Neste trabalho estudamos os efeitos quânticos de criação de partículas nos contextos cosmológico e da eletrodinâmica clássica. Com o estudo da criação de partículas em campos elétricos externos constantes e oscilatórios, os conhecidos efeitos Schwinger estático e oscilatório, respectivamente, foi possível estudar a polarização do vácuo quântico e a conseqüente criação de partículas do vácuo. Esse é um efeito não-perturbativo no contexto da eletrodinâmica e sua observação seria de grande importância para validar essa teoria nesse regime. Esse efeito, no entanto, ainda não foi detectado experimentalmente, uma vez que são necessários campos elétricos muito fortes no caso estático. Contudo, no caso oscilatório existe um experimento em andamento para a detecção de um efeito quântico análogo, o chamado efeito Casimir dinâmico. A verificação do efeito Casimir dinâmico é importante para a validação do efeito Schwinger oscilatório, dado que o movimento das condições de contorno é equivalente à aplicação de um campo externo. Finalmente, foi estudada a criação de partículas em um contexto cosmológico, sendo o caso estático a criação de partículas na era inflacionária do universo. O caso dinâmico corresponde ao período de reaquecimento ou, mais precisamente, em seu período inicial conhecido como pré-aquecimento, em que a criação de partículas é mais eficiente. A criação de partículas na era inflacionária ocorre devido à expansão acelerada do universo. Por meio de uma transformação canônica simples foi possível observar que o mecanismo de criação de partículas nesse período é o mesmo da criação de partículas do efeito Schwinger estático. Da mesma forma, para o pré-aquecimento, em que a criação de partículas é dada pela oscilação do inflaton em torno de seu mínimo do potencial, o mecanismo de criação resume-se a um mecanismo de ressonância paramétrica descrito por uma equação de Mathieu. Esse é exatamente o mesmo mecanismo que descreve a criação de partículas no caso do efeito Schwinger oscilatório. Dessa forma, os mecanismos de criação de partículas nos casos do efeitos Schwinger estático e oscilatório são análogos aos mecanismo de criação inflacionário e do pré- aquecimento. A detecção experimental desse efeito em um experimento terrestre seria uma grande confirmação para esses mecanismos cosmológicos. / In this work we study the quantum effects of particle creation in the cosmological context and of the classical electrodynamics. With the studies of the particle creation in constant and oscillatory external electrical fields, the known static and oscillatory Schwinger effects, respectively, it was possible to study the polarization of the quantum vacuum and the consequently creation of particles from the vacuum. This is a non perturbative effect in the context of the electrodynamics and its observation would be of great importance to validate this theory in this regime. This effect, however, was not yet experimentally detected, since very strong electric fields are necessary in the static case. However, in the oscillatory case there is an experiment currently in development for the detection of an analog quantum effect called dynamical Casimir effect. The verification of the dynamical Casimir effect is important for the validation of the oscillatory Schwinger effect, since the movement of the boundary conditions is equivalent to the application of a external field. Finally, it was studied the creation of particles in a cosmological context, being the static case the creation of particles in the inflationary epoch of the universe. The dynamical case corresponds to the reheating period or, more precisely, its initial stage known as preheating, in which the creation of particles is more efficient. The creation of particles in the inflationary epoch occurs due to the accelerated expansion of the universe. By means of a simple canonical transformation it was possible to observe that the mechanism of particle creation in this period is the same of the one in Schwinger effect. In the same way, for the preheating, in which the particle creation is given by the oscillation of the inflaton field around the minimum of its potential, the mechanism of creation happens due to the parametric resonance described by a Mathieu equation. This is exactly the same mechanism that describes the oscillatory Schwinger effect. In this way, the mechanisms of particle creation in the cases of the static and oscillatory Schwinger effects are analogs to the creation mechanisms from inflation and from preheating. The experimental detection of this effect in a terrestrial experiment would be a confirmation for this cosmological mechanisms.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Mathematics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by John A. Dickinson. / Ph.D.

This dissertation explores philosophical issues that arise in the practice of contemporary particle physics. The first chapter is a historical sketch of how the particle physics community came to believe that the quantum field theories used in particle physics are best understood as effective field theories -- quantum field theories that become inapplicable at some short distance scale and incorporate this inevitable breakdown in their mathematical framework. The second chapter argues that, contrary to the claims of many philosophers, the unique mathematical resources and empirical successes of effective field theories means that they can and should be equipped with a realist interpretation. The third chapter turns to an issue that has been at the center of particle physics for the last 40 years, the problem of naturalness, and draws on the renormalization group to provide a univocal understanding of ``naturalness'' that can make sense of the way it is employed in physical practice. Finally in the fourth chapter I critically examine recent philosophical work which argues that different physical scales in quantum field theory enjoy a sense of autonomy, arguing that the formal results on which this recent work is based cannot support the conclusions with which they have been saddled.

Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Physics. / Microfiche copy available in Archives and Science. / Includes bibliographical references. / by C.Y. Lo. / Sc.D.

É fato sabido que, de todas as interações fundamentais da Natureza que conhecemos, a gravitacional é a que acopla mais fracamente com a matéria. Isso sugere que na maior parte dos processos físicos a força gravitacional desempenha papel diminuto. Por outro lado, na Teoria Quântica de Campos é atribuído ao estado de vácuo uma rica estrutura, a qual é indispensável para uma descrição consistente da Natureza. No entanto, implicações experimentais diretas dessa estrutura são muito sutis e requerem sistemas cuidadosamente projetados para serem observadas, como é o caso do Efeito Casimir. À luz dos fatos mencionados acima, é de se esperar que na fusão minimamente consistente entre Gravitação e Mecânica Quântica, a chamada Teoria Quântica de Campos em Espaços-tempos Curvos, efeitos relacionados a perturbação do vácuo pelo campo gravitacional sejam muito difíceis de serem observados. De fato, a despeito de sua importância conceitual, o efeito de evaporação de buracos negros é praticamente impossível de ser observado para sistemas astrofísicos. No curso deste doutoramento, todavia, foi mostrado que essa crença é falsa e que é possível que existam situações em que a evolução bem comportada do espaço-tempo força a densidade de energia de vácuo a tornar-se dominante sobre a densidade de energia clássica que gera o espaço-tempo de fundo. Uma vez despertado, o vácuo passaria a dirigir a evolução do sistema gravitacional, o que poderia ter consequências inesperadas em contextos astrofísicos. Qualquer que seja seu destino, é razoável esperar que a retroação do vácuo aja sobre o sistema gravitacional de forma a cessar as instabilidades. Com essa simples observação é possível concluir que quando o vácuo adormece novamente processos de criação de partículas em profusão podem ser engendrados. / It is well known that the gravitational interaction is the weakest among the fundamental forces in Nature. This fact suggests that Gravity plays a minor part in the majority of physical process. On the other hand, in Quantum Field Theory a rich structure is attributed to the vacuum state, which is imperative for a consistent description of the more basic processes in Nature. Nevertheless, the direct experimental implications of this structure are very subtle and their observation requires specially designed systems, as in the case of the Casimir Effect. Therefore, it is reasonable to expect that effects related to the perturbation of the quantum vacuum by gravitational fields, described by the framework of Quantum Field Theory in Curved Space-times, would be hard to be observed. This is the case of the black hole evaporation effect. In spite of its conceptual importance, this effect is virtually impossible to be observed for astrophysical black holes. Notwithstanding, here it is argued that this belief is false and that there exist well-behaved space-time evolutions where the vacuum energy density of free quantum fields is forced by the very same background space-time to become dominant over the classical energy density. Once it has been awakened, the quantum vacuum would overrule the dynamics of the entire gravitational system, which may bear some unexpected astrophysical implications. Whatever turns out to be the fate of the system, it seems reasonable to conjecture that the vacuum backreaction will act in order to cease the quantum instabilities. Through this simple observation it is possible to conclude that when the vacuum falls dormant particles are released as consequence.

São vistos os modelos O (N) x O (N) e Thirring da Teoria Quantica de Campos na expansão 1/N. No modelo O (N) x O (N) construímos o potencial efetivo e determinamos o estado fundamental em duas e quatro dimensões. Ainda em quatro dimensões avaliamos os efeitos da temperatura sobre o modelo. Tratamos o modelo de Thirring em (2 + 1) dimensões nos aspectos concernentes a geração dinâmica do termo de Chern-Simons, espectro de partícula, solução da equação de Schwinger-Dyson para a auto-energia dos férmions e simetrias discretas e contínuas. / We study the models O(N) x O(N) and Thirring of the Quantum Field Theory in the context of the l/N expansion. In the O(N) x O(N) model we construct the effective potential and obtain the ground state in two and four dimensions. AIso in four dimensions we study the temperature effects on the model. We analyze the Thirring model in (2 + 1) dimensions in the aspects that concerns to dynamic generation of the Chern-Simons term, particle spectrum, solutions of the Schwinger-Dyson equation for the fermions self-energy and discret and continuous symmetries.

This thesis uses a variety of numerical and statistical techniques to perform high precision calculations in high energy physics using quantum field theory. It introduces the experimental motivation for the calculation of B meson form factors and includes a discussion of previous work. It then describes the modern theoretical framework describing these phenomena, outlining quantum chromodynamics and electroweak theory, and then illustrating the procedure of gauge fixing, the quantum effective action and background field gauge which is required for subsequent perturbative work. Details of the basic methodology of lattice quantum field theory are given as well as the specific formulation of the relativistic theory and nonrelativistic approximations used in this work to describe quantum chromodynamics. A comprehensive calculation of the zero recoil B to D* form factor is then presented, using state of the art lattice techniques with relativistic charm sea quarks and light sea quarks with correct physical masses, leading to a discussion of the dominant sources of uncertainty and possible resolutions of experimental tensions. Also included is preliminary work towards the full calculation of nonzero recoil matrix elements, with the aim of outlining possible future work. Finally, this thesis presents the computation of parameters correcting for radiative one loop phenomena and corrections to the kinetic coupling parameters in nonrelativistic quantum chromodynamics in order to achieve a desirable level of precision in future calculations. This is done using Monte-Carlo integration to evaluate integrals from diagrams generated using automated lattice perturbation theory in background field gauge in order to match the coefficients of the effective action between the lattice and the continuum.

São vistos os modelos O (N) x O (N) e Thirring da Teoria Quantica de Campos na expansão 1/N. No modelo O (N) x O (N) construímos o potencial efetivo e determinamos o estado fundamental em duas e quatro dimensões. Ainda em quatro dimensões avaliamos os efeitos da temperatura sobre o modelo. Tratamos o modelo de Thirring em (2 + 1) dimensões nos aspectos concernentes a geração dinâmica do termo de Chern-Simons, espectro de partícula, solução da equação de Schwinger-Dyson para a auto-energia dos férmions e simetrias discretas e contínuas. / We study the models O(N) x O(N) and Thirring of the Quantum Field Theory in the context of the l/N expansion. In the O(N) x O(N) model we construct the effective potential and obtain the ground state in two and four dimensions. AIso in four dimensions we study the temperature effects on the model. We analyze the Thirring model in (2 + 1) dimensions in the aspects that concerns to dynamic generation of the Chern-Simons term, particle spectrum, solutions of the Schwinger-Dyson equation for the fermions self-energy and discret and continuous symmetries.

This thesis investigates the use of Daubechies wavelets to separate scales in local quantum field theory. Field theories have an infinite number of degrees of freedom on all distance scales. Quantum field theories are believed to describe the physics of subatomic particles. These theories have no known mathematically convergent approximation methods. Daubechies wavelet bases can be used separate degrees of freedom on different distance scales. Volume and resolution truncations lead to mathematically well-defined truncated theories that can be treated using established methods. This work demonstrates that flow equation methods can be used to block diagonalize truncated field theoretic Hamiltonians by scale. This eliminates the fine scale degrees of freedom. This may lead to approximation methods and provide an understanding of how to formulate well-defined fine resolution limits.

This thesis deals with the concepts of a very interesting phenomenon in quantum physics, the Casimir effect. Here the effect is investigated in detail and its importance to other areas of physics is analysed. The Casimir effect is produced by disturbing the vacuum energy when material boundaries or background fields are introduced in the vacuum. The usual approach to this effect is the vacuum fluctuation that has been studied in the past in relation to the discussion of the zero-point energy as a result of the field resemblance to the quantum harmonic oscillators, where residual ground state energy must be considered. In this thesis a new method to study vacuum fluctuations is presented. This new approach to the problem which is more classical is based on the Heisenberg uncertainty principle and the very important fluctuation-dissipation theorem. The other aim of the thesis is to implement a new algorithm for regularizing the Casimir energy for a massive scalar field. Unlike the previous works on this problem by other authors that give approximate results, this attempt will produce precise results. My method is based on a new regularization procedure that allows us to employ the very reliable dimensional regularization scheme in place of a more mathematically complicated zeta-function regularization procedure. In order to achieve this goal I will deal with the problem by using the Euler-Maclaurin summation formula. The result will be a regularized Casimir energy for the case of a massive scalar field. This model may be used for the other geometrical boundaries and different fields. / Doctor of Philosophy (PhD)