Performance of the ATLAS Calorimeter 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 measuring the energy of photons, electrons, jets, taus and neutrinos, via missing transverse energy measurement. The ATLAS trigger system selects from the huge amount of events produced every second, those few that must be recorded for physics analysis (less than one out of 40 thousand can be kept). The selection process is performed in 3 levels with increasing complexity and resolution. The first level is hardware based, seeding the two other software levels called together the High-Level Trigger. 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. The performance of the trigger system actually improved in 2012 with respect to the 2011 data taking period despite the higher luminosity levels.