BF Actions for the Husain-Kuchar Model

We show that the Husain-Kuchar model can be described in the framework of BF theories. This is a first step towards its quantization by standard perturbative QFT techniques or the spin-foam formalism introduced in the space-time description of General Relativity and other diff-invariant theories. The actions that we will consider are similar to the ones describing the BF-Yang-Mills model and some mass generating mechanisms for gauge fields. We will also discuss the role of diffeomorphisms in the new formulations that we propose.Comment: 21 pages (in DIN A4 format), minor typos corrected; to appear in Phys. Rev.

Nonmesonic weak decay spectra of Λ4^4_\LambdaHe

To comprehend the recent Brookhaven National Laboratory experiment E788 on $^4_\Lambda$He, we have outlined a simple theoretical framework, based on the independent-particle shell model, for the one-nucleon-induced nonmesonic weak decay spectra. Basically, the shapes of all the spectra are tailored by the kinematics of the corresponding phase space, depending very weakly on the dynamics, which is gauged here by the one-meson-exchange-potential. In spite of the straightforwardness of the approach a good agreement with data is acheived. This might be an indication that the final-state-interactions and the two-nucleon induced processes are not very important in the decay of this hypernucleus. We have also found that the $\pi+K$ exchange potential with soft vertex-form-factor cutoffs $(\Lambda_\pi \approx 0.7$ GeV, $\Lambda_K \approx 0.9$ GeV), is able to account simultaneously for the available experimental data related to $\Gamma_p$ and $\Gamma_n$ for $^4_\Lambda$H, $^4_\Lambda$He, and $^5_\Lambda$He.Comment: 12 pages, 4 figures, 1 table, submitted for publication; v2: major revision, 18 pages, one author added, table, figures and bibliography change

Nuclear masses, deformations and shell effects

We show that the Liquid Drop Model is best suited to describe the masses of prolate deformed nuclei than of spherical nuclei. To this end three Liquid Drop Mass formulas are employed to describe nuclear masses of eight sets of nuclei with similar quadrupole deformations. It is shown that they are able to fit the measured masses of prolate deformed nuclei with an RMS smaller than 750 keV, while for the spherical nuclei the RMS is, in the three cases, larger than 2000 keV. The RMS of the best fit of the masses of semi-magic nuclei is also larger than 2000 keV. The parameters of the three models are studied, showing that the surface symmetry term is the one which varies the most from one group of nuclei to another. In one model, isospin dependent terms are also found to exhibit strong changes. The inclusion of shell effects allows for better fits, which continue to be better in the prolate deformed nuclei regionComment: 10 pages, 8 tables, Proc. of the XXXIV Nuclear Physics Symposium, January 4-7 2011, Cocoyoc, Morelos, Mexico. IOP Journal of Physics: Conference Series (in press

On the propagation speed of evanescent modes

The group-velocity of evanescent waves (in undersized waveguides, for instance) was theoretically predicted, and has been experimentally verified, to be Superluminal (v_g > c). By contrast, it is known that the precursor speed in vacuum cannot be larger than c. In this paper, by computer simulations based on Maxwell equations only, we show the existence of both phenomena. In other words, we verify the actual possibility of Superluminal group velocities, without violating the so-called (naive) Einstein causality. (Subject classes: General physics, Classical physics, Optics, Special Relativity; PACS nos.: 73.40Gk, 03.80+z, 03.65Bz; Keywords: evanescent waves; tunnelling photons; Hartman effect; group velocity; Superluminal waves; precursors; transient waves; front velocity; Maxwell equations; electromagnetic waves; computer simulations; Special Relativity; Extended Relativity).Comment: plain LaTeX file (14 pages), plus 15 figures in .jp

Unitary evolution of free massless fields in de Sitter space-time

We consider the quantum dynamics of a massless scalar field in de Sitter space-time. The classical evolution is represented by a canonical transformation on the phase space for the field theory. By studying the corresponding Bogoliubov transformations, we show that the symplectic map that encodes the evolution between two instants of time cannot be unitarily implemented on any Fock space built from a SO(4)-symmetric complex structure. We will show also that, in contrast with some effectively lower dimensional examples arising from Quantum General Relativity such as Gowdy models, it is impossible to find a time dependent conformal redefinition of the massless scalar field leading to a quantum unitary dynamics.Comment: 20 pages. Comments and references adde

Hilbert space structure of covariant loop quantum gravity

We investigate the Hilbert space in the Lorentz covariant approach to loop quantum gravity. We restrict ourselves to the space where all area operators are simultaneously diagonalizable, assuming that it exists. In this sector quantum states are realized by a generalization of spin network states based on Lorentz Wilson lines projected on irreducible representations of an SO(3) subgroup. The problem of infinite dimensionality of the unitary Lorentz representations is absent due to this projection. Nevertheless, the projection preserves the Lorentz covariance of the Wilson lines so that the symmetry is not broken. Under certain conditions the states can be thought as functions on a homogeneous space. We define the inner product as an integral over this space. With respect to this inner product the spin networks form an orthonormal basis in the investigated sector. We argue that it is the only relevant part of a larger state space arising in the approach. The problem of the noncommutativity of the Lorentz connection is solved by restriction to the simple representations. The resulting structure shows similarities with the spin foam approach.Comment: 20 pages, RevTE

Simulation of delamination in composite materials using the serial/parallel mixing theory

Este artículo presenta un nuevo procedimiento con el que simular el fallo por delaminación en materiales compuestos. El método propuesto utiliza la teoría de mezclas serie/paralelo, desarrollada por Fernando Rastellini, para la caracterización constitutiva del material compuesto. Esta formulación obtiene el comportamiento del compuesto acoplando la respuesta constitutiva de los materiales componentes, fibra y matriz, imponiendo una relación de iso-deformación en la dirección de la fibra (dirección en paralelo) y una relación de iso-tensión en el resto de direcciones (direcciones en serie). Si bien la teoría de mezclas serie/paralelo permite simular los materiales componentes con cualquier ecuación constitutiva, el presente trabajo utiliza una formulación de daño para caracterizar la matriz y una formulación elástica para la fibra. Se mostrará que es posible, utilizando la teoría de mezclas serie/paralelo y una formulación de daño, simular el fallo del compuesto por delaminación. Además, éste se obtiene de forma natural, sin necesidad de definir elementos especiales en los que propagar la fractura, ni ser necesario utilizar costosas técnicas numéricas como el remallado o elementos de contacto. Con el procedimiento propuesto el fallo constitutivo de la matriz conduce a una reducción de la capacidad portante del compuesto en las direcciones en serie (entre otras, la dirección de las tensiones tangenciales) sin eliminar la capacidad resistente en la dirección de la fibra. Se probará que un material compuesto con este comportamiento corresponde a un material delaminado. La última sección de este artículo muestra la simulación del End Notch Failure Test y los resultados obtenidos son comparados con resultados experimentales. La similitud de los resultados numéricos y experimentales demuestra la validez del método propuesto para resolver problemas de delaminación en materiales compuestos.This paper presents a new procedure to deal with the delamination problem found in laminated com- posites, based in a continuum mechanics formulation. The procedure proposed obtains the composite constitutive performance with the Serial/Parallel mixing theory, developed by F. Rastellini. This theory characterizes composite materials by coupling the constitutive behaviour of the composite components, imposing an iso-strain relation among the components in the fibre (or parallel) direction and an iso-stress relation in the remaining directions (serial directions). The proposed procedure uses a damage formula- tion to characterize the constitutive behaviour of matrix component in order to obtain the stress-strain performance of this material. With these two formulations, the delamination phenomenon is characterized naturally by the nume- rical simulation, being unnecessary the definition of special elements or computationally expensive techniques like the definition of contact elements or mesh separation. Matrix failure, as a result of the stress state found in it, leads to a reduction of the stiffness and strength capacity of the composite in its serial directions, among them, the shear component. This stiffness reduction provides a composite performance equivalent to what is found in a delaminated material. To prove the ability of the formulation proposed to solve delamination problems, the End Notch Fai- lure test is numerically simulated and the results obtained are compared with experimental ones. The agreement found in the results with both simulations, numerical and experimental, validate the proposed methodology to solve the delamination problem.Peer Reviewe

Minisuperspace Examples of Quantization Using Canonical Variables of the Ashtekar Type: Structure and Solutions

The Ashtekar variables have been use to find a number of exact solutions in quantum gravity and quantum cosmology. We investigate the origin of these solutions in the context of a number of canonical transformations (both complex and real) of the basic Hamiltonian variables of general relativity. We are able to present several new solutions in the minisuperspace (quantum cosmology) sector. The meaning of these solutions is then discussed.Comment: 23 pages, latex, 3 figures (uuencoded, separate file

On the structure of the space of generalized connections

We give a modern account of the construction and structure of the space of generalized connections, an extension of the space of connections that plays a central role in loop quantum gravity.Comment: 30 pages, added references, minor changes. To appear in International Journal of Geometric Methods in Modern Physic