S-Matrix Poles Close to Thresholds in Confined Geometries

We have studied the behavior of the S-matrix poles near threshold for quantum waveguides coupled to a cavity with a defect. We emphasize the occurrence of both dominant and shadow poles on the various sheets of the energy Riemann surface, and show that the changes of the total conductivity near threshold as the cavity's width changes can be explained in terms of dominant to shadow pole transitions.Comment: 10 pages, 5 figure

Non-localities in nucleon-nucleus potentials

Two causes of non-locality inherent in nucleon-nucleus scattering are considered. They are the results of two-nucleon antisymmetry of the projectile with each nucleon in the nucleus and the dynamic polarization potential representation of channel coupling. For energies $\sim 40 - 300$ MeV, a g-folding model of the optical potential is used to show the influence of the knock-out process that is a result of the two-nucleon antisymmetry. To explore the dynamic polarization potential caused by channel coupling, a multichannel algebraic scattering model has been used for low-energy scattering.Comment: 12 pages, 11 figures, submitted to EPJ

Fano Resonances in Stubbed Quantum Waveguides with Impurities

We consider T--shaped, two--dimensional quantum waveguides containing attractive or repulsive impurities with a smooth, realistic shape, and study how the resonance behavior of the total conductance depends upon the strength of the defect potential and the geometry of the device. The resonance parameters are determined locating the relevant S-matrix poles in the Riemann energy surface. The total scattering operator is obtained from the S-matrices of the various constituent segments of the device through the *-product composition rule. This allows for a numerically stable evaluation of the scattering matrix and of the resonance parameters.Comment: 23 pages, 6 figure

The pion-three-nucleon problem with two-cluster connected-kernel equations

It is found that the coupled piNNN-NNN system breaks into fragments in a nontrivial way. Assuming the particles as distinguishable, there are indeed four modes of fragmentation into two clusters, while in the standard three-body problem there are three possible two-cluster partitions and conversely the four-body problem has seven different possibilities. It is shown how to formulate the pion-three-nucleon collision problem through the integral-equation approach by taking into account the proper fragmentation of the system. The final result does not depend on the assumption of separability of the two-body t-matrices. Then, the quasiparticle method a' la Grassberger-Sandhas is applied and effective two-cluster connected-kernel equations are obtained. The corresponding bound-state problem is also formulated, and the resulting homogeneous equation provides a new approach which generalizes the commonly used techniques to describe the three-nucleon bound-state problem, where the meson degrees of freedom are usually suppressed.Comment: 20 pages, REVTeX, with 3 COLOR figures (PostScript

Practical approximation scheme for the pion dynamics in the three-nucleon system

We discuss a working approximation scheme to a recently developed formulation of the coupled piNNN-NNN problem. The approximation scheme is based on the physical assumption that, at low energies, the 2N-subsystem dynamics in the elastic channel is conveniently described by the usual 2N-potential approach, while the explicit pion dynamics describes small, correction-type effects. Using the standard separable-expansion method, we obtain a dynamical equation of the Alt-Grassberger-Sandhas (AGS) type. This is an important result, because the computational techniques used for solving the normal AGS equation can also be used to describe the pion dynamics in the 3N system once the matrix dimension is increased by one component. We have also shown that this approximation scheme treats the conventional 3N problem once the pion degrees of freedom are projected out. Then the 3N system is described with an extended AGS-type equation where the spin-off of the pion dynamics (beyond the 2N potential) is taken into account in additional contributions to the driving term. These new terms are shown to reproduce the diagrams leading to modern 3N-force models. We also recover two sets of irreducible diagrams that are commonly neglected in 3N-force discussions, and conclude that these sets should be further investigated, because a claimed cancellation is questionable.Comment: 18 pages, including 5 figures, RevTeX, Eps

Métricas para medir la calidad de la propuesta didáctica de un curso virtual

El Programa Permanente de Educación a Distancia (EaD) de la Facultad de Ciencias Exactas de la Universidad Nacional de San Juan, ante la necesidad de contar con mecanismos de seguimiento y evaluación que aseguren la calidad de sus ofertas educativas, ha elaborado una metodología para formar a los docentes interesados en proponer cursos de grado y/o posgrado en la modalidad no presencial. Una de las fases de esta metodología consiste en la Evaluación de la Calidad del uso de la propuesta realizada por los docentes respecto de la interactividad alumno/material e interacción entre los distintos actores del proceso educativo. El objetivo de este documento es describir el modelo de evaluación propuesto por el programa EaD de la Facultad de Ciencias Exactas, para evaluar la calidad de los procesos de interactividad e interacción. Los Criterios, Estándares e Indicadores del modelo, se sustentan en propuestas de la Filosofía de la Calidad Total y de los Sistemas Benchmarking. Se realiza además un análisis de la información obtenida, a partir de la aplicación de las métricas del modelo de evaluación, en cursos de grado y posgrado propuestos en la mencionada institución.Red de Universidades con Carreras en Informática (RedUNCI

Métricas para medir la calidad de la propuesta didáctica de un curso virtual

El Programa Permanente de Educación a Distancia (EaD) de la Facultad de Ciencias Exactas de la Universidad Nacional de San Juan, ante la necesidad de contar con mecanismos de seguimiento y evaluación que aseguren la calidad de sus ofertas educativas, ha elaborado una metodología para formar a los docentes interesados en proponer cursos de grado y/o posgrado en la modalidad no presencial. Una de las fases de esta metodología consiste en la Evaluación de la Calidad del uso de la propuesta realizada por los docentes respecto de la interactividad alumno/material e interacción entre los distintos actores del proceso educativo. El objetivo de este documento es describir el modelo de evaluación propuesto por el programa EaD de la Facultad de Ciencias Exactas, para evaluar la calidad de los procesos de interactividad e interacción. Los Criterios, Estándares e Indicadores del modelo, se sustentan en propuestas de la Filosofía de la Calidad Total y de los Sistemas Benchmarking. Se realiza además un análisis de la información obtenida, a partir de la aplicación de las métricas del modelo de evaluación, en cursos de grado y posgrado propuestos en la mencionada institución.Red de Universidades con Carreras en Informática (RedUNCI

From bound states to resonances: analytic continuation of the wave function

Single-particle resonance parameters and wave functions in spherical and deformed nuclei are determined through analytic continuation in the potential strength. In this method, the analyticity of the eigenvalues and eigenfunctions of the Schroedinger equation with respect to the coupling strength is exploited to analytically continue the bound-state solutions into the positive-energy region by means of Pade' approximants of the second kind. The method is here applied to single-particle wave functions of the $^{154}Sm$ and $^{131}Eu$ nuclei. A comparison of the results with the direct solution of the Schroedinger equation shows that the method can be confidently applied also in coupled-channel situations requiring high numerical accuracy.Comment: 13 pages, 3 figure

The pd <--> pi+ t reaction around the Delta resonance

The pd pi+ t process has been calculated in the energy region around the Delta-resonance with elementary production/absorption mechanisms involving one and two nucleons. The isobar degrees of freedom have been explicitly included in the two-nucleon mechanism via pi-- and rho-exchange diagrams. No free parameters have been employed in the analysis since all the parameters have been fixed in previous studies on the simpler pp pi+ d process. The treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of the reaction, with continuum calculations for the initial p-d state and accurate solutions of the three-nucleon bound-state equation. The integral cross-section was found to be quite sensitive to the NN interaction employed while the angular dependence showed less sensitivity. Approximately a 4% effect was found for the one-body mechanism, for the three-nucleon dynamics in the p-d channel, and for the inclusion of a large, possibly converged, number of three-body partial states, indicating that these different aspects are of comparable importance in the calculation of the spin-averaged observables.Comment: 40 Pages, RevTex, plus 5 PostScript figure

Spin observables for the pd <-> pi+ t process around the Delta resonance

The proton analyzing power Ay0 and the deuteron tensor analyzing power T20 are evaluated for the pd pi+ t process, in the energy region around and above the Delta resonance. These calculations extend a previous analysis of the excitation function and differential cross-section, based on a model embodying one-- and two-body p-wave absorption mechanisms and isobar excitation. The three-nucleon bound state and the pd scattering state are evaluated through Faddeev techniques for both the Bonn and Paris potentials. The spin variables exhibit a greater sensitivity to the number of included three-nucleon partial waves than the cross-sections, while the role played by the initial-- or final-state interactions appears to be small. The results for the tensor analyzing power at backward angles show a non-negligible dependence on the potentials employed, consistently with what has been previously found for the cross-sections. The calculation of spin observables gives a clear indication that other reaction mechanisms (presumably s-wave two-body absorption) have to be included in the model, in order to reproduce the experimental data below the Delta-resonance, in analogy with the simpler pp pi+ d process.Comment: 14 pages, REVTeX, plus 6 figs., PostScript (PRC, to be published