Performance of the ATLAS Calorimeter Trigger with 7 TeV collision data

Since the start of the LHC physics programme earlier this year, the ATLAS detector has been collecting proton-proton collisions at a 7 TeV center of mass Energy. As the LHC luminosity rises the ATLAS trigger system must become increasingly selective to reduce the event rate from a design bunch crossing rate of 40 MHz to about 200 Hz for recording. To achieve this the trigger algorithms must meet challenging requirements in terms of speed and selectivity. The trigger is hardware based at level-1 and uses software algorithms running on a farm of commercial processors at the two higher trigger levels. The calorimeter-based software algorithms have been designed with a common part optimized for fast access to detector data and subsequent stages tailored for maximum selectivity for specific signatures such as electrons, photons, jets, taus and missing total energy. We present the physics performance achieved during 2010 data taking, highlighting the key performance aspects for the different signatures. Event features reconstructed by the Trigger are compared with offline reconstruction and with expectations from Monte Carlo simulations. Emphasis is given to distributions of calorimeter variables for energy determination and particle identification and the measured selection efficiency at the different trigger levels. Rate stability, processing time and data access performance during different periods of data-taking are also presente d. The results presented demonstrate that the calorimeter based trigger is functioning correctly and is effective in selecting data for the ATLAS physics programme.