Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (M.Sc.)--Saint Mary's University, 2008. / Includes abstract and appendix. Supervisor: David Clarke. Includes bibliographical references (leaves 88-90).

Th. doct.-ing.--Nancy, I.N.P.L., 1978.

A measurement using the ATLAS detector has been made of the fraction of events in proton-proton collisions at √s = 7 TeV that do not contain additional jets in the rapidity region bounded by a di-jet system. This provided a strong test of perturbative QCD in the new energy regime of the Large Hadron Collider. Additional measurements have been made of the mean jet multiplicity in the rapidity bounded region and using a variety of different event selections. These observation were compared to recent next to leading order dijet predictions produced by POWHEG and a variety of leading order generators.

The electron density in a plasma jet has been determined as a function of position in the jet by measurements of the coherent scattering of laser light. The electron density profiles thus obtained are compared with profiles obtained from measurements of spectral line intensities. The results of these two diagnostic methods are seen to agree within experimental error. For the experiment described here, the scattering is from correlated motion of the electrons, and the scattered light spectrum has distinct electron satellites. It is shown that the width of these satellites, observed in earlier work by Chan and Nodwell (1966), may be accounted for by the electron density gradients in the plasma. This thesis gives a short review of techniques and relevant theory, with emphasis on experimental problems. A brief discussion of perturbation of the plasma by the laser light is also included. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Exploiting the substructure of jets observed at the LHC to better understand and interpret the experimental data has recently been a very active area of research. In this thesis we study the substructure of high-pt QCD jets, which form a background to many new physics searches. In particular, we explore in detail the perturbative distributions of a certain class of observables known as non-global jet shapes. More specifically, we identify and present state-of-the-art calculations, both at fixed-order and to all-orders in the perturbative expansion, of a set of large logarithms known as non-global logarithms. Hitherto, these logarithms have been largely mis-treated, and in many cases ignored, in the literature despite being first pointed out more than a decade ago. Our work has triggered the interest of many groups, particularly Soft and Collinear Effective Theory (SCET) groups, and led to a flurry of papers on non-global logarithms and related issues. Although our primary aim is to provide analytical results for hadron-hadron scattering environments, it is theoretically instructive to consider the simpler case of e+e- annihilation. We thus examine, in chapters 4, 5 and 6, the the said jet shapes in the latter environment and compute the full next-to-leading logarithmic resummation of the large logarithms present in the distribution for various jet definitions. We exploit the gained experience to extend our calculations to the more complex hadronic environment in chapter 7. We provide state-of-the-art resummation of the jet mass observable in vector boson + jet and dijet QCD processes at the LHC up to next-to-leading logarithmic accuracy. The resultant distribution of the former (vector boson + jet) process agrees well, after accounting for hadronisation corrections, with standard Monte Carlo event generators and potential comparisons to data from the LHC will be made soon.

The formation process and characteristics of sprays from annular liquid jet breakup in moving gas streams have been investigated. In the first part of the thesis, a linear instability analysis is carried out for the instability and breakup of annular liquid jets. A dispersion relation has been derived and solved numerically by using Muller's method. Temporal instability analysis shows that two independent unstable modes, para-sinuous and para-varicose, exist for the annular jet instability. The para-sinuous mode outgrows the para-varicose one at relatively low gas-liquid density ratios and large Weber numbers as typically encountered in the twin-fluid atomization. The curvature of the annular jet promotes the jet instability and may not be neglected for the breakup processes of annular liquid jets. Not only the velocity difference across each interface but also the absolute velocity of each fluid is important for the jet instability. Co-flowing gas at high velocities is found to significantly improve atomization performance. A mesh-searching method has been developed to determine absolute mode of instability. The numerical results indicate that both absolute and convective instability exist for para-sinuous and para-varicose modes under certain flow conditions. Para-sinuous unstable waves outgrow para-varicose ones, and hence dominate the jet instability according to both absolute and convective instability analysis. The liquid viscosity has a simple stabilizing effect on the jet instability while the gas inertial force shows fairly complex influence on the absolute instability of the jet. The convective growth rates for various inner gas velocities indicate that not only the velocity difference between, but also the absolute velocity of the liquid and gas, determine the jet breakup process. In the second part of this thesis, experimental investigations have been conducted for the breakup process of annular water jets exposed to an inner air stream by photographic technique, and the characteristics of the resultant sprays by Phase Doppler Particle Analyzer. Two annular nozzles of the same structure but different dimensions are designed and constructed especially to provide smooth contraction for the liquid flow. The test apparatus is constructed to produce the annular liquid sheets or sprays of good quality. Flow visualization reveals that there exist three regimes. i.e., bubble formation, annular jet formation and atomization regime for the jet breakup process. Within the bubble formation regime, the jet breakup characteristics measured from the photographs taken under various liquid and gas velocities show that uniform bubbles are observed for various air-to-water velocity ratios. The jet breakup and wave lengths decrease with the air-to-water velocity ratio. The measurements are compared with the predictions by the linear instability analysis, and fair agreement is obtained. Spray characteristics measured by a Phase Doppler Particle Analyzer indicate that using atomizing air enhances the jet breakup process and improves the atomization performance by producing fine sizes of droplets and increasing the uniformity of drop sizes. The drop axial velocity has a jet-type distribution in the radial direction, and decreases monotonically along the spray axis. Increase in the water and air velocities results in higher drop axial velocity. The droplet size described by its Sauter mean diameter (SMD) reaches a minimum value at the central region of the spray and increases towards the spray edge. The SMD has a complex variation along the spray axis. / Graduate

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