Performance of the ATLAS Level--1 Calorimeter Trigger in high luminosity proton--proton collisions at the LHC

The ATLAS Level-1 Calorimeter trigger is one of the main components of the first level trigger of the ATLAS experiment at the LHC. Its input stage, the PreProcessor, is a mixed analogue/digital system which receives pre-summed analogue signals from the calorimeters and determines the calibrated energies and the corresponding bunch crossing which are sent to the digital processors. The complete processing chain is performed in a digital, pipelined system, where programmable algorithms are performed in parallel within a fixed latency of 2 mus. The real-time output consists of counts of high-ET physics objects, such as jets, electron/photon and tau candidates, and global energy sums. While the trigger system has been operational from the time of the very first LHC data taking, many configurable parameters such as timing and energy calibration were optimized based on studies of high luminosity proton-proton collisions. After two years of successful operation close-to-ideal performance of the trigger has been established. During the proton-proton data taking period in 2011 and 2012 the running conditions changed substantially: compared to 2010, the instantaneous luminosity increased by a factor of 15, with up to 25 pile-up interactions per bunch crossing. Parameters like e.g. noise cuts needed to be adjusted to account for the different conditions. An overview of the performance of the calorimeter trigger hardware under these challenging conditions will be presented, along with the experiences and resulting optimizations for the data taking period in 2012.