Status of the Atlas Liquid Argon Calorimeter and its Performance after Three Years of LHC Operation

The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider(LHC) at CERN. Liquid argon sampling calorimeters are used for all electromagnetic calorimetry covering the pseudo- rapidity region up to 3.2, as well as for hadronic calorimetry in the range 1.5-4.9. The electromagnetic calorimeters use lead as passive material and are characterized by an accordion geometry that allows a fast and uniform azimuthal response without any gap. Copper and tungsten were chosen as passive material for the hadronic calorimetry; whereas a classic plate geometry was adopted at large polar angles, an innovative one based on cylindrical electrodes with thin argon gaps was designed for the coverage at low angles, where the particles flow is higher. All detectors are housed in three cryostats kept at approximately 89 K. After installation in 2004-2006, the calorimeters were extensively commissioned over the three-year period prior to first collisions in 2009, using cosmic rays and single LHC beams. Since then, around 27 fb-1 of data have been collected at centre of mass energies of 7-8 TeV. During all these stages, the calorimeter has been operating almost optimally, with performance very close to specifications. The talk will cover all aspects of these first years of operation, including the calibration efforts and the data quality assessment procedure. The excellent performance achieved will also be briefly reviewed, especially in the context of the recently announced discovery of the Higgs boson.