Design and expected performance of the ATLAS trigger and event selection

The LHC and ATLAS present the T/DAQ system with a highly challenging environment: the unprecedented LHC rate of 109 interactions per second with large and complex detectors with channels to read out; the bunch crossing rate of 40 MHz requires a decision every 25 ns, while the event storage rate is limited to MB/s. Within these constraints, the ATLAS T/DAQ system must separate rare physics signatures from the overwhelming rate of background events. This paper gives an overview of the T/DAQ system and describes how the high level triggers are being designed to meet these challenges. Data bandwidth and processing times in the higher level triggers are reduced by region of interest guidance from the first level trigger and sequential steps in the reconstruction process. Flexibility is paramount in order to adapt to the changing luminosity, backgrounds and physics goals. This is achieved by simple, inclusive trigger menus and modular software design. Algorithms have been developed which provide the flexibility to control the trigger rates.