Measurement of the Higgs boson production in the H-> τ τ -> τlepτhad decay channel at √s = 13 TeV with the ATLAS detector at the LHC

This thesis presents a measurement of the Standard Model Higgs boson production in the H-> τ τ decay with particular emphasis on the final state in which one tau decays hadronically and the other one decays leptonically, referred to as the H-> τ τ -> τlepτhad channel . The analysis uses a dataset corresponding to 36.1 fb-1 collected by the ATLAS detector in the 2015 and 2016 Run 2 data taking period at a center of mass energy √s = 13 TeV. The decay of the Higgs boson into a tau lepton pair is currently the only accessible channel to establish the Higgs-Yukawa coupling to leptons. In this context, the H-> τ τ -> τlepτhad channel plays an important role due to the high branching ratio and the moderate background. A cut-based analysis has been performed to enhance the sensitivity to the signal considering the kinematic features of the two dominant Higgs boson production modes: gluon fusion and vector-boson fusion. The results of a profile likelihood fit to the di-tau mass distribution will be presented considering the τlepτhad standalone fit, as well as the combination of all possible tau pair decay channels. From the combined fit, a signal strength μ = 1.08+0.175-0.171 (stat.)+0.265-0.222 (syst.) and the cross section times branching ratio σHττ (VBF) = 0.28+0.09-0.09 (stat.) +0.10-0.09 (syst.) pb, σHττ (ggH) = 2.97+1.03-1.01 (stat.) +1.67-1.23 (syst.) pb values have been measured. The results are in good agreement with the Standard Model prediction within the uncertainties. The observed (expected) significance, considering only the Run 2 data, is 4.36 (4.13) standard deviations from the background-only hypothesis. The combination of Run 1 and Run 2 data gives a significance of 6.37 (5.43), which is above the 5 standard deviations observation threshold.