Concept, design and performance of the second level triggers of the H1 detector

Since the end of the 1995 HERA running period the second level (L2) trigger system of the H1 experiment has been in operation. The L2 decision is presently derived from a combination of two independent hardware systems; one is based on the output of neural networks while the other decides on the grounds of topological event signatures in the detector. In contrast to the first level trigger (L1), which is fully pipelined and operates at the HERA bunch crossing rate of 10.4 MHz, the decision of the L2 systems is delivered within 20 μs of the preceding L1 keep signal, allowing sufficient time for transmission of trigger information and computation of the L2 trigger response. This scheme significantly reduces the trigger rate, with minimal impact on detector deadtime for L1 rates below 1 kHz. The L2 trigger has proven to be flexible and highly effective in isolating physics processes from background