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Tests of the current understanding of physics at the highest energies achievable in man-made experiments are performed at CERN’s Large Hadron Collider. In the theory of the strong force within the Standard Model of particle physics - Quantum ChromoDynamics or QCD - confined quarks and gluons from the proton-proton scattering manifest themselves as groups of collimated particles. These particles are clustered into physically measurable objects called hadronic jets. As jets are widely produced at hadron colliders, they are the key physics objects for an early "rediscovery of QCD". This thesis presents the first jet measurement from the ATLAS Collaboration at the LHC and confronts the experimental challenges of precision measurements. Inclusive jet cross section data are then used to improve the knowledge of the momentum distribution of quarks and gluons within the proton and of the magnitude of the strong force.
Dijet production is measured in proton--proton collisions with a center-of-mass energy of 7 TeV at the LHC. Double-differential dijet cross sections are presented as functions of the dijet mass and rapidity separation of the two highest-transverse-momentum jets in an event. The data sample collected in 2011 by the ATLAS detector is considered, corresponding to an integrated luminosity of 4.5 fb -1. Jets are defined using the anti-kt algorithm, with two choices of the jet radius parameter, R = 0.4 and R = 0.6. The results are compared with next-to-leading-order (NLO) perturbative calculations, as well as NLO Monte Carlo generator predictions. In both cases, electroweak effects are accounted for. A method for the quantitative comparison of theory predictions with data is presented, employing a frequentist technique. This allows for strong statements on which sets of parton distribution functions best describe the data. A theory of quark compositeness modeled by contact interactions is confronted by the unfolded data using the CLs technique, an example of the versatility of the measurement.
This book reviews the latest experimental results on jet physics from proton-proton collisons at the LHC. Jets allow to determine the strong coupling constant over a wide range of energies up the highest ones possible so far, and to constrain the gluon parton distribution of the proton, both of which are important uncertainties on theory predictions in general and for the Higgs boson in particular.A novel approach in this book is to categorize the examined quantities according to the types of absolute, ratio, or shape measurements and to explain in detail the advantages and differences. Including numerous illustrations and tables the physics message and impact of each observable is clearly elaborated.
Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of √s = 7TeV using the ATLAS detector at the large hadron collider. The measurements are presented as a function of the three-jet mass (mjjj), in bins of the sum of the absolute rapidity separations between the three leading jets (
This thesis is based on the first data from the Large Hadron Collider (LHC) at CERN. Its theme can be described as the classical Rutherford scattering experiment adapted to the LHC: measurement of scattering angles to search for new physics and substructure. At the LHC, colliding quarks and gluons exit the proton collisions as collimated particle showers, or jets. The thesis presents studies of the scattering angles of these jets. It includes a phenomenological study at the LHC design energy of 14 TeV, where a model of so-called large extra dimensions is used as a benchmark process for the sensitivity to new physics. The experimental result is the first measurement, made in 2010, by ATLAS, operating at the LHC start-up energy of 7 TeV. The result is compatible with the Standard Model and demonstrates how well the physics and the apparatus are understood. The first data is a tiny fraction of what will be accumulated in the coming years, and this study has set the stage for performing these measurements with confidence as the LHC accumulates luminosity and increases its energy, thereby probing smaller length scales.