Measurement of the H→ττ Production Cross-Section in Hadronic Final States in 13TeV Proton-Proton Collisions with the ATLAS Detector

The couplings of the Standard Model Higgs boson to fermions provide valuable insights into the nature of electroweak symmetry breaking in the Yukawa sector. This thesis summarises the measurement of the Higgs boson production cross section in decays to two hadronically decaying $\tau$-leptons. The measurement is based on 36.1 fb$^{−1}$ proton-proton collision data at a centre-of-mass energy of 13 TeV recorded in 2015 and 2016 with the Atlas detector at the Large Hadron Collider. A cut-based and a multivariate analysis approach are investigated, based on the reconstructed $\tau\tau$-mass and boosted decision trees, respectively. The two dominant Higgs boson production modes, the gluon-gluon fusion and vector-boson fusion, are targeted for a mass hypothesis of $m_H$ = 125 GeV. Their characteristic event topologies are exploited to separate Higgs decays from background processes. The main background sources arise from $Z/\gamma^*$ and multijet events. The background estimation relies on both simulation and data-driven methods. An excess of events over the predicted background was found, corresponding to an observed (expected) significance of 4.15 (3.56) standard deviations for the multivariate analysis. The signal strength for the multivariate analysis was measured as $μ_{\mathrm{MVA}}$ = 1.15±0.39 and is compatible within total uncertainties with the cut-based result of $μ_{\mathrm{CBA}}$ = 0.73 ± 0.34. The measured total cross section times branching ratio for the $H\rightarrow\tau_{\mathrm{had}}\tau_{\mathrm{had}}$ hadhad process is $\sigma_{\mathrm{CBA}}$ = 1.06 ± 0.49 pb for the cut-based analysis and $\sigma_{\mathrm{MVA}}$ = 1.67 ± 0.57 pb for the multivariate approach, quoted with the respective total uncertainty. This result constitutes a direct evidence of the Higgs boson couplings to fermions and confirms the Standard Model prediction for a Higgs boson mass of $m_H$ = 125 GeV.