Physical Design of Main Drift Chamber Trigger for BES III Experiment

This article describes the design and simulation of the scheme of Main Drift Chamber ( MDC) trigger for BES III (Beijing Spectrometer III). The algorithm of MDC trigger includes 3 steps: track segments finding (TSF), track finding (TF) and trigger condition formation of MDC tracks. These trigger conditions are studied carefully and the trigger parameters are optimized through simulation. Trigger efficiencies vs. p(t), vertex position r-psi and z are presented. Trigger efficiency exceeds 99% for J/psi events and some important physics channels. The rejection of beam-related backgrounds exceeds 99.99% and the rate of cosmic rays is about 18Hz.