PhD thesis defense by Almut Pingel

Title: Tau lepton identification and studies of associated Higgs boson production with the ATLAS detector

In 2012, the ATLAS detector at the LHC recorded 20 fb−1 of proton-proton collisions at a centre-of-mass energy of 8 TeV. A very good particle identification is essential to understand the data. The thesis focuses on the identification of hadronically decaying tau leptons. Boosted decision trees (BDTs) are used to reject backgrounds that mimic the tau signature in the detector. The algorithm is made robust against run conditions with many simultaneous proton-proton interactions per bunch crossing (pile-up). Furthermore, the dependence of the identification method on the tau momentum is studied, and the usage of additional algorithms that explore the pion content of the tau decay.

The second part of the thesis is a search for a Higgs boson that is produced in association with a Z boson. The Z boson decays into a pair of electrons or muons, while the Higgs boson decays into a pair of tau leptons. The background is estimated from simulated samples and adjusted using data driven techniques. No deviation from the Standard Model is found and a limit is set on the cross section of the process, based on the Higgs boson decay modes τ(had)τ(had), τ(e)τ(had), τ(μ)τ(had) and

τ(e) τ(μ). An upper observed (expected) limit of 8.96 (6.18+8.94−4.45) times the SM cross section is set at 95% confidence level for a Higgs boson mass of 125 GeV.