Date of Award

Fall 2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Engineering Physics

First Advisor

Markus Wobisch

Abstract

The strong force is one of the four fundamental forces and its strength is given by the coupling constant αs. The theory that describes the strong interaction is Quantum Chromodynamics (QCD) and it explains the interactions between quarks and gluons. The strong coupling constant is the only free parameter in the QCD Lagrangian if the quark masses are fixed. Determinations of αs provide direct tests of perturbative QCD calculations. The collimated sprays of particles originating from the quark and gluon interactions are called jets. The ratio of jet cross sections are sensitive to α s and are used in the determination of α s. One such quantity is RΔR, which is the ratio of a three-jet cross section to the inclusive jot cross section. RΔR is the average number of neighbouring jets for jets from an inclusive sample. It is measured as a function of the transverse momentum of the inclusive jets for different distances ( ΔR) to the neighbouring jets in the plane of rapidity and azimuthal angle, and for different transverse momentum requirements for the neighbouring jets. The measurement is based on a data set corresponding to an integrated luminosity of ∼20 fb-1 collected with the ATLAS detector at the Large Hadron Collider in pp collisions at [special characters omitted] = 8 TeV in 2012. The results are compared to the predictions of a perturbative QCD calculation in next-to-leading order in the strong coupling constant corrected for non-perturbative effects. Theory is giving a good description of the data over the whole kinematic range of the measurement.

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