A first level trigger subsystem CIP2k for the H1 experiment at HERA

Abstract

The ep-collider HERA at DESY and the high energy physics experiment H1 were upgraded in the years 2000–2002. A specific luminosity three times higher than before has recently been observed. To cope with the higher luminosity and the more complex background situation a set of five new cylindrical multiwire proportional chambers (CIP2k) were built providing information to distinguish between beam-gas induced background events and true ep-interactions within 2.3 microseconds on a first level trigger stage using all channels of the chambers. The five cylindrical chambers with a cathode pad readout have each a 16 fold segmentation of the azimuth angle and a segmentation of about 120 in z along the beam axis. The total number of readout channels amounts to 9600. Groups of 60 cathode pads are read out via striplines by a custom designed readout chip (CIPix). The on-chamber electronics digitizes and multiplexes the data and sends it via optical links to the trigger and data acquisition system at a rate of about 10 GBit/s per optical link. A histogram of the vertex distribution along the beam axis of the tracks crossing the chamber is calculated by the trigger system. From this histogram trigger informations are derived dividing the histogram in regions of background and physics events. The tracking algorithm is based on bit pattern comparison of pad data. Field-programmable gate arrays (FPGA) were chosen to accomplish this complex and highly parallelized task. In addition the FPGAs contain pipelines to store the chamber data making it possible to read out the information if a trigger occurs. The trigger, data acquisition and high voltage system are controlled and monitored by the experiment control system (ECS) based on commercial software called PVSSII.