Measurement of the Higgs-Boson production cross-section and test of CP invariance in Vector-Boson fusion production with H->tau tau decays in the fully-leptonic final state with the ATLAS detector at the LHC

Abstract

This thesis presents the measurement of the Higgs-boson production cross-section in the H->tau tau fully-leptonic final state based on data taken with the ATLAS detector at sqrt(s)=13 TeV with an integrated luminosity of 36.1 fb^{-1}. The analysis is based on the classification of events into categories, which target the dominant Higgs-boson production modes in gluon-fusion (ggF) and vector-boson fusion (VBF). The H->tau tau signal is extracted by performing a maximum likelihood fit in the reconstructed di-tau mass. In the fully-leptonic final state, an excess of H->tau tau signal events over the background-only hypothesis is observed with a significance of 2.2 standard deviations, while 1.2 standard deviations are expected. The obtained Higgs-boson production cross-section times branching ratio is 6.76^{+2.80}_{-2.78} pb, which is consistent with the SM prediction within two standard deviations. The combined analysis with the semi-leptonic and fully-hadronic final states yields a Higgs-boson production cross-section times branching ratio of 3.77^{+1.06}_{-0.95} pb. This result is consistent with the SM prediction within the uncertainties. The observed signal excess over the background-only hypothesis corresponds to a significance of 4.4 standard deviations, while 4.14 standard deviations are expected. In combination with the analysis of the data taken at 7 TeV and 8 TeV with integrated luminosities of 4.5 fb^{-1} and 20.3 fb^{-1}, respectively, the observed significance amounts to 6.4 standard deviations, while 5.4 standard deviations are expected. This result allows to establish the discovery of H->tau tau decays with the ATLAS experiment. Furthermore, this thesis presents a search for CP-violating interactions of the Higgs boson with weak gauge-bosons HVV in the VBF production mode with subsequent H->tau tau decays. Additional CP-odd contributions to the CP-even HVV coupling stucture are described by the parameter dtilde in the framework of an effective field theory with dtilde=0 corresponding to the SM prediction. The VBF Higgs-boson signal is separated from the background processes, where no CP violation is expected, by exploiting a multivariate approach. The CP-odd Optimal Observable is used in order to determine the dtilde-parameter and construct its central confidence level (CL) intervals. In the fully-leptonic final state, the obtained dtilde-parameter is -0.03 and no CL intervals for dtilde can be derived. The combined analysis with the semi-leptonic and fully-hadronic final states measures dtilde to be -0.01 and obtains a 68 % CL interval of [-0.090,0.035] compared to the expected 68 % CL interval for the SM hypothesis of [-0.035,0.033]. No 95 % CL interval for dtilde is observed, while the expected 95v% CL interval for the SM hypothesis is [-0.21,0.15]. These results are consistent with the SM prediction of a CP-even Higgs boson and show no sign of CP-violating HVV interactions.