Searching for the Higgs Boson Produced in Association with a Pair of Top Quarks in Multilepton Final States Using the ATLAS Detector at the LHC

This thesis details a search for the associated production of a Higgs boson with a pair of top quarks using final states with multiple leptonic objects. An integrated luminosity of 36.1 fb−1 of proton-proton collision data was used, collected during 2015 and 2016 at a centre-of-mass energy of √s = 13 TeV with the ATLAS detector at the LHC. The specific final state studied in this thesis is the 3l+1τ channel which includes three light leptons (electrons or muons) and one hadronically-decaying τ-lepton. The 3l+1τ channel is primarily sensitive to the Higgs boson decaying to two τ-leptons, with additional sensitivity to the H → W+W− and H → ZZ decays. The 3l+1τ channel measured a ttH signal strength of μ = 1.6 (+1.8, -1.3) with an observed (expected) significance of 1.3 (0.9) standard deviations relative to the Standard Model background-only hypothesis. The combined ttH multilepton analysis also found an excess of events above the background-only expectation corresponding to an observed (expected) significance of 4.1 (2.8) standard deviations. The combined observed ttH signal strength of μ = 1.6 (+0.5, -0.4) is compatible with the Standard Model expectation to within approximately one standard deviation. This result shows evidence for ttH production.

Department dc:contributor.department

Physics

Year dc:date.issued

2019

Author dc:creator

• DeMarco, David Anthony

Advisor dc:contributor.advisor

• Orr, Robert S

• ATLAS
• Boson
• Higgs
• LHC
• Quark
• Top

Handle dc:identifier.uri

http://hdl.handle.net/1807/97028

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/97028