A 40 MHz pipelined trigger for $K^{0} \to 2 \pi^{0}$ decays for the CERN NA48 experiment

A first level trigger system based on a 40 MHz digital pipeline has been developed for the CERN NA48 experiment, aiming at measuring CP violation in $K^{0} \to 2\pi$ decays. The outputs of the 13340 cells of the 10 m$^{3}$ liquid krypton calorimeter are summed into 64 X and 64 Y projection strips and continuously digitised with 40 MHz FADCs. This information is used to reconstruct at each clock cycle and for the two calorimeter projections, the number of clusters, the impact time of each of them (with a precision of about 3 ns), their total energy and the first and second moments of the energy distribution. Based on the quantities listed above, a programmable look-up table system subsequently computes online the longitudinal position of the kaon decay vertex and performs an event selection. The system is described.