4,342 research outputs found
The quadratic spinor Lagrangian, axial torsion current, and generalizations
We show that the Einstein-Hilbert, the Einstein-Palatini, and the Holst
actions can be derived from the Quadratic Spinor Lagrangian (QSL), when the
three classes of Dirac spinor fields, under Lounesto spinor field
classification, are considered. To each one of these classes, there corresponds
a unique kind of action for a covariant gravity theory. In other words, it is
shown to exist a one-to-one correspondence between the three classes of
non-equivalent solutions of the Dirac equation, and Einstein-Hilbert,
Einstein-Palatini, and Holst actions. Furthermore, it arises naturally, from
Lounesto spinor field classification, that any other class of spinor field
(Weyl, Majorana, flagpole, or flag-dipole spinor fields) yields a trivial
(zero) QSL, up to a boundary term. To investigate this boundary term we do not
impose any constraint on the Dirac spinor field, and consequently we obtain new
terms in the boundary component of the QSL. In the particular case of a
teleparallel connection, an axial torsion 1-form current density is obtained.
New terms are also obtained in the corresponding Hamiltonian formalism. We then
discuss how these new terms could shed new light on more general
investigations.Comment: 9 pages, RevTeX, to be published in Int.J.Mod.Phys.D (2007
Avaliação econômica da cafeicultura adubada em Buritis, Rondônia, 2007.
O objetivo deste estudo foi avaliar o desempenho econômico de um sistema de produção de café convencional adubado no Município de Buritis, RO.bitstream/CPAF-RO-2010/14499/1/338-cafeiculturaemburitis.pd
Ambient magnetic field amplification in shock fronts of relativistic jets: an application to GRB afterglows
Strong downstream magnetic fields of order of G, with large
correlation lengths, are believed to cause the large synchrotron emission at
the afterglow phase of gamma ray bursts (GRBs). Despite of the recent
theoretical efforts, models have failed to fully explain the amplification of
the magnetic field, particularly in a matter dominated scenario. We revisit the
problem by considering the synchrotron emission to occur at the expanding shock
front of a weakly magnetized relativistic jet over a magnetized surrounding
medium. Analytical estimates and a number of high resolution 2D relativistic
magneto-hydrodynamical (RMHD) simulations are provided. Jet opening angles of
, and ambient to jet density ratios of
were considered. We found that most of the amplification is
due to compression of the ambient magnetic field at the contact discontinuity
between the reverse and forward shocks at the jet head, with substantial
pile-up of the magnetic field lines as the jet propagates sweeping the ambient
field lines. The pile-up is maximum for , decreasing with
, but larger than in the spherical blast problem. Values obtained for
certain models are able to explain the observed intensities. The maximum
correlation lengths found for such strong fields is of cm, orders of magnitude larger than the found in previous
works.Comment: mnras accepte
Lucratividade da agricultura.
O artigo considera a concentração do valor da produção e a renda líquida. Procura mostrar que a elevada concentração da produção está fortemente ligada à modernização da agricultura. Daí decorre que o Brasil enfrenta problema de difusão de tecnologia, qual seja, o de fazer a tecnologia chegar aos milhões de estabelecimentos que contribuíram muito pouco para a produção.Na publicação: Eliseu Alves
Desafios para a competitividade internacional.
São inúmeros os fatores competitivos e pré-competitivos que prejudicam os ganhos de eficiência na cadeia produtiva de leite e penalizam uma maior inserção do Brasil no cenário internacional. No entanto, mesmo com todas essas dificuldades, historicamente, a produção seguiu se expandindo
Dark spinor models in gravitation and cosmology
We introduce and carefully define an entire class of field theories based on
non-standard spinors. Their dominant interaction is via the gravitational field
which makes them naturally dark; we refer to them as Dark Spinors. We provide a
critical analysis of previous proposals for dark spinors noting that they
violate Lorentz invariance. As a working assumption we restrict our analysis to
non-standard spinors which preserve Lorentz invariance, whilst being non-local
and explicitly construct such a theory. We construct the complete
energy-momentum tensor and derive its components explicitly by assuming a
specific projection operator. It is natural to next consider dark spinors in a
cosmological setting. We find various interesting solutions where the spinor
field leads to slow roll and fast roll de Sitter solutions. We also analyse
models where the spinor is coupled conformally to gravity, and consider the
perturbations and stability of the spinor.Comment: 43 pages. Several new sections and details added. JHEP in prin
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