Standard model and exotic physics with the top quark at ATLAS

Abstract

The top quark is the most massive fundamental particle in the Standard Model of particle physics. Only experimentally observed in 1995, it can be used as a precise test of Standard Model predictions, and it could lend insight to the problem of what lies beyond the Standard Model. This thesis presents a measurement of top-quark pair production using data collected at a center-of-mass energy √s=7 TeV in 2011, and a search for production of vector-like quarks using data collected at √s=8 TeV in 2012. Both datasets were recorded by the ATLAS detector, a multipurpose proton-proton collider located at the CERN LHC outside of Geneva Switzerland.

The top-quark pair production cross-section is measured as a function of four different variables and the results are presented as normalized, differential spectra. The variables considered are the transverse momentum of the top quark, and the mass, rapidity, and transverse momentum of the top-quark pair system. Events are selected in the lepton+jets channel, and the measured spectra are corrected for detector resolution and efficiency. The final results are compared with predictions from various Monte Carlo generators, theoretical calculations and proton parton distribution functions and found to be in reasonable agreement. Data is found to be softer than all predictions, particularly for high values of top-quark transverse momentum and the top-quark pair invariant mass.

The search for vector-like quarks focuses on new heavy quarks that decay with a large branching ratio to a Z boson and a third generation Standard Model quark. Events are selected with at least two leptons (electrons or muons), and two of the leptons are required to reconstruct a Z boson with high transverse momentum. No significant excess of events is observed above the Standard Model prediction. Upper limits on the masses of vector-like T and B quarks are derived for various branching ratio hypotheses.