Performance of the ATLAS Calorimeter High-Level Trigger in the LHC Run 1 Data Taking period

The ATLAS detector operated very successfully at the LHC Run 1 data taking period collecting a large number of events used for the discovery of the Higgs boson as well as for the search for beyond the Standard Model physics. In the main search channels related to the finding of the Higgs, the ATLAS calorimeter system played a major role in measuring the energy of photons, electrons, jets, taus as well as global missing transverse energy. The ATLAS trigger system selects the physics of interest from the huge amount of events produced every second, those few that must be recorded for analysis (less than one out of 40 thousand can be kept). The selection process is performed in three levels. The first one uses hardware based algorithms to select events using information with coarse granularity from the detectors. The next levels are designed in software. The Level 2 (L2) works only in the regions that caused the first level to fire, the so called Regions Of Interest (RoI), calculating special features like the energy and shower shape variables of clusters of calorimeter cells. The third level called Event Filter (EF) can access the full event data and run offline-like algorithms for the selection process. The L2 and EF are called altogether the High-Level Trigger (HLT). The paper will describe details of the calorimeter based HLT algorithms with special emphasis on the algorithms used for missing transverse energy and jet detection which were improved as the instantaneous luminosity of the accelerator increased orders of magnitude. In particular, the RoI concept, a key design point of the L2 had to be broken when global quantities, such as the missing transverse energy were considered for implementation. The performance of the trigger system actually improved in the 2012 data taking period despite the higher luminosity levels attained by the LHC.