Search for neutral bosons decaying into the fully hadronic di-tau final state with the ATLAS detector at the LHC

This thesis presents a search for neutral bosons, such as new Higgs and Z' bosons, predicted by theories extending the Standard Model of particle physics. The search is performed in the di-tau analysis channel, where both tau leptons decay hadronically. Promising candidates of such theories are the Minimal Supersymmetric Standard Model (MSSM) and the non-universal G(221) model, which predict large couplings to tau leptons in large regions of their parameter space. Proton–proton collisions produced by the Large Hadron Collider (LHC) at a center-of-mass energy of 13 TeV, recorded with the ATLAS detector in 2015 and 2016, are analyzed for this search. The data correspond to an integrated luminosity of 36.1 fb^-1. This search relies on sophisticated algorithms for the reconstruction of hadronic tau decays from their decay products. This thesis presents a novel approach employing multivariate techniques to significantly improve existing algorithms, which became the default for reconstruction of hadronic tau decays in ATLAS since 2017. Additionally, the new method can provide useful information for subsequent tau identification algorithms. The MSSM extends the Higgs sector of the Standard Model by four additional Higgs bosons. Of particular interest for this thesis are the neutral CP-even H and CP-odd A bosons. The search for these bosons is performed in the mass range of 0.2 TeV to 2.25 TeV in two orthogonal categories depending on the number of identified b-quarks, each preferring one of the two considered production modes via gluon–gluon fusion or b-associated production. The data are in good agreement with the Standard Model prediction. Upper limits are set on the cross-section times branching fraction using a confidence level (CL) of 95 % independently for both production modes. Most stringent observed limits are found for a resonance mass of 1.5 TeV to be 4.94 fb and 3.65 fb for gluon–gluon fusion and b-associated production, respectively. The results are further interpreted in the hMSSM, mhmax and mhmod scenarios. Observed upper limits in the hMSSM scenario on tanβ are found to be between 4.6 at mA = 0.25 TeV and 41.4 at mA = 1.5 TeV. The search for additional Z' bosons is performed independently on the number of identified b-quarks in the mass range of 0.2 TeV to 4 TeV. As for the search for additional Higgs bosons no significant hint for new physics has been observed. 95 % CL observed upper limits are set on the cross-section times branching fraction for Z' bosons in the Sequential Standard Model (SSM) benchmark scenario between 20.5 pb at mZ' = 0.2 TeV and 7.74 fb at mZ' = 1.75 TeV. The observed upper limit for the highest considered mass of mZ' = 4 TeV is found to be 16 fb. Z' bosons in the SSM and the non-universal G(221) model are excluded at 95 % CL for masses below 2353 GeV and 2232 GeV, respectively.