Probing Quantum Chromodynamics with the ATLAS Detector: Charged-Particle Event Shape Variables and the Dijet Cross-Section

Quantum chromodynamics, QCD, the theory of the strong interaction is split into two regimes. Scattering processes of the proton constituents, the partons, with a high momentum transfer $Q^2$ can be calculated and predicted with perturbative calculations. At low momentum transfers between the scattering particles perturbation theory is not applicable anymore, and phenomenological methods are used to describe the physics in this regime. The ATLAS experiment at the Large Hadron Collider, LHC, provides the possibility to analyze QCD processes at both ends of the momentum scale. Two measurements are presented in this thesis, emphasizing one of the two regimes each: The measurement of charged-particle event shape variables in inelastic proton–proton collisions at a center-of-mass energy of $\sqrt{s}$ = 7 TeV analyses the transverse momentum flow and structure of hadronic events. Due to the, on average, low momentum transfer, predictions of these events are mainly driven by non-perturbative models. Three event shapes are studied: The transverse thrust, the thrust minor and the transverse sphericity. Data recorded during low pile-up beam conditions in early 2010, corresponding to an integrated luminosity of 168 ${\mu}$b, is analyzed. Charged particles with a transverse momentum above 500 MeV are used to compute the event shape variables. In addition to differential event shape distributions in $p_T$ of the highest-transverse- momentum particle, the evolution of each event shape variable as a function of charged particle multiplicity and summed transverse momentum is presented. Predictions from different Monte Carlo models are compared with data and they show significant deviations from data. Using the results to tune the free parameters of the phenomenological models can improve future Monte Carlo predictions and give additional insight to the non-perturbative part of the QCD. The measurement of the double-differential dijet cross section as function of the dijet mass and rapidity separation of the two highest-transverse-momentum jets in an event, the leading and the subleading jet, probes the QCD up to the highest scales. Proton–proton collisions at $\sqrt{s}$ = 8 TeV recorded during 2012, corresponding to an integrated luminosity of 20.3 fb$^{−1}$, are analyzed. Jets are identified using the anti-kt jet clustering algorithm with the radius parameters R = 0.4 and R = 0.6. The cross section is measured for events with leading jet transverse momentum above 100 GeV and sub-leading jet transverse momentum above 50 GeV within |y| < 2.8. Several NLO predictions are calculated, using four different parton distribution functions, and corrected for non-perturbative effects. The measured results are corrected for detector effects and compared to the four NLO predictions. A good agreement within the systematic and statistical uncertainties between data and theory is observed for dijet masses below ∼ 3 TeV. Above that, deviations are observed between the data and perturbative predictions. Miscellaneous behaviour for theory predictions using different PDF sets is visible in the results. Including the results of the measurement into future PDF determinations can improve their performance.