Generalized Parton Distributions of ^3He

A realistic microscopic calculation of the unpolarized quark Generalized Parton Distribution (GPD) $H_q^3$ of the $^3He$ nucleus is presented. In Impulse Approximation, $H_q^3$ is obtained as a convolution between the GPD of the internal nucleon and the non-diagonal spectral function, describing properly Fermi motion and binding effects. The proposed scheme is valid at low values of $\Delta^2$, the momentum transfer to the target, the most relevant kinematical region for the coherent channel of hard exclusive processes. The obtained formula has the correct forward limit, corresponding to the standard deep inelastic nuclear parton distributions, and first moment, giving the charge form factor of $^3He$. Nuclear effects, evaluated by a modern realistic potential, are found to be larger than in the forward case. In particular, they increase with increasing the momentum transfer when the asymmetry of the process is kept fixed, and they increase with the asymmetry at fixed momentum transfer. Another relevant feature of the obtained results is that the nuclear GPD cannot be factorized into a $\Delta^2$-dependent and a $\Delta^2$-independent term, as suggested in prescriptions proposed for finite nuclei. The size of nuclear effects reaches 8 % even in the most important part of the kinematical range under scrutiny. The relevance of the obtained results to study the feasibility of experiments is addressed.Comment: 23 pages, 8 figures; Discussion in section II enlarged; discussion in section IV shortened. Final version accepted by Phys. Rev.

Three-dimensional turbulence vorticity: Numerical and experimental modeling

We show in this paper how a system of equations of motion, diffusion and continuity that present the effects of vorticity through a vorticity transfer length scale may be used to model 2D-3D vorticity behaviour. The local turbulent vorticity is separated from the large-scale flow following the Reynolds decomposition (Reynolds 1894, Taylor 1931)—∇×v +∇×v—similar to the technique used for velocity. The system of equations extended through the terms ∇× v and ∇× v is solved numerically using a purely statistical local method that details the role of vorticity transport in the turbulence behaviour of the flow. Moreover, this numerical model that shows the temporal evolution of both fields, 3D velocity and 3D vorticity is used to investigate the propagation of turbulent perturbations that arise from the development of a vortex placed in the centre of the numerical domain. Even with a small mesh (60 × 60 × 120), the results show the propagation of vorticity-related waves both in the plane and in the vertical. The numerical results are compared with experiments performed in a stratified flow, where velocity and vorticity are measured with PIV as turbulence behind a grid decay, these experiments have been performed both in a rotating frame of reference and with no rotation and show features also detected in the numerical simulations when the assumption of a quasi–two-dimensional flow is used

Axial Transition Form Factors and Pion Decay of Baryon Resonances

The pion decay constants of the lowest orbitally excited states of the nucleon and the $\Delta(1232)$ along with the corresponding axial transition form factors are calculated with Poincar\'e covariant constituent-quark models with instant, point and front forms of relativistic kinematics. The model wave functions are chosen such that the calculated electromagnetic and axial form factors of the nucleon represent the empirical values in all three forms of kinematics, when calculated with single-constituent currents. The pion decay widths calculated with the three forms of kinematics are smaller than the empirical values. Front and instant form kinematics provide a similar description, with a slight preference for front form, while the point form values are significantly smaller in the case of the lowest positive parity resonances.Comment: 18 pages, 5 figures. Slightly revised, accepted in Phys. Rev.

Filtered deterministic waves and analysis of the fractal dimension of the components of the wind velocity

The difficulty in developing models for waves in turbulent flows is a key problem in the analysis of the complexity of turbulence. We present a method to find and filter perturbations that are generated by the flow of deterministic waves from the power spectrum in the atmospheric boundary layer (ABL). The perturbation model proposed assumes that the amplitude and frequency of such waves decay with time exponentially. For illustrative purposes, we apply the technique to three time series of wind velocities obtained with a sonic anemometer. This analytical procedure allows us to filter waves of the proposed structure with a 99% significance level in the power spectrum. We have applied the same method to 540 such wind series, all painting similar results. We then compare the fractal dimension of the original series to those from which the waves have been removed. We find that the fractal dimension of the filtered waves is slightly less than that of the original series. Finally, we consider the fractal dimension of the studied series as a function of the length-scales and dissipation rate of kinetic energy per unit mass. Our results suggest an increase of fractal dimension with both length-scale and dissipation rate of kinetic energy

Effect of debris size on the tribological performance of thermally sprayed coatings

This research aims to assess the effect of the debris particle size on the tribological performance and lubrication regime parameters of a Ni-based alloy coating. This is a key industrial problem, and its resolution can contribute to better machine endurance and proper maintenance. The debris particles are simulated by hard Al2O3 particles of size ranging from nanometers to 45 μm and dispersed in an oil lubricant. The coating studied is NiCrBSi deposited by flame spraying technique followed by the Surface Flame Melting (SFM) process. The counterpart disk sample was fabricated from quenched and tempered F-5220 steel (in line with A681(O1) ASTM). This pair was tested under linear sliding contact. Our results show that the addition of alumina particles contributes to a significant increase in wear, particularly for the largest particles (micrometric size). In the case of micrometric particles, it is possible to observe the formation of higher surface roughness, numerous microgrooves, and plastic flow of NiCrBSi coating perpendicular to the sliding direction, resulting in higher loss of volume. It was found that the actual surface roughness (obtained as a function of the debris particle size) allows better identification and prediction of the lubrication regime for wear processes instead of the traditional approach that uses the initial surface roughness as a parameter

Integral priors and constrained imaginary training samples for nested and non-nested Bayesian model Comparison

© 2013 International Society for Bayesian Analysis This document is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 This document is the accepted version of a published work that appeared in final form in Bayesian AnalysisIn Bayesian model selection when the prior information on the parameters of the models is vague default priors should be used. Unfortunately, these priors are usually improper yielding indeterminate Bayes factors that preclude the comparison of the models. To calibrate the initial default priors Cano et al. (2008) proposed integral priors as prior distributions for Bayesian model selection. These priors were defined as the solution of a system of two integral equations that under some general assumptions has a unique solution associated with a recurrent Markov chain. Later, in Cano et al. (2012b) integral priors were successfully applied in some situations where they are known and they are unique, being proper or not, and it was pointed out how to deal with other situations. Here, we present some new situations to illustrate how this new methodology works in the cases where we are not able to explicitly find the integral priors but we know they are proper and unique (one-sided testing for the exponential distribution) and in the cases where recurrence of the associated Markov chains is difficult to check. To deal with this latter scenario we impose a technical constraint on the imaginary training samples space that virtually implies the existence and the uniqueness of integral priors which are proper distributions. The improvement over other existing methodologies comes from the fact that this method is more automatic since we only need to simulate from the involved models and their posteriors to compute very well behaved Bayes factors

Self-organization and social science

Abstract Complexity science and its methodological applications have increased in popularity in social science during the last two decades. One key concept within complexity science is that of self-organization. Self-organization is used to refer to the emergence of stable patterns through autonomous and self-reinforcing dynamics at the micro-level. In spite of its potential relevance for the study of social dynamics, the articulation and use of the concept of self-organization has been kept within the boundaries of complexity science and links to and from mainstream social science are scarce. These links can be difficult to establish, even for researchers working in social complexity with a background in social science, because of the theoretical and conceptual diversity and fragmentation in traditional social science. This article is meant to serve as a first step in the process of overcoming this lack of cross-fertilization between complexity and mainstream social science. A systematic review of the concept of self-organization and a critical discussion of similar notions in mainstream social science is presented, in an effort to help practitioners within subareas of complexity science to identify literature from traditional social science that could potentially inform their research

Detrimental adsorbate fields in experiments with cold Rydberg gases near surfaces

We observe the shift of Rydberg levels of rubidium close to a copper surface when atomic clouds are repeatedly deposited on it. We measure transition frequencies of rubidium to S and D Rydberg states with principal quantum numbers n between 31 and 48 using the technique of electromagnetically induced transparency. The spectroscopic measurement shows a strong increase of electric fields towards the surface that evolves with the deposition of atoms. Starting with a clean surface, we measure the evolution of electrostatic fields in the range between 30 and 300 \mum from the surface. We find that after the deposition of a few hundred atomic clouds, each containing ~10^6 atoms, the field of adsorbates reaches 1 V/cm for a distance of 30 \mum from the surface. This evolution of the electrostatic field sets serious limitations on cavity QED experiments proposed for Rydberg atoms on atom chips.Comment: 4 pages, 3 figures Submitted to Phys. Rev.

Magnetotransport in the Kondo model with ferromagnetic exchange interaction

We consider the transport properties in an applied magnetic field of the spin S=1/2 Kondo model with ferromagnetic exchange coupling to electronic reservoirs, a description relevant for the strong coupling limit of underscreened spin S=1 Kondo impurities. Because the ferromagnetic Kondo interaction is marginally irrelevant, perturbative methods should prove accurate down to low energies. For the purpose of this study, we use a combination of Majorana diagrammatic theory with Density Matrix Numerical Renormalization Group simulations. In the standard case of antiferromagnetic Kondo exchange, we first show that our technique recovers previously obtained results for the T-matrix and spin relaxation at weak coupling (above the Kondo temperature). Considering then the ferromagnetic case, we demonstrate how the low-energy Kondo anomaly splits for arbitrary small values of the Zeeman energy, in contrast to fully screened Kondo impurities near the strong coupling Fermi liquid fixed point, and in agreement with recent experimental findings for spin S=1 molecular quantum dots.Comment: 14 pages, 13 figures, minor changes in V