The ATLAS Liquid Argon Calorimeter at the LHC

The Liquid Argon (LAr) Calorimeter is a key detector component in the ATLAS experiment at the LHC. It has been installed in the ATLAS cavern and filled with liquid argon since 2006. Its performance has been obtained using random triggers, calibration data, cosmic muons, LHC beam splash events (from september 2008 and november 2009 LHC running) and collision events (from november and december 2009 LHC running). The properties of each read-out channels such as pedestal, noise and gain have been measured and the high stability of the LAr electronics over several months of data taking is shown. Calibration data are stored into a database and used at reconstruction level (online and offline). The quality of the energy reconstruction at the first trigger level has also been studied. The method used to predict the ionization pulse shape is presented and the prediction is compared to ionization signals from cosmic muons. The uniformity of the calorimeter response has also been measured. Splash events have been used to estimate the calorimeter timing. Moreover, the missing transverse energy resolution has been evaluated using collision events and compared to the expected resolution.