CONCEPT, DESIGN AND PERFORMANCE OF THE 2ND LEVEL TRIGGERS OF THE H1 DETECTOR

Since the end of the 1995 HERA running period the second level (L2) tr igger system of the H1 experiment has been in operation. The L2 decisi on is presently derived from a combination of two independent hardware systems; one is based on the output of neural networks while the othe r decides on the grounds of topological event signatures in the detect or. In contrast to the first level trigger (LI), which is fully pipeli ned and operates at the HERA bunch crossing rate of 10.4 MHz, the deci sion of the L2 systems is delivered within 20 mu s of the preceding L1 keep signal, allowing sufficient time for transmission of trigger inf ormation and computation of the L2 trigger response. This scheme signi ficantly reduces the trigger rate, with minimal impact on defector dea dtime for L1 rates below 1 kHz. The L2 trigger has proven to be flexib le and highly effective in isolating physics processes from background .