Performance of the ATLAS Liquid Argon Calorimeter after three years of LHC operation and plans for a future upgrade

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 1.4-4.9. 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 87 K. After installation in 2004-2006, the calorimeters were extensively commissioned over the three years 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 a unprecedented center of mass energy between 7 TeV and 8 TeV. During all these stages, the calorimeter and its electronics have been operating almost optimally, with performances very close to the specification ones. Plans to upgrade the LHC collider to up to 7 times the original design luminosity of 10^34 cm^2s^-1 may require a further improvement of the LAr calorimetry in ATLAS. In particular, the increased particle flux in the LAr forward calorimeter (FCal) can degrade its performance. Therefore, a replacement with a new FCal with smaller LAr gaps or an additional Mini-FCal are considered.The performance of these new calorimeters is being studied in highest intensity particle beams. The talk will cover all aspects of these first years of operation. The excellent performance achieved will be especially detailed in the context of the discovery announced in July 2012 of a new boson, whose characteristics are consistent with the ones of the famous Higgs boson. The future plans to preserve this performance until the end of the LHC program will be also presented.