Calibration of the ATLAS electromagnetic calorimeter at the per mill level

Full text: ATLAS is one of the experiments built to explore new physics at the TeV with proton-proton collisions at the Large Hadron Collider. The ATLAS experiment will briefly described with emphasis on the ATLAS Liquid Argon electromagnetic calorimeter, this talk will concentrate on the performance and calibration of the electron reconstruction during the 2011 and 2012 data taking periods. Over the past years a calibration method for the energy scale of the liquid argon calorimeter using the distribution of the ratio of the electron energy measured by the calorimeter itself and the momentum measured by the Inner Tracking detector was developed. Results of this powerful calibration method using electrons from W and Z decays and combining the calorimetry and tracking information, will be presented. It will be shown how scale factors in ne pseudo-rapidity bins can be extracted with a precision at the per mill level. These results will be compared to other calibration methods used in ATLAS . It will be demonstrated how the linearity of the response of the electromagnetic calorimeter can be measured in situ and how the passive material in front of the calorimeter can be estimated. Finally, the impact of this precise calibration of the electromagnetic calorimeter on selected physics measurements will be presented. (author)