Quantum gauge boson propagators in the light front

Gauge fields in the light front are traditionally addressed via the employment of an algebraic condition $n\cdot A=0$ in the Lagrangian density, where $A_{\mu}$ is the gauge field (Abelian or non-Abelian) and $n^\mu$ is the external, light-like, constant vector which defines the gauge proper. However, this condition though necessary is not sufficient to fix the gauge completely; there still remains a residual gauge freedom that must be addressed appropriately. To do this, we need to define the condition $(n\cdot A)(\partial \cdot A)=0$ with $n\cdot A=0=\partial \cdot A$. The implementation of this condition in the theory gives rise to a gauge boson propagator (in momentum space) leading to conspicuous non-local singularities of the type $(k\cdot n)^{-\alpha}$ where $\alpha=1,2$. These singularities must be conveniently treated, and by convenient we mean not only matemathically well-defined but physically sound and meaningfull as well. In calculating such a propagator for one and two noncovariant gauge bosons those singularities demand from the outset the use of a prescription such as the Mandelstam-Leibbrandt (ML) one. We show that the implementation of the ML prescription does not remove certain pathologies associated with zero modes. However we present a causal, singularity-softening prescription and show how to keep causality from being broken without the zero mode nuisance and letting only the propagation of physical degrees of freedom.Comment: 10 page

Cosmological perturbations in FRW model with scalar field within Hamilton-Jacobi formalism and symplectic projector method

The Hamilton-Jacobi analysis is applied to the dynamics of the scalar fluctuations about the Friedmann-Robertson-Walker (FRW). The gauge conditions are found from the consistency conditions. The physical degrees of freedom of the model are obtain by symplectic projector method. The role of the linearly dependent Hamiltonians and the gauge variables in Hamilton-Jacobi formalism is discussed.Comment: 11 page

Bianchi Type I Cosmology in N=2, D=5 Supergravity

The dynamics and evolution of Bianchi type I space-times is considered in the framework of the four-dimensional truncation of a reduced theory obtained from the N=2,D=5 supergravity. The general solution of the gravitational field equations can be represented in an exact parametric form. All solutions have a singular behavior at the initial/final moment, except when the space-time geometry reduces to the isotropic flat case. Generically the obtained cosmological models describe an anisotropic, expanding or collapsing, singular Universe with a non-inflationary evolution for all times.Comment: revised version to appear in PR

Pastagens de setária: formação e manejo.

bitstream/item/138544/1/COT-23.pd

Capim setária: características e aspectos produtivos.

O capim setaria (Setaria anceps Stapf ex Massey ou Setaria sphacelata (Schumach.) Moss var. sericea (Strapf Clayton), cujas cultivares mais utilizadas no formação de pastagens no Brasil são Kazungula e, em menor escala, Nandi e Norok, vem despertando um crescente interesse no meio pecuário, pelas suas boas características forrageiras, especialmente pela sua considerável capacidade de crescimento durante boa parte do período seco. Resultados experimentais e observações a nível de fazendas tem confirmado os bons níveis de produtividade e suporte da espécie verificados em outros paises, destacando-a ainda como uma gramínea resistente ao pisoteio e, aparentemente, as cigarrinhas das pastagens. Neste trabalho, são apresentados uma descrição sucinta das três cultivares acima citadas e resultados de pesquisas, especialmente os obtidos nas condições brasileiras, com relação as suas exigências de clima e de solo e quanto ao estabelecimento, valor nutritivo, respostas a fertilizantes, produtividade e manejo. As informações reunidas permitem concluir que, embora a setaria seja uma gramínea tropical e medianamente exigente em fertilidade, apresenta condições de adaptação e boa produtividade, em grande parte dos diferentes tipos de solo e de clima existentes no Brasil, destacando-se especialmente nas regiões de clima subtropical e tropical ameno, onde a época de frio não muito prolongada, e a ocorrência de geadas severas não e freqüente.bitstream/item/104675/1/Capim-setaria.pd

Cylindrically symmetric, static strings with a cosmological constant in Brans-Dicke theory

The static, cylindrically symmetric vacuum solutions with a cosmological constant in the framework of the Brans-Dicke theory are investigated. Some of these solutions admitting Lorentz boost invariance along the symmetry axis correspond to local, straight cosmic strings with a cosmological constant. Some physical properties of such solutions are studied. These strings apply attractive or repulsive forces on the test particles. A smooth matching is also performed with a recently introduced interior thick string solution with a cosmological constant.Comment: 8 pages, Revtex; Published versio