| dc.description.abstract | As heavy as an atom of gold but much smaller than a proton, the top quark is the most massive fundamental particle known in the universe and an integral part of current particle physics research. The characteristics of the top quark, such as its high mass—close to the scale of electroweak symmetry breaking—and its dominance in the running of the recently discovered Higgs boson’s self-coupling, make the top quark a cornerstone of current research. Studies of the top quark are probes of new physics, essential tools to understand the Higgs boson, and valuable tests of the Standard Model. This thesis presents two measurements of top quark pair production events collected with the ATLAS Detector at the CERN Large Hadron Collider (LHC) in proton-proton collisions at a center-of-mass energy 7-8 TeV. Both measurements select top-antitop quark events with an opposite-sign electron-muon pair and one or two b-tagged jets in the final state.
A study is first presented of the jet activity arising from quark and gluon radiation produced in association with top quark pairs, in events with an opposite-sign electron-muon pair as well as at least two b-tagged jets in the final state. It includes the complete 2012 ATLAS data sample of 20.3 fb-1 integrated luminosity of proton-proton collision data at 8 TeV. The fraction of events that does not contain additional jet activity in a central rapidity region is measured as a function of a) the minimum transverse momentum of any additional jet in the event, and b) the minimum scalar transverse momentum sum of all additional jets in the event, and the results given for four central rapidity regions and four regions of the invariant mass of the electron, muon and two b-jet system. Compensation for detector effects is applied to the experimental measurement and the results compared at the particle level to simulations by several next-to-leading order (NLO) and leading order (LO) Monte Carlo generators. The resulting gap fraction measurements, in comparison with simulation, can be used to tune Monte Carlo modeling of quantum chromodynamic (QCD) radiation and reduce associated modeling uncertainties in ATLAS physics measurements involving top quark pair production, such as top quark measurements and Higgs boson measurements with top quark backgrounds.
Secondly, the top quark pole mass (mpole) was extracted from top quark pair top events having an opposite-sign electron-muon pair in the final state and one or two b-tagged jets. This measurement includes the complete 2012 ATLAS data sample, as well as the complete 2011 ATLAS data sample of 4.6 fb-1 integrated luminosity of proton-proton collision data at 7 TeV. This is combined with the theoretical cross section prediction including QCD corrections at next-to-next-to-leading order (NNLO+NNLL), with three different sets of parton distribution functions. The results are:
top quark pole mass (7 TeV) = 171.40+2.58-2.56 GeV
top quark pole mass (8 TeV) = 174.10 ± 2.64 GeV
top quark pole mass (7-8 TeV) = 172.87+2.51-2.63 GeV
in which the uncertainties include data statistics, experimental systematic effects, the knowledge of integrated luminosity and LHC beam energy, as well as PDF, QCD scale, and strong coupling constant uncertainties. An examination of consistency between the 7 and 8 TeV results found agreement within approximately 1.6 sigma. | en_US |
