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Abstract: The measurement of the Higgs-boson coupling to tau-leptons, based on an integrated luminosity of 36.1 ifb recorded at a centre-of-mass energy of 13 TeV by the ATLAS detector, is presented in this thesis. The analysis considers all combinations of leptonic and hadronic decays of tau-leptons, and targets the main two production modes of the Higgs boson, gluon Fusion and Vector-Boson Fusion. The inclusive cross section of the H->tautau decay channel is measured as 3.77 +1.06-0.95 (+0.60-0.59 (Stat.) +0.87-0.74 (Syst.)) pb, which corresponds to a deviation from the background-only hypothesis with an observed (expected) significance of 4.4 (4.1) standard deviations. In addition, the measurement of cross sections in the framework of Simplified Template Cross Sections is discussed. All measurement are in agreement with predictions of the Standard Model. The combined measurement of Higgs-boson properties is also presented, based on up to 79.8 ifb of data recorded at sqrt(s) = 13 TeV by the ATLAS detector. An observation of the four main production modes (ggF, VBF, VH and ttH+tH) and of four of the five main decay channels (H->gammagamma, H->ZZ, H->WW and H->bb) of the Higgs boson is achieved. In particular, the deviation from the background-only hypothesis with an observed (expected) significance of 6.5 (5.3) standard deviations for the VBF production mode is the first observation of this process by a single experiment. Measurements in the scope of the kappa-framework and of Simplified Template Cross Sections are also discussed. All the performed measurements are compatible with the Standard-Model expectations
The Compact Muon Solenoid (CMS) detector has successfully operated with the Large Hadron Collider (LHC) collisions since 2010, leading to the Higgs boson discovery in 2012 by the CMS and ATLAS collaborations. The analyses of data taken during Run 2 (2016-2018) has already led to many interesting results about the properties of the Higgs boson and for the most part they are compatible in the context of the standard model (SM) so far. These studies are important because the SM appears to be an approximation of a more general theory. Precise studies of the Higgs boson's interactions with the known matter and force particles are essential to reveal the microscopic deviations from the SM prediction. One of such interactions is Yukawa couplings of the Higgs boson and fermions. The H → [tau tau] decay is the most sensitive fermionic decay mode that allows direct probing of this interaction. This thesis presents a study of the Higgs boson properties by observing its decay to a pair of tau leptons and testing evidence of non-SM interactions in the Higgs boson production. The results exploit the data collected by the CMS detector during LHC Run-2, in proton-proton collisions with a center-of-mass energy of 13 TeV. Firstly, the Higgs bosons' properties were explored by two complementary approaches by measuring its cross sections. Thanks to the observation of the H → [tau tau] decay in 2016, we can measure the total cross section with higher precision. Furthermore, in the first analysis, the cross sections of different Higgs productions were scrutinized using fine selections within each Higgs production mode which are mutually exclusive using the Simplified Template Cross Section (STXS) framework, proposed by the LHC Higgs Cross Section Working Group. In the second analysis, the first-ever measurement of the inclusive and differential fiducial cross sections for the Higgs boson production decaying to a pair of tau leptons was presented. In the third analysis, a search for anomalous couplings in the Higgs production vertex, including Charge-Parity (CP) violating couplings, is also performed targeting Higgs boson production via gluon fusion in association with two jets and vector boson fusion. None of these analyses has found any hint of new physics beyond the SM, but stringent limits on anomalous Higgs boson couplings to data were set and cross sections of the Higgs bosons in smaller phase spaces were precisely measured.
This thesis presents a 5.5 standard deviation observation of the Higgs boson decaying to fermions using the data collected at the LHC at 13 TeV center-of-mass energy. The studied dataset corresponds to an integrated luminosity of 35.9 inverse femtobarns. The best fit signal strength for the H to tau tau process is measured to be mu = 1.24 ^{+0.29}/{-0.27}, consistent with standard model predictions. Unique event categories are used targeting the leading Higgs boson production processes, gluon fusion, vector boson fusion, and associated production. This provides signal regions sensitive to Higgs boson couplings to both fermions and vector bosons. These two Higgs boson couplings are measured and are consistent with standard model predictions within one standard deviation. This 5.5 standard deviation observation of the H to tau tau process and the consistency of the Higgs boson couplings with the standard model provide confirmation of the Higgs boson Yukawa couplings to fermions. This is evidence that the Higgs field provides mass for the tau lepton in addition to the vector bosons.