Abstract: On the 4 th of July 2012 the observation of a new neutral particle was announced by the ATLAS [1] and CMS [2] Collaborations. This particle is by now generally considered as the Higgs boson H predicted [3, 4] in the Standard Model of particle physics. Its mass of m H = 125.09 ± 0.24 GeV [5] implies a rich set of final states which allow for experimental studies in regards to this particle and searches for deviations from the properties predicted by the Standard Model. In this thesis a measurement of the gluon-fusion and vector-boson fusion production cross sections times H→WW* branching ratio is presented using final states with one electron and one muon. The measurement is based on proton- proton collisions with an integrated luminosity of 36.1 fb -1 √ recorded with the ATLAS detector at the CERN Large Hadron Collider ( LHC ) at s = 13 TeV in 2015 and 2016. Signal-like events are selected in categories with different jet multiplicities. The results are extracted by means of a binned maximum-likelihood fit. To this end, either distributions in multiple discriminant variables or the response of a multivariate classifier are used depending on the category. The results are found to be well compatible with the Standard Model predictions. This analysis is extrapolated to estimate the future precision of such a measurement at the end of the High-Luminosity LHC programme [6]. This extrapolated analysis is combined with analogously extrapolated analyses targeting other production and decay modes of the Higgs boson. Based on this combination the expected precision is determined for future measurements of couplings between the Higgs boson and other particles. Thereby an estimation is given for the anticipated sensitivity of these analyses to signs of physics beyond the Standard Model. In addition studies are presented to identify sources of electron charge misidentification in the reconstruction algorithms employed in the ATLAS Collaboration.

Abstract: This Letter presents direct searches for lepton flavour violation in Higgs boson decays, and , performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy TeV, corresponding to an integrated luminosity of . No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% () and 0.28% () for and , respectively

Abstract: The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in tt ̄ events. The topology of the t→Wb decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 fb−1, were collected in proton-proton collisions during the years 2015-2017 at a centre-of-mass energy s√= 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum

Abstract: A test of CP invariance in Higgs boson production via vector-boson fusion is performed in the decay channel. This test uses the Optimal Observable method and is carried out using of proton-proton collision data collected by the ATLAS experiment at the LHC. Contributions from CP-violating interactions between the Higgs boson and electroweak gauge bosons are described by an effective field theory, in which the parameter governs the strength of CP violation. No sign of CP violation is observed in the distributions of the Optimal Observable, and is constrained to the interval [−0.090, 0.035] at the 68% confidence level (CL), compared to an expected interval of based upon the Standard Model prediction. No constraints can be set on at 95% CL, while an expected 95% CL interval of for the Standard Model hypothesis was expected

Abstract: Constraints on selected mediator-based dark matter models and a scalar dark energy model using up to 37 fb−1s√ = 13 TeV pp collision data collected by the ATLAS detector at the LHC during 2015-2016 are summarised in this paper. The results of experimental searches in a variety of final states are interpreted in terms of a set of spin-1 and spin-0 single-mediator dark matter simplified models and a second set of models involving an extended Higgs sector plus an additional vector or pseudo-scalar mediator. The searches considered in this paper constrain spin-1 leptophobic and leptophilic mediators, spin-0 colour-neutral and colour-charged mediators and vector or pseudo-scalar mediators embedded in extended Higgs sector models. In this case, also s√ = 8 TeV pp collision data are used for the interpretation of the results. The results are also interpreted for the first time in terms of light scalar particles that could contribute to the accelerating expansion of the universe (dark energy)