Neste trabalho buscamos explorar os efeitos do campo magnético, criado em colisões de íons pesados não-centrais, em estados ligados. Motivados pelo caso do átomo de hidrogênio, onde a presença do campo magnético deixa o estado mais ligado,estudamos o caso dos mésons B e D. Inicialmente trabalhamos com a equação de Schrödinger e utilizamos o modelo de potencial de Cornell que faz uma boa descrição da espectroscopoia de mésons pesados. Em seguida, utilizando as regras de soma da QCD, procuramos introduzir o efeito do campo magnético no cálculo da massa de mésons. O ponto de partida foi substituir o propagador livre dos quarks pelo propagador de Schwinger, que corresponde ao propagador de um férmion na presença de um campo magnético externo e constante. Tratamos do limite de campo fraco, onde eB « m2 « M2 e o de campo muito forte, onde eB » M2, sendo m a massa do quark leve e M a massa do quark pesado. Em todos os casos observamos uma tendência da queda na massa do méson devido ao campo magnético. Esta mudança poderia ter diversas implicações fenomenológicas, como por exemplo na taxa de produção do quarkonium, que ilustramos utilizando o modelo de evaporação de cor. / At the present work, we explore the effects of the magnetic field on bound states. Such field is generated by non-central heavy ion collisions. Since the presence of magnetic field results in more deeply bounded states for the hydrogen, we will analyze the cases of mesons B and D. Initially we investigate the Cornell potential in the Schrödinger equation, which is usually a good description of the spectroscopy of heavy mesons. Next, as we examine the model using QCD sum rules, we tried to introduce the magnetic field and its consequences in the meson mass calculation. The starting point was to substitute the free propagator of quarks by the Schwinger propagator, which corresponds to the propagator of a fermion in the presence of an external constant magnetic field. We explored the weak field limit, where eB « m2 « M2 and of very strong fields, where eB » M2, for m being the mass of the light quark and M the mass of the heavy quark. In all cases we observed a trend of decreased meson mass due to the magnetic field. These variations could have many phenomenological implications; for instance, the production rates of quarkonium, illustrated by the method of color evaporation.

Neste trabalho, nós estudamos a hadroprodução de charmonium em experimentos de alvo fixo com os três modelos: modelo de singleto de cor (CSM), modelo de evaporação de cor (CEM) e QCD não relativística (NRQCD). Os ajustes feitos aos dados experimentais nos permitiram concluir que o CSM subestima o valor da seção de choque de produção dos estados de charmonium J/ e (2S) e, portanto, não descreve adequadamente a hadroprodução nestes casos. Os modelos CEM e NRQCD por outro lado descreveram satisfatoriamente os dados, no entanto, o fato dos resultados experimentais possuírem grandes incertezas não nos permitiu comparar diretamente os dois modelos e desta forma concluir qual dos dois é mais adequado. / ln this work we evaluate the charmonium production at fixed-target experiments with three different models: the color singlet model (CSM), the color evaporation model (CEM) and the nonrelativistic quantum chromodynamics (NRQCD). We verify that the results of CSM under-preditcs the production cross-section for J / and (2S), thus, in this case, the CSM is an inadequate model. In addition, we find that the two models CEM and NRQCD satisfactorily predict the charmonium hadro-production. However, due to the fact that the experimental uncertainties are large we can not compare the two models.

The quark-connected part of the hadronic light-by-light scattering contribution to the muon’s anomalous magnetic moment is computed using lattice QCD with chiral fermions. We report several significant algorithmic improvements and demonstrate their effectiveness through specific calculations which show a reduction in statistical errors by more than an order of magnitude. The most realistic of these calculations is performed with a near-physical, 139 MeV pion mass on a (5.5 fm)³ spatial volume using the 48³ × 96 Iwasaki gauge ensemble of the RBC/UKQCD Collaboration.

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.

<p>In this thesis, we study effective field theories for doubly heavy baryons and lattice QCD. We construct a chiral Lagrangian for doubly heavy baryons and heavy mesons that is invariant under heavy quark-diquark symmetry at leading order and includes the leading O(1/m_Q ) symmetry violating operators. The theory is used to predict the electromagnetic decay width of the J = 3/2 member of the ground state doubly heavy baryon doublet. Numerical estimates are provided for doubly charm baryons. We also calculate chiral corrections to doubly heavy baryon masses and strong decay widths of low lying excited doubly heavy baryons. We derive the couplings of heavy diquarks to weak currents in the limit of heavy quark-diquark symmetry, and construct the chiral Lagrangian for doubly heavy baryons coupled to weak currents. Chiral corrections to doubly heavy baryon zero-recoil semileptonic decay for both unquenched and partially quenched QCD are calculated. This theory is used to derive chiral extrapolation formulae for measurements of the doubly heavy baryon zero-recoil semileptonic decay form factors in lattice QCD simulations. Additionally, we study the pion physics on lattice using chiral perturbation theory. For finite volume field theories with discrete translational invariance, conserved currents can obtain additional corrections from infrared effects. We demonstrate this for pions using chiral perturbation theory coupled to electromagnetism in a periodic box. Gauge invariant single particle effective theories are constructed to explain these results. We use chiral perturbation theory to study the extraction of pion electromagnetic polarizabilities from lattice QCD. Chiral extrapolation formulae are derived for partially quenched and quenched QCD simulations. We determine finite volume corrections to the Compton scattering tensor of pions.</p> / Dissertation

Thesis (Ph. D. in Physics)--Vanderbilt University, May 2004. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 115-121).

<p>Quantum chromodynamics (QCD), the theory of quarks and gluons, is known to be</p><p>the correct description of strong nuclear interactions. At high energy and momenta,</p><p>one can use QCD directly to compute quantities of physical interest related to the</p><p>strong force. At low energies and momenta, one should use a different description in</p><p>terms of the degrees of freedom relevant at that scale. Two approaches to achieve</p><p>this end are effective field theories and gauge/gravity dualities. The former involves</p><p>a field theory more or less like QCD itself, but with states which are composites</p><p>of quarks and gluons. Then a perturbative expansion is made not in terms of the</p><p>gauge coupling but instead in terms of the momentum of the fields. This approach</p><p>dates back to the 1970s and is on firm theoretical footing. Gauge/gravity dualities</p><p>are a newer and less understood technique, which relates the physics of the strong</p><p>interactions to a different but likely equivalent theory in a higher dimensional space-</p><p>time, where the quantity of interest can be computed more readily. We employ</p><p>both effective field theories and gauge/gravity dualities to study the physics of ex-</p><p>otic quarkonium states, that is bound states containing a heavy quark-antiquark pair</p><p>which nevertheless cannot be be understood working only with the standard quark</p><p>model of hadrons. Candidates for such states, long speculated to exist, have recently</p><p>been observed at particle colliders, so that the theory of exotic quarkonium is now</p><p>of great experimental importance.</p> / Dissertation

No description available.

We present the techniques for the calculation of one- and two-loop integrals contributing to the virtual corrections to 2→2 scattering of massless particles. First, tensor integrals are related to scalar integrals with extra powers of propagators and higher dimension using the Schwinger representation. Integration By Parts and Lorentz Invariance recurrence relations reduce the number of independent scalar integrals to a set of master integrals for which their expansion in є = 2 — D/2 is calculated using a combination of Feynman parameters, the Negative Dimension Integration Method, the Differential Equations Method, and Mellin-Barnes integral representations. The two-loop matrix-elements for light-quark scattering are calculated in Conventional Dimensional Regularisation by direct evaluation of the Feynman diagrams. The ultraviolet divergences are removed by renormalising with the MS scheme. Finally, the infrared singular behavior is shown to be in agreement with the one anticipated by the application of Catani's formalism for the infrared divergences of generic QCD two-loop amplitudes.