Precision Measurements of QCD Radiation in Top-Antitop and Z+Jets Final States at ATLAS

This thesis presents precision measurements and phenomenological studies of quark and gluon radiation in $t \bar{t}$ and Z + jets final states. A measurement of $t \bar{t}$ production with a veto on additional jet activity is performed using 2.1 fb$^{−1}$ of proton-proton collision data with a centre-of-mass energy of 7 TeV, collected by the ATLAS detector. Jet veto efficiency measurements are performed in several regions of rapidity and corrected for the effects of finite detector resolution and efficiency. A total experimental uncertainty of less than 5% is achieved in all distributions. Two phenomenological studies are performed on the additional quark and gluon radiation produced in association with boosted $t \bar{t}$ systems. In the first study it is shown how a measurement of the jet veto efficiency can be used to identify the colour of a TeV-scale resonance decaying to $t \bar{t}$. The second follow-up study describes how the performance of several top-tagging algorithms is affected by the colour structure of an event. Measurements of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion are performed using 20.3 fb$^{−1}$ of proton-proton collision data collected by ATLAS at 8 TeV. Detector-corrected differential cross sections, and dis- tributions sensitive to radiation produced in addition to the Z$_{jj}$ system, are measured in five fiducial regions with varying sensitivity to the electroweak component of the Z$_{jj}$ cross section. Data are compared to MC predictions and are in reasonable agreement in the majority of cases. The electroweak Z$_{jj}$ cross section is extracted and found to be in good agreement with theory predictions. Limits are also placed on anoma- lous triple gauge couplings. The commissioning of an event filter isolated muon trigger is also presented. The trigger became one of the primary muon triggers used during the 2012 data taking.