Search for Dark Matter in events with missing transverse momentum and a Higgs boson decaying into bottom quarks with the ATLAS detector

Many astrophysical observations indicate the existence Dark Matter, which is not predicted by the Standard Model (SM) of particle physics. Many theories beyond the Standard Model (BSM) predict new heavy particles called WIMPs (Weakly Interacting Massive Particles). One way to search for WIMP dark matter is through its pair-production in collider experiments. A possible signature, in this case, is the associated production of a WIMP pair with a Higgs boson decaying to a pair of b-quarks. The WIMPs leave the detector without interacting, leading to events with a large amount of missing transverse momentum and two highly-energetic b-jets due to the decay of the Higgs boson. The search presented in this thesis uses such events based on data from proton-proton collisions recorded with the ATLAS detector from 2015 to 2018 at √s=13 TeV. A profile likelihood fit is performed to extract a possible signal. No significant excess above the SM predictions is observed, and the results are interpreted in terms of the Z'2HDM and 2HDM+a benchmark models. In addition, model-independent limits are set on the cross-section for new physics at 95% confidence level (CL). The thesis also presents a reinterpretation of the results in terms of the Dark-Higgs model. All physics analyses rely on the reconstruction of physics objects and the high-precision measurements of their energy and momentum. The outermost part of the ATLAS Inner Detector, Transition Radiation Tracker (TRT), contributes significantly to the momentum measurement of the charged particles by extending the length of the measured trajectories. This thesis describes studies of the drift-time calibration of the TRT. These studies examine the dependence of the calibration parameters on the transverse momentum of reconstructed tracks and pile-up using simulated events. Furthermore, the effect of additional drift-time corrections in straw tubes filled with Argon-based gas mixture is studied.