Configuration of the ATLAS Trigger

The ATLAS detector at CERN's LHC will be exposed to proton-proton collisions at a rate of 40 MHz. In order to reduce the data rate, only potentially interesting events are selected by a three-level trigger system. Its first level is implemented in electronics and firmware, and aims at reducing the data rate to about 75 kHz. The second and third trigger levels are based on software and reduce the data rate to about 200 Hz. To prepare the full trigger chain for the online event selection according to a certain strategy, a system is being set up that provides the corresponding information to all parts of the trigger chain, e.g. values of hardware registers in the first-level trigger or steering parameters in the high-level trigger algorithms. The same information is used to configure offline trigger simulation and to run offline analysis. This article gives an overview of the system and its main components.