Room 203, Graduate Student Centre (6371 Crescent Road)
Add to Calendar 2019-08-27T12:30:00 2019-08-27T14:30:00 Final PhD Oral Examination (Thesis Title: “Search for the Production of Higgs Bosons in Association with Top Quarks and Decaying into Bottom Quark Pairs with the ATLAS Detector”) Event Information: Abstract:The Standard Model of particle physics (SM) describes mass generation of fundamental particles via the Brout-Englert-Higgs mechanism. It predicts Yukawa interactions between the Higgs boson and fermions, with interaction strengths proportional to the fermion masses. The largest Yukawa coupling is that of the top quark, and its value has implications in particle physics and cosmology. As the SM is not a complete theory of nature, detailed measurements of its predictions are a mandatory step towards improving the understanding of nature. This dissertation presents a search for Higgs boson production in association with a top quark pair, a process directly sensitive to the top quark Yukawa coupling. The search uses 36.1 fb−1 of data at √s = 13 TeV, collected with the ATLAS detector at the Large Hadron Collider (LHC) in 2015 and 2016. It is designed for Higgs boson decays to bottom quarks, and decays of the top quark pair resulting in final states with one or two electrons or muons. The discrimination between the signal Higgs boson production process and background processes, dominated by the production of top quark pairs, is performed with multivariate analysis techniques. The matrix element method is used and optimized for this search. Possible machine learning extensions of the method are investigated to help overcome its large computational demand. The obtained ratio of the measured cross-section for the signal Higgs boson production process to the prediction of the SM is μ = 0.84 (+0.64, -0.61). The expected sensitivity of an extension of the search, using 139.0 fb−1 of data collected between 2015 and 2018, is 3.3σ. Data collected between 2016 and 2018 is also used in a measurement of the ATLAS muon trigger system efficiency. A statistical combination of searches for Higgs bosons produced in association with top quark pairs is performed, including the search for Higgs boson decays to bottom quarks and additional final states. The combination results in the observation of this Higgs boson production process with an observed significance of 5.4σ, compared to an expected sensitivity of 5.5σ. It experimentally establishes top quark Yukawa interactions in the SM. Event Location: Room 203, Graduate Student Centre (6371 Crescent Road)
Event Time: Thursday, August 29, 2019 | 9:30 am - 11:30 am
Add to Calendar 2019-08-29T09:30:00 2019-08-29T11:30:00 Departmental Oral Examination (Thesis Title: "Spin-Orbit Coupling in Iridates") Event Information: Abstract:Transition-metal oxides (TMOs) are a widely studied class of materials with fascinating electronic properties and a great potential for applications. Sr2IrO4 is such a TMO, with a partially filled 5d t2g shell. Given the reduced Coulomb interactions in these extended 5d orbitals, the insulating state in Sr2IrO4 is quite unexpected. To explain this state, it has been proposed that spin-orbit coupling (SOC) entangles the t2g states into a filled jeff = 3/2 state and a half-filled jeff = 1/2 state, in which a smaller Coulomb interaction can open a gap. This new scheme extends filling and bandwidth, the canonical control parameters for metal-insulator transitions, to the relativistic domain. Naturally the question arises whether in this case, SOC can in fact drive such a transition. In order to address this question, we have studied the behaviour of Sr2IrO4 when substituting Ir for Ru or Rh. Both of these elements change the electronic structure and drive the system into a metallic state. A careful analysis of filling, bandwidth, and SOC, demonstrates that only SOC can satisfactorily explain the the transition. This establishes the importance of SOC in the description of metal-insulator transitions and stabilizing the insulating state in Sr2IrO4. It has furthermore been proposed that the jeff = 1/2 model in Sr2IrO4 is an analogue to the superconducting cuprates, realizing a two-dimensional pseudo-spin 1/2 model. We test this directly by measuring the spin-orbital entanglement using circularly polarized spin-ARPES. Our results indicate that there is a drastic change in the spin-orbital entanglement throughout the Brillouin zone, implying that Sr2IrO4 can not simply be described as a pseudo-spin 1/2 insulator, casting doubt on direct comparisons to the cuprate superconductors. We thus find that the insulating ground state in Sr2IrO4 is mediated by SOC, however, SOC is not strong enough to fully disentangle the jeff = 1/2 state, requiring that Sr2IrO4 is described as a multi-orbital relativistic Mott insulator. Event Location: AMPEL 488
Room 309, Hennings Bldg.
Add to Calendar 2019-08-29T12:30:00 2019-08-29T14:30:00 Final PhD Oral Examination (Thesis Title: “Search for New High-Mass Phenomena in Events with Two Muons using the ATLAS Detector at the Large Hadron Collider”) Event Information: Abstract: Although elementary particles and their interactions are extremely well modeled by the Standard Model of particle physics, some experimental measurements cannot be explained entirely by this theory. Many extensions of the Standard Model predict the existence of new phenomena at high energies. In particular, new resonance models and contact interaction models leading to dimuon final states are numerous. This dissertation presents a search for new high-mass phenomena in events with two muons using the ATLAS detector at the Large Hadron Collider. The search results are found to be consistent with the Standard Model background prediction. Interpretations both in the context of resonant and non-resonant new physics models are carried out. In particular, lower limits on the mass of hypothetical Z' bosons are set between 4.0 TeV for the Z'SSM model and 3.3 TeV for the Z'ψ model, and lower limits on the contact interaction energy scale Λ are set between 18 TeV and 30 TeV, depending on the chiral structure of the contact interaction. In addition to data analysis at the energy frontier, the performance of muon reconstruction and identification within the ATLAS experiment is detailed. More precisely, calculations of muon trigger scale factors for high-pΤ muons using events containing a leptonically decaying W boson and jets are presented. A new muon identification working point is also investigated. Finally, as the ATLAS experiment enters its second long shutdown, the first layer of the endcap regions of the muon spectrometer will be replaced with the New Small Wheels (NSWs) in order to improve both the triggering and tracking capabilities of the ATLAS detector. One of the two main technologies used in the NSW is small-strip Thin Gap Chambers (sTGCs). Work carried out with the sTGC collaboration, which aims to characterize and integrate the NSW into the ATLAS detector in the coming years, is described. Particularly, results of various test beam campaigns carried out at Fermilab and at CERN are presented. Position resolution measurements of less than 50μm are obtained. Measurements using the latest electronics readout chain of the sTGC detectors under realistic conditions are also presented. Event Location: Room 309, Hennings Bldg.