the Pretrigger Board and the Message Generator. Each section consists of one Message Generator and a number of pretrigger boards. Two processes are running asynchronously in every section - pads' hit information filtering procedure and serialization-encoding process. Output of the first process is the input for the second one. Communication between two processes is being provided by FIFOs.

A pretrigger board executes filter procedure for raw hit information to diminish hit data rate to acceptable level. It receives pads' hit information and tests it for coincidences. Only that information is being passed to the serialization and encoding procedure which matches with the coincidence patterns. The message generator acquires data out of a group of pretrigger boards, completes pretrigger logic task, defines the position and direction of detected coincidence and generates messages to the appropriate FLT proces

sors.

A pretrigger board serves two rows of 96 pads each. Since the board serves a complete row of pads no readout overlapping occurs.

The high-pt level-0 trigger latency is not more than 0.5 ps.

The trigger logic has flexibility to the trigger algorithm - coincidence patterns can be easily reprogrammed.

Some additional facilities to provide monitoring and testing functions, are implemented either.

The trigger logic prototype has been developed and is being tested in the HERA-B experiment. Some results obtained are presented.

References

[1] E.Hartouni et al., Hera-B, An experiment to study CP violation in the B system using an internal target at the HERA proton ring, Design Report, DESY-PRC 95/01 (January 1995)

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The HERA-B Level-O high-pt trigger searches candidates for particles with large transverse momentum. Hit information from three superlayers of gaseous chambers with pad readout located in a magnet is being transfered to the trigger logic via optical link lines. Triple coincidence algorithm is performed by a dedicated logic electronics to select track candidates out of hit streams coming with typical rate of few 1012 hits/s. The paper describes the high-pt trigger logic electronics and some results obtained.
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