Reconstruction and Selection of Physics Objects in the ATLAS High Level Trigger

ATLAS has a three-level trigger system. The first level trigger is based mostly on information from the calorimeter and muon trigger chambers. The upper two trigger levels, known collectively as the High Level Trigger (HLT), are software-based. The trigger must reduce the event rate from an ultimate 40 MHz bunch-crossing rate to an average of about 200Hz to storage. The trigger selection is described by Menus, each of which contains more than 500 signatures. Each signature corresponds to a chain of algorithms which reconstruct and refine specific event features. As well as triggers using global event features, such as missing transverse energy, there are selections based on identifying candidate muons, electrons, photons, tau leptons or jets. Reconstruction of these features generally starts with a confirmation and refinement of the level-1 triggers, using information from the full granularity calorimeter and precision muon chambers. The subsequent trigger selection relies on combining information from the different detector systems, including information from the inner detector tracker, and applying selection cuts to identify candidate features. We give an overview of the performance of these trigger selections based on extensive online running during LHC collisions and describe the progress towards fully commissioning these triggers. Distributions of key selection variables based on calorimeter, muon and inner detector tracki ng information are shown calculated at the different trigger levels and are compared with offline reconstruction. We include examples of online triggering of Standard Model physics such as candidate W-boson decays. Comparisons between data and simulations are shown for some important selection variables, already illustrating a very good level of understanding of the detector and trigger performance. Finally we give a brief overview of plans for the evolution of trigger selections with increasing LHC luminosity.