The evolution and performance of the ATLAS calorimeter-based triggers in 2011 and 2012

In 2011 the ATLAS detector at the LHC collected approximately 5fb-1 of proton-proton collision data at a centre of mass energy of 7TeV. During the data-taking period the LHC conditions changed significantly, with the instantaneous luminosity increasing by a factor of 15 and the average number of interactions per bunch crossing (“pile up”) reaching 20. In early 2012 the LHC running conditions rapidly evolved to produce record instantaneous luminosities and pile-up. The efficient performance of the ATLAS trigger has therefore been vital to maintain the recorded data rate within its limits while minimising the loss of signal events. The ATLAS trigger is hardware based at Level-1 and uses software algorithms running on a farm of commercial processors in the Level-2 and Event Filter levels of the Higher Level Trigger (HLT). A large fraction of the ATLAS physics programme is covered by the calorimeter-based triggers, which can select events with candidate electrons, photons, jets, taus or those with large missing transverse energy. The ATLAS Level-1 Calorimeter trigger receives input from the ATLAS main calorimeters and determines the calibrated energies sent to the algorithmic trigger processors that identify the high-ET physics objects and global energy sums. The reconstruction at Level-2 is then seeded by the Level-1 result in Regions of Interest (ROIs) and the HLT calorimeter-based software algorithms perform the selection of electrons, photons, jets, taus and also events with missing transverse energy using all available detector data. We present the performance of the L1 calorimeter trigger hardware in 2011 and 2012, highlighting the achievements of the different signatures. For electrons and photons, at Level-1 the thresholds have been raised and configured separately in various rapidity regions to account for energy losses in the upstream material and hadronic isolation requirements have been implemented. For the tau trigger an electromagnetic isolation requirement was introduced. For the jet trigger, the ROI-based strategy has been extended with the possibility of unpacking the full calorimeter at Event Filter level and even at an intermediate level between Level-1 and Level-2. Additionally the use of calibrated energy scale at trigger level and noise cuts to reduce rate spikes have been introduced. In summary, this contribution gives an overview of the optimisation and performance of the calorimeter-based triggers, demonstrating the robustness of the trigger system in the high luminosity, high pile-up environment of the LHC in 2011 and 2012.