Search for $t\bar{t}H$ production and measurement of the $t\bar{t}$ cross-section with the ATLAS detector

  • Yang Qin

Student thesis: Phd


The Higgs boson and the top quark have been a focus in modern elementary particle physics research because of their special roles in the Standard Model (SM) of particle physics. The studies of both particles are crucial for revealing the unsolved puzzles of modern particle physics. The coupling between the Higgs boson and the top quark, the top-Yukawa coupling, is one of the fundamental parameters in the SM that can potentially direct the future development of the theory of elementary particle physics.This thesis presents two analyses on the Higgs boson and the top quark, using proton-proton ($pp$) collision data collected by the ATLAS detector during 2012 and 2015. A search for the SM Higgs boson produced in association with a top quark pair ($t\bar{t}H$) was performed using 20.3~fb$^{-1}$ of $pp$ collision data at a centre-of-mass energy of $\sqrt{s} = 8$~TeV. The search is designed to be primarily sensitive to the $H \rightarrow b\bar{b}$ decay mode. Events with one of two electrons or muons are used in the search. No significant excess of events is observed above the background predicted by the SM. An observed (expected) upper limit on the signal strength of 3.4 (2.2) times the SM prediction is obtained at 95\% confidence level. The $t\bar{t}H$ signal strength, represented by the ratio of the measured $t\bar{t}H$ cross-section to the SM prediction, is found to be $\mu = 1.5 \pm 1.1$ for a Higgs boson mass of $m_H = 125$~GeV. A measurement of the top quark pair ($t\bar{t}$) production cross-section was performed using 3.2~fb$^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$~TeV. The measurement uses events with an opposite-charge-sign electron-muon pair and exactly one and two jets originating from $b$ quarks. The inclusive $t\bar{t}$ production cross-section is measured to be $\sigma_{t\bar{t}} = 818 \pm 8 \, (\mathrm{stat}) \pm 27 \, (\mathrm{syst}) \pm 19 \, (\mathrm{lumi}) \pm 12 \, (\mathrm{beam}) \,\, \mathrm{pb}$, where the uncertainties arise from data statistics, analysis systematic effects, the integrated luminosity and the LHC beam energy. The total relative uncertainty is 4.4\%. The result is consistent with the theoretical prediction at the next-to-next-to-leading order accuracy in the strong coupling constant $\alpha_S$ of QCD, with the resummation of next-to-next-to-leading logarithmic (NNLL) soft gluon terms. A fiducial cross-section corresponding to the experimental acceptance of leptons is also measured.
Date of Award1 Aug 2017
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorYvonne Peters (Supervisor) & Alexander Oh (Supervisor)


  • ttbar
  • ttH
  • top-Yukawa coupling
  • Higgs boson
  • LHC
  • top quark
  • cross-section

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