Tesi: "Quantum chromodynamics"

1

Soerensen, P. H. "Soft divergences in quantum chromodynamics". Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384315.

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2

Von, Oertzen Detlof Wilhelm. "Transport coefficients in quantum chromodynamics". Doctoral thesis, University of Cape Town, 1990. http://hdl.handle.net/11427/22057.

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Relativistic kinetic theory provides a transport equation for classical, spinless, colored particles in a non-Abelian external field. We review the methods of solution used in the literature to find the transport coefficients for quark and gluon systems. Most authors use the relaxation time approximation of the Boltzmann equation to compute the transport coefficients, but this method has shortcomings in mixtures. We use the Chapman Enskog (CE) method to solve the classical transport equations for quarks and gluons for the transport coefficients. The differential crosssections describing the particle interaction are obtained from the lowest order scattering diagrams of quantum chromodynamics. We study a pure quark system, a pure gluon system and a quark antiquark (qq) mixture. For mixtures of quarks, antiquarks and gluons, we find the shear viscosity, heat conductivity and cross-coefficients. The coefficients pertaining to qq mixtures, namely the thermal diffusion, diffusion and Dufour coefficient, the viscosities and heat conductivity are obtained and the conductivity of a qq mixture in an external field is computed. We compare our transport coefficients to others in the literature by rewriting them in terms of characteristic relaxation times. Although our results are generally larger than others, they are of the same order of magnitude, with important implications for quark-gluon (QG) plasma signatures. The quark to gluon shear viscosity ratio is found to be ~5 times the number of quark flavors, emphasising the importance of quarks in dynamical QG calculations. The coefficients for a field-free qq mixture indicate no qq separation in the presence of a temperature gradient. In the CE method, the transport coefficients depend naturally on a logarithmic factor due to the divergent scattering cross-sections, reflecting the plasma shielding effects. This logarithm is evaluated by relating it to typical plasma parameters. We apply our results to the QG phase in the early universe and ultra-relativistic heavy ion collisions. A comparison of the QG to pion transport coefficients at the quark-hadron phase transition shows that the latter are ~10³ smaller. Dissipative effects increase the plasma lifetime, resulting in a longer high energy density and temperature plasma phase.

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3

Tsegaye, Takele Dessie. "Confinement Mechanisms in Quantum Chromodynamics". University of Cincinnati / OhioLINK, 2003. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1051373650.

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4

Detmold, William. "Nonperturbative approaches to quantum chromodynamics". Title page, contents and abstract only, 2002. http://web4.library.adelaide.edu.au/theses/09PH/09phd4817.pdf.

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5

Peláez, Arzúa Monica Marcela. "Infrared correlation functions in Quantum Chromodynamics". Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066491/document.

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Le but de cette thèse est l'étude des fonctions de corrélation des théries Yang-Mills dans le régime infrarouge. Il est connu que, à cause de l'invariance jauge, il est nécessaire de fixer la jauge pour calculer des valeurs moyennes analytiquement. La procedure de fixation gauge standard est la procedure de Faddeev-Popov (FP). Le Lagrangien de FP permet de faire des calculs perturbatifs pour la Chromodynamique Quantique dans le régime de hautes énergies dont les résultats sont comparés avec succès avec des expériences. Cependant, dans le régime de basses énergies, il se trouve que la constante de couplage, calculée avec la procedure antérieure, diverge. En conséquence, la théorie des perturbations standard n'est plus valide. D'autre part, les simulations du réseau trouvent que la constante de couplage est finie avec une valeur modérée même dans le régime infrarouge. Ceci suggère qu'il devrait exister une manière de faire des calculs perturbatifs également dans le régime infrarouge. Cette différence dans la constante de couplage peut être due au fait que la procedure de FP n'est pas bien justifiée dans ce régime. Nous proposons de modifier le Lagrangien de FP avec un terme massif pour les gluons. Cette modification est également justifiée par le fait que le réseau trouve un propagateur du gluon qui paraît massive aux basses énergies. Nous utilisons cette version massive pour calculer à une boucle les fonctions de corrélations à deux et trois points pour une configuration cinématique générale et en dimension quelconque dans la jauge de Landau. On trouve que les comparaisons de notre calcul à une boucle avec les résultat du réseau donnent, en géneral, un très bon accord
The aim of this thesis is to investigate the infrared behaviour of Yang-Mills correlation functions. It is known that the gauge invariance of the theory brings as a consequence the necessity of a gauge fixing procedure in order to compute expectation values analytically. The standard procedure for fixing the gauge is the Faddeev-Popov (FP) procedure which allows one to do perturbation theory in the ultraviolet regime. Perturbative calculations using the FP gauge fixed action successfully reproduce Quantum Chromodynamics observables measured by experiments in the ultraviolet regime. In the infrared regime the coupling constant of the theory computed with the above procedure diverges, and standard perturbation theory does not seem to be valid. However, lattice simulations show that the coupling constant takes finite and not very large value. This suggests that some kind of perturbative calculations should be valid even in the infrared regime. The theoretical justification for the FP procedure depends on the absence of Gribov copies and hence is not valid in the infrared regime (where such copies exist). To correct this we propose to add a mass term for the gluons in the gauge-fixed Lagrangian. The gluon mass term is also motivated by lattice simulations which observe that the gluon propagator behaves as it was massive in the infrared regime. We use this massive extension of the FP gauge fixed action to compute the one loop correction of the two- and three-point correlation functions in the Landau gauge for arbitrary kinematics and dimension. Our one-loop calculations are enough, in general, to reproduce with good accuracy the lattice data available in the literature

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6

Bass, Steven David. "Spin dependent effects in quantum chromodynamics /". Title page, contents and abstract only, 1992. http://web4.library.adelaide.edu.au/theses/09PH/09phb317.pdf.

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Thesis (Ph. D.)--University of Adelaide, Dept. of Physics and Mathematical Physics, 1993.
Copies of three of the author's previously published articles inserted as appendix B. Includes bibliographical references.

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7

Malaza, E. D. "Jet multiplicity distributions in quantum chromodynamics". Thesis, University of Cambridge, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382607.

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8

Dasgupta, M. "Power suppressed corrections in quantum chromodynamics". Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598295.

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The purpose of the research described in this dissertation, is to establish a bridge between perturbative and non-perturbative aspects of QCD. While it is remarkable that the factorisation theorems of QCD allow one the freedom to carry out perturbative calculations in most cases of interest, it cannot be forgotten that perturbation theory can never provide a complete description of strong interaction phenomena. The factorisation theorems themselves make allowance for non-perturbative effects by parametrising them through parton distributions and fragmentation functions, which has given rise to what has become known as QCD phenomenology. In this dissertation we adopt a phenomenological approach to the study of a certain class of non-perturbative effects which are manifested in terms that behave as an inverse power of the relevant hard scale. The idea underlying our approach is to examine the ambiguity of the perturbative series and then interpret it as representing non-perturbative effects. To make quantitative estimates we adopt an approach in which our predictions depend on supposedly universal parameters that can be extracted from experiment in the same spirit as parton distributions. These parameters are the moments of the strong coupling, which is assumed infrared finite. Within this approach we can relate the power corrections to different observables, due to the universality assumption. Knowledge of these parameters can be employed to constrain the form of the strong coupling at small scales which may allow a unique glimpse into the confinement domain. In this dissertation we make unambiguous predictions for power corrections to a wide variety of observables. These include DIS structure functions, fragmentation functions in both e⁺e^- annihilation and DIS, and event shape variables. In most cases we observe that there is good support for the predictions made here, from experimental data.

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9

Daniel, David John. "Towards phenomenology from lattice quantum chromodynamics". Thesis, University of Edinburgh, 1987. http://hdl.handle.net/1842/13558.

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10

Sheard, Stephen Noel. "Weak interactions in lattice quantum chromodynamics". Thesis, University of Edinburgh, 1988. http://hdl.handle.net/1842/14402.

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11

Chalmers, David Lindsay. "Numerical studies of lattice quantum chromodynamics". Thesis, University of Edinburgh, 1985. http://hdl.handle.net/1842/13347.

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12

Dharmaratna, Welathantri G. D. "Massive quark polarization in Quantum Chromodynamics subprocesses /". Thesis, Connect to Dissertations & Theses @ Tufts University, 1990.

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Thesis (Ph.D.)--Tufts University, 1990.
Adviser: Gary R. Goldstein. Submitted to the Dept. of Physics. Includes bibliographical references. Access restricted to members of the Tufts University community. Also available via the World Wide Web;

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13

Hierl, Dieter. "Lattice Quantum ChromoDynamics with approximately chiral fermions". kostenfrei, 2008. http://www.opus-bayern.de/uni-regensburg/volltexte/2008/1020/.

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14

Papadopoulos, Stavros. "Aspects of perturbative corrections in quantum chromodynamics". Thesis, McGill University, 1989. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=74317.

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The perturbative regime of Quantum Chromodynamics (QCD) is considered and certain aspects related with higher order corrections (HOC) are studied. Certain large correction terms in the perturbative expansion are determined, in particular for large transverse momentum $(p sb{T})$ direct photon production. The origin of these terms is specified and simple forms, called K-factors, are provided (soft gluon approach).
Furthermore, for processes initiated by 2 $ to$ 2 particle subprocesses the structure of the complete HOC is analyzed. It is shown that when structure functions and/or fragmentation functions are involved, there is a gauge invariant part that dominates HOC over a sizable kinematic range. Simple and general expressions are derived allowing an easy calculation of this part. Also, it is shown that, under certain approximations, this part reduces to the form of the simple K-factors.
Other aspects of HOC, in particular the dependence on the choice of scales, are considered. Using complete HOC, detailed analysis of recent and old data on large-$p sb{T}$ direct photon production is carried out. The dependence of the form of the gluon distribution on the choice of the scales (physical versus optimal scales) is discussed and it is concluded that appreciable ambiguity in this distribution still remains.

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15

Portelli, Antonin. "Nonpertubative quantum chromodynamics and isospin symmetry breaking". Thesis, Aix-Marseille, 2012. http://www.theses.fr/2012AIXM4110.

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Depuis les années 1930, on sait que le noyau des atomes est composé de deux types de particules: les protons et les neutrons. Ces deux particules sont très similaires: d'une part le neutron est subtilement plus lourd (un pour mille) que le proton et d'autre part le proton porte une charge électrique positive tandis que le neutron est neutre. La petite différence de masse entre le neutron et le proton fourni l'énergie suffisante pour autoriser désintégration où un neutron se désintègre en un proton en émettant un électron et un anti-neutrino électronique. Aussi, le fait que le proton ne se désintègre pas assure la stabilité de l'atome d'hydrogène. De plus, on sait empiriquement que les paramètres de la désintégration déterminent la composition des noyaux d'atomes stables plus lourds que l'hydrogène. Il est donc raisonnable de penser que si la différence de masse entre le neutron et le proton était de signe opposé ou seulement légèrement différente, l'Univers visible serait surement très différent de celui que l'on connait. Il est donc essentiel de comprendre l'origine de cette différence de masse à partir des principes premiers de la physique. C'est à ce problème, et à des problèmes liés à celui-ci, qu'essaye de répondre ce travail. Dans la compréhension actuelle de la physique, les neutrons et les protons sont des particules composées de particules élémentaires appelées quark up (symbole u) et quark down (symbole d). Le proton est un état lié uud et le neutron est un état lié udd. Les quarks up et down sont deux particules similaires: elles sont toutes deux légères (de l'ordre de quelques MeV) et leurs charges électriques sont différentes

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16

Shi, Zhifeng. "Multi-meson systems from Lattice Quantum Chromodynamics". W&M ScholarWorks, 2013. https://scholarworks.wm.edu/etd/1539623623.

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Systems of non-zero isospin chemical potential ( muI), where the chemical potential for up and down quarks is equal in magnitude but of opposite sign, do not suffer from the sign problem, and normal LQCD techniques can be successfully adapted to study such systems. From chiral perturbation theory (chiPT), in addition to the deconfined phase transition at high temperature at zero chemical potential, another phase transition from ordinary hadronic states to a Bose Einstein Condensate (BEC) state has been conjectured [1] at non-zero isospin chemical potential. Such a BEC phase is of phenomenological relevance in the interior of neutron stars.;In LQCD, one way to investigate non-zero isospin chemical potential system is from a grand canonical approach by directly working with fermion actions of targeted isospin chemical potentials. Another approach to isospin chemical potential is by explicitly constructing systems of fixed isospin density, and inferring the isospin chemical potential from its ground state energy. In Ref. [2], the first studies of nonzero isospin chemical potential system from this approach were presented, finding that the dependence of the isospin chemical potential on the isospin density agrees with predictions from Ref. [1] at low density. In this thesis, we studied systems with the quantum numbers of up to 72 pions with newly constructed algorithms, and clearly identified the conjectured phase transition from a pion gas to a BEC state at muI = 1.3 mpi at T ≈ 20 MeV for the first time.;Having numerically constructed a novel state of matter, a natural question to ask is how it can be investigated. The suppression of J/psi and Upsilon resonances [3] at non-zero temperature in heavy ion collision is an important diagnostic of the formation of a quark-gluon plasma. Such suppression effects have been experimentally observed at Super Proton Synchrotron (SPS), RHIC and LHC [3]. Heavy quarks are naturally also expected to be useful probes of phase transitions at non-zero baryon chemical potential and non-zero isospin chemical potential. In this thesis, we investigated both bottomonium and charmonium in media of non-zero isospin chemical potential.;The investigation of QCD at non-zero isospin density presented in this thesis provide a numerical window into a novel state of strongly interacting matter. This matter is difficult to create in experiment but may play an important role in dense astrophysical environments.

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17

Cook, Merritt S. "Exotic meson decay widths using lattice quantum chromodynamics". FIU Digital Commons, 2006. http://digitalcommons.fiu.edu/etd/2526.

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Most experiments in particle physics are scattering experiments, the analysis of which leads to masses, scattering phases, decay widths and other properties of one or multi-particle systems. Until the advent of Lattice Quantum Chromodynamics (LQCD) it was difficult to compare experimental results on low energy hadron- hadron scattering processes to the predictions of QCD, the current theory of strong interactions. The reason being, at low energies the QCD coupling constant becomes large and the perturbation expansion for scattering amplitudes does not converge. To overcome this, one puts the theory onto a lattice, imposes a momentum cutoff, and computes the integral numerically. For particle masses, predictions of LQCD agree with experiment, but the area of decay widths is largely unexplored. LQCD provides ab initio access to unusual hadrons like exotic mesons that are predicted to contain real gluonic structure. To study decays of these type resonances the energy spectra of a two-particle decay state in a finite volume of dimension L can be related to the associated scattering phase shift δ(k) at momemtum k through exact formulae derived by Lüscher. Because the spectra can be computed using numerical Monte Carlo techniques, the scattering phases can thus be determined using Lüscher's formulae, and the corresponding decay widths can be found by fitting Breit-Wigner functions. Results of such a decay width calculation for an exotic hybrid(h) meson (JPC = 1-+) are presented for the decay channel h → πa1. This calculation employed Lüscher’s formulae and an approximation of LQCD called the quenched approximation. Energy spectra for the h and πa1 systems were extracted using eigenvalues of a correlation matrix, and the corresponding scattering phase shifts were determined for a discrete set of πa1 momenta. Although the number of phase shift data points was sparse, fits to a Breit-Wigner model were made, resulting in a decay width of about 60 MeV.

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18

Fariborz, A. H. "Investigations in light-quark low-energy quantum chromodynamics". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/nq28486.pdf.

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19

Ford, I. J. "Aspects of pure quantum chromodynamics on large lattices". Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382631.

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20

Tonkin, S. P. "A study of the vacuum of quantum chromodynamics". Thesis, University of Oxford, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.375326.

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21

Nail, Graeme. "Quantum chromodynamics : simulation in Monte Carlo event generators". Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/quantum-chromodynamics-simulation-in-monte-carlo-event-generators(46dc6f2e-1552-4dfa-b435-9608932a3261).html.

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This thesis contains the work of two recent developments in the Herwig general purpose event genrator. Firstly, the results from an new implementation of the KrkNLO method in the Herwig event generator are presented. This method allows enables the generation of matched next-to-leading order plus parton shower events through the application of simple positive weights to showered leading order events. This simplicity is achieved by the construction Monte Carlo scheme parton distribution functions. This implementation contains the necessary components to simulation Drell-Yan production as well as Higgs production via gluon fusion. This is used to generate the first differential Higgs results using this method. The results from this implementation are shown to be comparable with predictions from the well established approaches of POWHEG and MC@NLO. The predictions from KrkNLO are found to closely resemble the original configuration for POWHEG. Secondly, a benchmark study focussing on the source of perturbative uncertainties in parton showers is presented. The study employs leading order plus parton shower simulations as a starting point in order to establish a baseline set of controllable uncertainties. The aim of which is to build an understanding of the uncertainties associated with a full simulation which includes higher-order corrections and interplay with non- perturbative models. The uncertainty estimates for a number of benchmark processes are presented. The requirement that these estimates be consistent across the two distinct parton show implementations in Herwig provided an important measure to assess the quality of these uncertainty estimates. The profile scale choice is seen to be an important consideration with the power and hfact displaying inconsistencies between the showers. The resummation profile scale is shown to deliver consistent predictions for the central value and uncertainty bands.

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22

Hutchcroft, David Edward. "Quantum chromodynamics with the OPAL detector at LEP". Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624733.

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23

Zimak, Petr. "Canonical quantization and quantum chromodynamics in a cavity". Doctoral thesis, University of Cape Town, 1986. http://hdl.handle.net/11427/22142.

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The canonical quantization formalism is applied to the Lagrange density of chromodynamics in a general covariant gauge. The physical states are characterized by their BRS-invariance. We develop the quantum theory of the interacting fields in the Dirac picture, based on the Gell-Mann and Low Theorem and the Dyson expansion of the time evolution operator. Subsequently, confinement is introduced phenomenologically by imposing, on the quark, gluon and ghost field operators, the linear boundary conditions of the M.I.T. bag model at the surface of a spherically symmetric and static cavity. Based on this formalism, we calculate, in the Feynman gauge, all non-divergent Feynman diagrams of second order in the strong coupling constant g. Explicit values of the matrix elements are given for low-lying quark and gluon cavity modes.

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24

Schreiber, Gunhild Ursula. "The gluon self-energy in cavity quantum chromodynamics". Doctoral thesis, University of Cape Town, 1991. http://hdl.handle.net/11427/17391.

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Bibliography: p. 78-79.
A numerical technique to regularize divergent loop diagrams in cavity quantum chromodynamics is discussed, which is closely related to free space dimensional regularization. In this cavity regularization method, the energy shift is expressed as the integral of a divergent spectral function, from which the divergence may be extracted by analogy to the free space expression. It is shown for the case of the self-energy of a gluon in a cavity that no new divergences arise due to the presence of the boundary, provided that the regularization can be achieved in such a way that no subtractions are necessary. In order to avoid such subtractions, the so-called method of separation is developed, in which the spectral forms in the cavity are separated in such a way that the divergences of the various terms cancel exactly. This method is in close analogy to the free space regularization method of separation where tadpole contributions are separated off from the rest of the momentum integrals. The technique is used to evaluate the self-energy of a gluon in a cavity, which turns out to be positive for both the quark loop and the gauge loops. The positive value obtained offers a possible explanation for the absence of gluonic exotic states.

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25

Ramlakan, Alastair. "Flavour SU(3) symmetry breaking in quantum chromodynamics". Master's thesis, University of Cape Town, 2000. http://hdl.handle.net/11427/6547.

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Includes bibliographical references.
The subject of this thesis is the determination of the size of SU(3) flavour symmetry breaking in the QeD vacuum, as measured by the ratio of the strange to non-strange quark vacuum condensates / (with <ūu>

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26

Murphy, Kevin Christoher Klink William H. "The structure of gluons in point form quantum chromodynamics". [Iowa City, Iowa] : University of Iowa, 2009. http://ir.uiowa.edu/etd/411.

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27

Von, Hippel Georg Matthias. "Aspects of quantum chromodynamics : high temperatures and heavy quarks". Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.615784.

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28

Munier, Stéphane. "Quantum chromodynamics at high energy and noisy traveling waves". Habilitation à diriger des recherches, Université Pierre et Marie Curie - Paris VI, 2011. http://tel.archives-ouvertes.fr/tel-00712413.

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When hadrons scatter at high energies, strong color fields, whose dynamics is described by quantum chromodynamics (QCD), are generated at the interaction point. If one represents these fields in terms of partons (quarks and gluons), the average number densities of the latter saturate at ultrahigh energies. At that point, nonlinear effects become predominant in the dynamical equations. The hadronic states that one gets in this regime of QCD are generically called "color glass condensates". Our understanding of scattering in QCD has benefited from recent progress in statistical and mathematical physics. The evolution of hadronic scattering amplitudes at fixed impact parameter in the regime where nonlinear parton saturation effects become sizable was shown to be similar to the time evolution of a system of classical particles undergoing reaction-diffusion processes. The dynamics of such a system is essentially governed by equations in the universality class of the stochastic Fisher-Kolmogorov-Petrovsky-Piscounov equation, which is a stochastic nonlinear partial differential equation. Realizations of that kind of equations (that is, "events" in a particle physics language) have the form of noisy traveling waves. Universal properties of the latter can be taken over to scattering amplitudes in QCD. This review provides an introduction to the basic methods of statistical physics useful in QCD, and summarizes the correspondence between these two fields and its theoretical and phenomenological implications.

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29

Perantonis, S. J. "Model calculations in quantum chromodynamics and other field theories". Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379929.

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30

Gibbs, P. E. "Applications of the Lanczos Algorithm to lattice quantum chromodynamics". Thesis, University of Glasgow, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372404.

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31

Fong, Che Ping. "Jet fragmentation at small momentum fractions in quantum chromodynamics". Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239209.

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32

Lim, Matthew Alexander. "Quantum chromodynamics and the precision phenomenology of heavy quarks". Thesis, University of Cambridge, 2019. https://www.repository.cam.ac.uk/handle/1810/285109.

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In this thesis we consider the phenomenology of the theory of strong interactions, Quantum Chromodynamics (QCD), with particular reference to the ongoing experimental program at the Large Hadron Collider in CERN. The current progress in precision measurement of Standard Model processes at the LHC experiments must be matched with corresponding precision in theoretical predictions, and to this end we present calculations at next-to-next-to-leading order in perturbation theory of observable quantities involving quarks and gluons, the strongly interacting particles of the SM. Such calculations form the most important class of corrections to observables and are vital if we are to untangle signals of New Physics from LHC data. We consider in particular the amplitudes for five parton interactions at 1- and 2-loop order and present full (in the 1-loop case) and partial (in the 2-loop case) analytic results in terms of rational functions of kinematic invariants multiplying a basis of master integrals. We address the problem of the solution of a system of integration-by-parts identities for Feynman integrals and demonstrate how some current difficulties may be overcome. We consider also the properties of the top quark, and present the NNLO, real-virtual contributions to the calculation of its decay rate. The results are presented as helicity amplitudes so that the full behaviour of the top spin is retained. These amplitudes constitute a necessary ingredient in the complete calculation of top quark pair production and decay at NNLO which will be an important theoretical input to many experimental analyses. Turning to a more phenomenological study, we consider the extraction of two important SM parameters, the top mass and the strong coupling constant, from measurements of top pair production at the ATLAS and CMS experiments. We compare with NNLO theory predictions and use a least-squares method to extract the values of the parameters simultaneously. We find best fit values of the parameters which are compatible with previous extractions performed using top data with the current world averages published by the Particle Data Group. We consider the issue of PDF choice and the circumstances in which a heavy quark can be considered a constituent of the proton. In particular, we look at the production of a Higgs boson in association with bottom quarks in four and five flavour schemes, in which the b may or may not be included in the initial state. We show that theoretical predictions in both schemes are well-motivated and appropriate in different scenarios, and moreover that results in the schemes are consistent provided a judicious choice of the renormalisation and factorisation scales is made. We suggest a typical scale choice motivated by considerations of consistency and find it to be somewhat lower than the typical hard scale of the process.

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Andrikopoulou, Anastasia. "Exclusive decays and hardronic wave functions in quantum chromodynamics". Thesis, Imperial College London, 1985. http://hdl.handle.net/10044/1/37617.

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34

Bell, J. M. "The structure of quantum chromodynamics at the symmetric point". Thesis, University of Liverpool, 2016. http://livrepository.liverpool.ac.uk/3001770/.

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This thesis contains a study on the structure of the vertex functions of Quantum Chromodynamics (QCD) in both linear and non-linear gauges. In particular we show results for the arbitrary linear covariant gauge at two loops as well as renor- malizing the one loop non-linear Curci-Ferrari gauge and maximal abelian gauge (MAG). The full minimal subtraction MS and momentum subtraction (MOM) scheme renormalization of QCD is performed in all three gauges. This is carried out for an arbitrary colour group at one loop for the maximal abelian gauge and at two loops for the arbitrary linear covariant and Curci-Ferrari gauges. From the n loop MS results the (n + 1) loop β-functions and anomalous dimensions can be constructed in the respective gauges for each MOM scheme. This is demonstrated in all of the gauges considered. In addition to analysing the vertex functions at the symmetric subtraction point for both the MS and MOM schemes, we also consider an operator insertion into the quark 2-point function at the asymmetric point with an interpolating parameter. This requires a new configuration setup and introduces new master integrals which we determine. The scalar, vector and tensor operators are considered along with W2 and ∂W2, the twist-2 Wil- son operators for moment n = 2. The operator renormalization is performed at two loops in the MS and modified regularization invariant (RI′) scheme, both of which are preferred schemes of the lattice. Following the construction of the conversion function for the scalar operator for checking purposes, the amplitudes are presented for all other operators in the MS scheme.

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35

Brown, Zachary S. "Heavy flavor interactions and spectroscopy from lattice quantum chromodynamics". W&M ScholarWorks, 2015. https://scholarworks.wm.edu/etd/1539623998.

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In the present work, spectroscopy and interactions of hadrons containing heavy quarks is investigated. In particular, a focus is placed on properties of exotic heavy hadronic states, including doubly and triply heavy baryons and doubly heavy tetraquark states. The framework in which these calculations are carried out is provided by lattice quantum chromodynamics, a discrete formulation of the modern theory of the strong interaction. The main body of the thesis had two main project focuses. For the first project, an extensive calculation of the mass spectrum of doubly and triply heavy baryons including both charm and bottom quarks is carried out. The wide range of quark masses in these systems require that the various flavors of quarks be treated with different lattice actions. We use domain wall fermions for 2+1 flavors (up down and strange) of sea and valence quarks, a relativistic heavy quark action for the charm quarks, and non-relativistic QCD for the heavier bottom quarks. The calculation of the ground state spectrum is presented and compared to recent models. For the second project, the interaction potential of two heavy-light mesons in lattice QCD is used to study the existence of tetraquark bound states. The interaction potential of the tetraquark system is calculated on the lattice with 2+1 flavors of dynamical fermions with lattice interpolating fields constructed using colorwave propagators. These propagators provide a method for constructing all-to-all spatially smeared the interpolating fields, a technique which allows for a better overlap with the ground state wavefunction as well as reduced statistical noise. Potentials are extracted for 24 distinct channels, and are fit with a phenomenological non-relativistic quark model potential, from which a determination of the existence of bound states is made via numerical solution of the two body radial Schrodinger equation.

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36

Murphy, Kevin Christoher. "The structure of gluons in point form quantum chromodynamics". Diss., University of Iowa, 2009. https://ir.uiowa.edu/etd/411.

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This dissertation investigates part of the strong nuclear force in point form QCD. The quark sector is neglected to focus on gluons and their self-interactions. The structure of gluons is investigated by building up a field theory for massless particles. Single gluon states are defined, and a condition is implemented to make the wave function inner product positive definite. The transformation between gluon and classical gluon fields generates a differentiation inner product, and the kernels allow for transition between momentum and position space. Then, multiparticle gluon states are introduced as symmetric tensor products of gluon Hilbert spaces generated by creation and annihilation operators. In order to assure that the resulting Fock space inner product is positive definite, an annihilator condition is needed and gauge transformations are introduced. The four momentum operator consists of the stress-energy tensor integrated over the forward hyperboloid. The free gluon four momentum operator introduced via the Lagrangian and stress-energy tensor is shown to be equivalent to that generated by gluon irreducible representations when acting on the physical Fock space. Next the vacuum problem is discussed, where the vacuum state is the state that is annihilated by the the four momentum operator and is invariant under Lorentz and color transformations. To find such a state, the vacuum problem is simplified by considering a one degree of freedom model. The Hamiltonian for such a model, the one dimensional energy operator, is solved under a variety of different ansatzes. It is shown that the Hamiltonian has a continuous eigenvalue spectrum, and that the vacuum can be constructed in a way that eliminates the interaction term of the Hamiltonian. This one dimensional vacuum model is adapted to the full problem where it is shown that such a result cannot be replicated.

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37

Maelger, Jan. "Perturbative perspectives on the Phase diagram of Quantum ChromoDynamics". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLX050/document.

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L'étude du diagramme des phases de la Chromodynamique Quantique (QCD) et des transitons associées (déconfinement et restoration de la symétrie chirale) représentent des défis majeurs de la Physique moderne et nombreuses sont les approches théoriques qui visent à en sonder les multiples facettes. Du fait de l'intensité de l'interaction forte dans les régimes d’énergie pertinents pour les transitions susmentionnées, ces approches sont en général de nature non-perturbative, la théorie des perturbations étant réputée inapplicable à ces échelles. Il est, cependant, bien établi que le point de départ de la théorie usuelle des perturbations, basée sur la procédure de fixation de jauge de Faddeev-Popov, est ambigu à ces échelles (ambiguïté de Gribov). Dans ce contexte, une approche perturbative modifiée, basée sur le Lagrangien de Curci et Ferrari, a été proposée, via l’ajout phénoménologique d'un terme de masse effectif pour le gluon en jauge de Landau. Cette approche a été testée avec succès, notamment dans sa capacité à reproduire les fonctions de corrélation de la théorie Yang-Mills (et QCD dans la limite de quarks lourds) et la thermodynamique à temperature et potentiel chimique non nuls.Dans cette thèse, nous avons testé la robustesse de ces résultats en évaluant la structure de phase de la QCD avec quarks lourds au deuxième ordre de la théorie des perturbations dans le modèle de Curci-Ferrari et en comparant nos résultats à ceux d'approches nonperturbatives. Nos résultats indiquent que, dans ce régime de quarks lourds, le diagramme de phases est contrôlée perturbativement. Nous avons égalementétendu notre étude au cas de la QCD avec quarks légers en utilisant un schéma de resommation qui exploite la présence de petits paramètres dans le régime infrarouge de la QCD. Dans le secteur des quarks, cette démarche donne lieu à la resommation des fameux diagrammes dits "arc-en-ciel”. Ici, nous généralisons ce formalisme à temperature et densité non nulles et en presence d'un champ de fond gluonique. Nous réalisons une toute première étude qualitative des prédictions du modèle CF concernant l’existence possible d’un point critique dans le diagramme de phases de QCD sur la base d’une version simplifiée des équations générales ainsi obtenues
Unravelling the structure of the QCD phase diagram and its many aspects such as (de)confinementand chiral symmetry breaking, is one of the big challenges of modern theoretical physics, and manyapproaches have been devised to this aim. Since perturbation theory is believed to cease feasibilityat low energy scales, these approaches treat the relevant order parameters, the quark condensate andthe Polyakov loop, non-perturbatively. However, it is also well-established that the starting point forperturbation theory, the Fadeev-Popov gauge-fixing procedure, is inherently ill-defined in the infrareddue to the presence of Gribov ambiguities. In this context, a modified perturbative approach based onthe Curci-Ferrari Lagrangian has been introduced, where a phenomenologically motivated effective gluonmass term is added to the Landau gauge-fixed action. Prior to the beginning of the thesis, this approach hasproven extremely fruitful in its descriptions of (unquenched) Yang-Mills correlation functions and thermodynamics at (non)zero temperature and density.Throughout the thesis we extend this analysis to the entire phase structure of QCD and QCD-liketheories and test the validity of the model in various regimes of interest. For instance, to further aprevious one-loop study in the regime of heavy quark masses, we have computed the two-loop quarksunset diagram in the presence of a non-trivial gluon background in a finite temperature and densitysetting. We come to the conclusion that the physics underlying center symmetry is well-described by our perturbative model with a seemingly robust weak-coupling expansion scheme. Furthermore, we study the regime of light quarks by means of a recently proposed resummation scheme which exploits the presence of actual small parameters in the Curci-Ferrari description of infrared QCD. In the quark sector, this leads to the renown rainbow equations. We extend this first-principle setup to nonzero temperature, chemical potential, and gluon background. We perform a first qualitative analysis of the prediction of the model concerning the possible existence of a critical endpoint in the QCD phase diagram by using a simplified version of these general equations

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38

Ashley, Jonathan D. "Investigations in non-perturbative QCD". Title page, abstract and table of contents only, 2004. http://hdl.handle.net/2440/37959.

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In this thesis we review two methods for studying the non-pertubative region of QCD: the effective field theory, chiral perturbation theory (χPT), and the cloudy bag model, a successful chiral quark model of hadron structure. We use information from both of these sources to construct a simple extrapolation formula in the pion mass, mπ, for the nucleon electromagnetic form factors, which combines the correct non-analytic chiral behaviour predicted by (χPT), with the correct large mπ behaviour. This formula is applied to recent quenched lattice QCD results to extrapolate to the physical regime. Given the simple nature of the extrapolation scheme, our results compare surprisingly well with experiment. We also employ a simple chiral quark model (the hedgehog) to examine the volume and pion mass dependence of the axial coupling constant, ga, along with the hedgehog baryon mass. Our results for ga reveal large volume dependence at low pion masses.
Thesis (M.Sc.)--School of Chemistry and Physics, 2004.

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39

Shcheredin, Stanislav. "Simulations of lattice fermions with chiral symmetry in quantum chromodynamics". Doctoral thesis, [S.l.] : [s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=97410907X.

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40

Miravitllas, Mas Ramon. "Asymptotic expansions, resurgence and large order behaviour of quantum chromodynamics". Doctoral thesis, Universitat Autònoma de Barcelona, 2019. http://hdl.handle.net/10803/667932.

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En les teories quàntiques de camps, les prediccions numèriques d’observables físics només es poden calcular amb expansions pertorbatives en potències de les constants d'acoblament, els paràmetres que determinen la força de les interaccions entre camps. Mentre que l’èxit predictiu de la teoria quàntica de camps no es pot negar, aquests càlculs pertorbatius estan plens de divergències. D’una banda, els coeficients de l’expansió pertorbativa es calculen a partir d’integrals de loops que són divergents la majoria de les vegades. Algunes d’aquestes divergències estan associades a termes no físics que es poden sostreure. En altres casos, s’aplica un procés de renormalització per cancelar aquestes divergències, però això suposa l'elecció d’un conveni teòric (escala i esquema) de la qual els observables físics no poden dependre. D’altra banda, un cop les integrals de loops han sigut renormalitzades, l’expansió resultant encara suma a una resposta infinita per tots els valors no nuls de la constant d'acoblament. Això succeeix perquè els coeficients de l’expansió creixent factorialment amb l’ordre. Tot i així, aquestes expansions es poden entendre com expansions asimptòtiques, que codifiquen el comportament de l’observable en el límit quan la constant d'acoblament s’acosta a zero, i l’observable es pot aproximar truncant l’expansió a un terme òptim. Aquest segon tipus de divergència no está limitat, de fet, a la teoria quàntica de camps, sinó que pot apareixer en diferents contextos de les matemàtiques i la física: per exemple, en expansions pertorbatives dels valors propis de l’energia d’un sistema de la mecànica quàntica, o com a solucions formals d’una equació diferencial. A la part I d’aquesta tesi, l’objecte principal d’estudi és la constant d'acoblament forta i les expansions pertorbatives d’observables físics a la quàntica chromodinàmica. Primer, discitum breument com les divergències de loops d’un gluó propagant-se a l’espai amb correccions quàntiques poden ser absorbides dins de la constant d'acoblament forta durant el procés de renormalització. Aquest procés, no obstant, implica el cost d’introduir dependències en l’escala i l’esquema dins la constant d'acoblament, per tant, aquesta no és un observable físic de la teoria. Això motiva una redefinició de la constant d'acoblament tal que la seva dependència en l’esquema es redueix a un sol paràmetre. Després utilitzem aquesta redefinició de la constant d'acoblament en anàlisis fenomenològics d’observables físics associats a dispersions electró-positró, i a la desintegració del Higgs i del tau en hadrons. Demostrem que eleccions apropiades d’aquest paràmetre d’esquema pot donar lloc a millores substancials de les prediccions pertorbatives d’aquests observables. A la part II, discutim les divergències d’expansions asimptòtiques en el context d’integrals de camí. Convencionalment, el mètode de la sumació de Borel asigna una resposta finita a les expansions divergents. Tot i així, la suma de Borel podria no contenir tota la informació d’una funció, perquè a aquesta li poden faltar correccions exponencialment petites. Llavors considerem una petita variació de la sumació de Borel, on una transformada de Borel generalitzada (una transformada de Laplace inversa) és seguida d’una transformada de Laplace direccionals. Aquestes eines ens permet donar, potser, millors respostes a problemes típics de la sumació de Borel, com la pèrdua de les correccions exponencials i les ambigüitats de la sumació de Borel. A més, definim ressurgència com una connexió entre la discontinuïtat d’una funció i els coeficients de la seva expansió asimptòtica. A partir d’aquesta definició, podem reduir el problema de ressurgència a un problema de correccions exponencials perdudes en les expansions asimptòtiques i podem relacionar diferents formes d’entendre la ressurgència que es troben a la literatura.
For realistic quantum field theories, numerical predictions of physical observables can only be calculated from perturbative expansions in powers of the couplings, the parameters that determine the strength of the field interactions. While the predictive success of quantum field theory is undeniable, these perturbative computations are plagued with divergences. On one hand, the coefficients of the perturbative expansion are computed from loop integrals that are divergent most of the times. Some of these divergences are associated with unphysical terms that can be subtracted. In other cases, a renormalisation procedure is applied to cancel these divergences, but this entails a choice of theoretical conventions (scale and scheme) which physical observables cannot depend on. On the other hand, once the loop integrals have been renormalised, the resulting expansion still sums to an infinite answer for all non-vanishing values of the coupling. This is due to the fact that the coefficients of the expansion grow factorially with the order. Still, these expansions can be understood as asymptotic expansions, which encode the limiting behaviour of the observable for small coupling, and whose truncation to an optimal term yields numerical approximations of the observable. This second kind of divergence is in fact not limited to quantum field theories, but it may arise in different contexts of mathematics and physics: for instance, in perturbative approximations to the energy eigenvalues of a quantum mechanic system, or in formal solutions to differential equations. In part I of this dissertation, the main object of study is the strong coupling constant and the perturbative expansions of physical observables in quantum chromodynamics. First, we briefly discuss how the loop divergence of the quantum corrected gluon propagator can be absorbed inside the strong coupling constant during the renormalisation. This process, however, comes at the cost of introducing scale and scheme dependences into the coupling, therefore it is not a physical observable of the theory. This motivates a coupling redefinition whose scheme dependence is reduced to a single parameter. We then use this coupling redefinition in phenomenological analysis of physical observables associated to electron-positron scattering, and to Higgs and tau decays into hadrons. We demonstrate that appropriate choices of this scheme parameter can lead to substantial improvements in perturbative predictions of these observables. In part II, we discuss the divergence of asymptotic expansions in the context of path integrals. Conventionally, the method of Borel summation assigns a finite answer to the divergent expansion. Still, the Borel sum might not encode the full information of a function, because it misses exponentially small corrections. We then consider a slight variation of the conventional Borel summation, in which a generalised Borel transform (an inverse Laplace transform) is followed by a directional Laplace transform. These tools allow us to give perhaps better answers to typical problems in Borel summation: missing exponential corrections and ambiguities in the Borel summation. In addition, we define resurgence as a connection between the discontinuity of a function and the coefficients of its asymptotic expansion. From this definition, we can reduce resurgence to the problem of missing exponential corrections in asymptotic expansions and correlate different approaches to resurgence found in the literature.

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41

Budczies, Jan. "The color-flavor transformation and its applications to quantum chromodynamics". [S.l. : s.n.], 2002. http://deposit.ddb.de/cgi-bin/dokserv?idn=964923106.

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42

Enberg, Rikard. "Quantum chromodynamics and colour singlet exchange in high energy interactions". Doctoral thesis, Uppsala University, Department of Nuclear and Particle Physics, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-3385.

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Abstract (sommario):

Quantum chromodynamics (QCD) is the fundamental theory in elementary particle physics that describes the strong interaction in terms of exchanges of force-carrying, colour-charged particles known as gluons. Although well-established through experimental verifications, there are fundamental unsolved problems in the theory.

In this thesis, some novel aspects of strong interaction dynamics are studied in the context of colour singlet exchange processes — interactions where complex systems of gluons with no net colour charge are exchanged. Both perturbative and non-perturbative QCD methods are used, as well as Monte Carlo computer simulations.

Soft colour interactions in the final state of a high energy collision can lead to effective colour singlet exchange. Non-perturbative models for such interactions are shown to give a good description of diffractive production of W, Z, bb, J/ψ and jets in pp collisions at the Tevatron. Predictions are given for diffractive Higgs boson and prompt photon production at hadron colliders.

Rapidity gaps between jets is a new phenomenon which is studied with an improved perturbative calculation of hard colour singlet exchange using the BFKL equation, taking into account previously neglected contributions and non-leading logarithmic corrections. Including also underlying soft rescattering effects, the complete model reproduces well data from the Tevatron.

Diffractive vector meson production through hard colour singlet exchange in γp collisions is studied in the framework of the conformal invariant non-forward solution of the BFKL equation. Expressions for helicity-dependent amplitudes are derived, and the results show good agreement with data on J/ψ and ρ production from the ep collider HERA.

These studies lead to a deeper knowledge of complex gluon dynamics, and therefore advance our understanding of QCD.

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43

Heybrock, Simon [Verfasser], e Tilo [Akademischer Betreuer] Wettig. "Computational methods for Quantum Chromodynamics / Simon Heybrock. Betreuer: Tilo Wettig". Regensburg : Universitätsbibliothek Regensburg, 2013. http://d-nb.info/1035808641/34.

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44

Knowles, Ian Garfield. "The treatment of spin in hard processes using quantum chromodynamics". Thesis, University of Cambridge, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.317835.

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45

Bilson-Thompson, Sundance Osland. "Investigation of topology, instantons, and the Nahm transform in lattice QCD using highly improved operators". Title page, contents and abstract only, 2002. http://web4.library.adelaide.edu.au/theses/09PH/09phb5996.pdf.

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46

Waugh, Robert George. "Measurement of event shape variables in deep inelastic scattering". Thesis, University of Glasgow, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.301504.

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47

Reader, Matthew Thomas. "Extracting Λ MS from experiment". Thesis, Durham University, 1994. http://etheses.dur.ac.uk/5363/.

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48

L'Heureux, Pierre. "Parton picture of soft pp and p̄p reactions". Thesis, McGill University, 1985. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=71998.

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We study a model for inclusive central particle production in pp and pp reactions based on parton model concepts and lowest-order QCD. In this picture, two-gluon fusion creates a heavy excited state (fireball) whose decay (described by statistical considerations) leads to the creation of light hadrons with low transverse momenta. Agreement with experimental data is found to be good. This first model is then embedded (providing energy dependence) in a two-component eikonal model used to study elastic diffraction and unifying parton model and impact parameter formulations. We discuss the inclusion of a real part for the elastic amplitude, the removal of multiple diffraction zeros and the behavior of this second model up to the multi-TeV energy range (the Froissart bound is not saturated). We find excellent agreement with a large body of experimental data.

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49

Turnock, Graham. "Applications of resummation techniques to electron-positron annihilation". Thesis, University of Cambridge, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.259591.

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50

Turner, Miriam Frances. "Tests of QCD at the Z'0 resonance". Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239763.

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Tesi sul tema "Quantum chromodynamics"

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