The trigger system for the NA48 experiment at CERN

The trigger system of the NA48 experiment, which has been taking data at the CERN SPS since July 1997 is presented. The aim of the experiment is to study direct CP-violation, through the high precision measurement of charged and neutral two pion decay modes of neutral kaons. To reach a sensitivity of $2 \times 10^{-4}$ on the CP violating parameter $\Re(\epsilon'/\epsilon)$, the experiment must collect a data sample with very high statistics. This requires a high rate data acquisition system and imposes tight requirements on the trigger. For both charged and neutral decay modes, dedicated pipelined branches have been implemented. The charged branch processes information from the magnetic spectrometer, while the neutral branch processes data from the liquid krypton electromagnetic calorimeter. The results from the two branches are managed and merged by a central decision system, which transmits the trigger requests to the read-out devices. A description of the three sections is given.