DI-TAU TOPOLOGIES AT ATLAS: PREPARATORY STUDIES AND SEARCH FOR HIGGS BOSON DECAYS TO TWO TAU LEPTONS DURING LHC RUN-I

The study of di-tau topologies is a very important part of the physics program of the ATLAS experiment, operating at the Large Hadron Collider (CERN). After the discovery of the Higgs boson in July 2012, the study of its decays to two tau leptons has turned out to be crucial for its characterisation, offering sensitivity to important aspects of the theoretical predictions, such as the coupling of the Higgs boson to fermions and scenarios beyond to the Standard Model. In this thesis several aspects of the study of di-tau topologies during LHC Run-I are presented. Preparatory studies include the measurement of the Z → τ+τ− process cross-section, which was measured with an uncertainty of ∼ 10% on 36 pb−1 of pp collision data collected at √s = 7 TeV. Studies crucial to the high performance of reconstruction and identification of hadronically decaying tau leptons at ATLAS are presented. They include contributions to the calibration and validation of the ATLAS Pixel detector clustering algorithms and the development of an in-situ method for the determination of the energy scale of hadronically decaying tau leptons. Their pseudorapidity intercalibration was cross-checked to 3% on a dataset corresponding to 4.26 fb−1 of pp collisions collected at √s = 7TeV. Validation and preliminary studies for the improvement of the ATLAS standard di-tau mass reconstruction algorithm are also presented. A cut-based analysis for the search of H → ττ → τlepτhad process on a dataset corresponding to 20.3 fb−1 of pp collisions collected at √s = 8TeV is presented. This analysis shares several points with and complements the ATLAS multi-variate result, which provided first evidence for H → τ+τ− decays in ATLAS.