Measurements of final states with $\tau$ leptons in proton-proton collisions using the ATLAS detector at the LHC

This monograph describes first analyses of processes with τ leptons in final state that have been performed with proton-proton collision data at the centre-of-mass energy of √ s = 7 TeV collected with the ATLAS detector at the LHC. Described studies are based on early data, recorded in 2010 and corresponding to an integrated luminosity of 35 pb−1 . Presented Higgs boson(s) searches, requiring higher statistics samples, are based on more data, corresponding to an integrated luminosity of 1.06 fb−1 and collected in 2010 and the first half of 2011. The reconstruction algorithms and identification methods for hadronically decaying τ leptons in the ATLAS experiment are described in detail. Validation of those algorithms in data as well as the first attempts to estimate the rate of the mis-identification of Quantum Chromodynamics jets or electrons as τ candidates are also presented. A dedicated Chapter is devoted to the first measurements of Z → ττ and W → τν production cross sections. The use of the latter process for determination of the hadronic τ decay identification efficiency is also reported. The early analyses of Higgs boson(s) searches with τ leptons in final states, presented in this document, cover studies of both the Standard Model and Minimal Supersymmetric Standard Model neutral Higgs boson(s) decaying into the H → ττ final state as well as Minimal Supersymmetric Standard Model charged Higgs boson decays, H + → τν. No significant excess over the expected background is observed in any of these studies. Nevertheless, even though performed on limited statistics, they provided improved exclusion limits as compared to those obtained by previous experiments.