Nucl. Instrum. Methods Phys. Res., A Nucl. Instrum. Methods Phys. Res., A 0167-5087 ATLAS trigger and data acquisition Scannicchio D A CERN 2010 617 306 309 The ATLAS trigger system is based on three levels of event selection that selects the physics of interest from an initial bunch crossing rate of 40 MHz to an output rate of View the MathML source compatible with the offline computing power and storage capacity. During nominal LHC operations at a luminosity of View the MathML source, decisions must be taken every 25 ns. The LHC is expected to begin operations with a peak luminosity of View the MathML source with far fewer number of bunches, but quickly ramp up to higher luminosities. Hence, the ATLAS Trigger and Data Acquisition system needs to adapt to the changing beam conditions preserving the interesting physics and detector requirements that may vary with these conditions. research-article