Energy Measurement with the ATLAS Electromagnetic Calorimeter at the Per Mill Accuracy Level

The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider (LHC) at CERN. It is made up of various sub-detectors to measure the properties of all the particles produced at the proton-proton collision. Over the last three years of running around 20 x 10^14 collisions of proton data have been recorded. Liquid argon (LAr) sampling calorimeters are used for all electromagnetic calorimetry and for hadronic calorimetry in the end-caps. The Inner Detector, on the other hand, measures the transverse momentum of charged particles down to a momentum of 0.5 GeV. This thesis deals with the absolute measurement of the energy in the electromagnetic calorimeter and the improvement of the systematic uncertainties. A method using the ratio of the energy E in the calorimeter and the momentum measurement p in the Inner Detector (E/p) was used to extract the energy scale of the electromagnetic LAr calorimeter for electrons and positrons. To investigate and further reduce the systematic uncertainties of the extracted energy scale correction, several effects were studied. The calorimeter’s linearity had to be measured - for the first time in ATLAS - in five regions of the detector. Energy loss due to material effects upstream of the calorimeter had to be investigated. Differences in the shower development between MC simulation and data, along with its energy dependence, were shown. The uniformity of the energy response was measured with respect to time, pile-up and detector geometry. The uncertainties on the energy scale relative to the different sampling energies in the calorimeter had to be estimated. One very important, fundamental measurement within the Standard Model of particle physics is the measurement of the mass of the W boson. To measure the mass of the W boson the linearity of the electron energy measurement in a region from 20 to 80 GeV is crucial. Using the derived energy scale and linearity from the E/p ratio the impact on the W mass measurement was shown. The goal was to estimate uncertainties for this measurement, which aims to reach an accuracy smaller than 0.02%.