Accurate nucleon electromagnetic form factors from dispersively improved chiral effective field theory

We present a theoretical parametrization of the nucleon electromagnetic form factors (FFs) based on a combination of chiral effective field theory and dispersion analysis. The isovector spectral functions on the two-pion cut are computed using elastic unitarity, chiral pion-nucleon amplitudes, and timelike pion FF data. Higher-mass isovector and isoscalar t-channel states are described by effective poles, whose strength is fixed by sum rules (charges, radii). Excellent agreement with the spacelike proton and neutron FF data is achieved up to Q^2 \sim 1 GeV^2. Our parametrization provides proper analyticity and theoretical uncertainty estimates and can be used for low-Q^2 FF studies and proton radius extraction.Comment: 5 pages, 3 figures, 2 table

Effective and neutral stresses in soils using boundary element methods

The evaluation of neutral pressures in soil mechanics problems is a fundamental step to evaluate deformations in soils. In this paper, we present some results obtained by using the boundary element method for plane problems, describing the undrained situation as well as the consolidation problem

Multiple imputation approach for interval-censored time to HIV RNA viral rebound within a mixed effects Cox model

This is the peer reviewed version of the following article: “Alarcón-Soto, Y, Langohr K., Fehér, C., García, F., and Gómez, G. (2018) Multiple imputation approach for interval-censored time to HIV RNA viral rebound within a mixed effects Cox Model.Biometrical journal, December 13th ”which has been published in final form at [doi: 10.1002/bimj.201700291]. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.We present a method to fit a mixed effects Cox model with interval-censored data. Our proposal is based on a multiple imputation approach that uses the truncated Weibull distribution to replace the interval-censored data by imputed survival times and then uses established mixed effects Cox methods for right-censored data. Interval-censored data were encountered in a database corresponding to a recompilation of retrospective data from eight analytical treatment interruption (ATI) studies in 158 human immunodeficiency virus (HIV) positive combination antiretroviral treatment (cART) suppressed individuals. The main variable of interest is the time to viral rebound, which is defined as the increase of serum viral load (VL) to detectable levels in a patient with previously undetectable VL, as a consequence of the interruption of cART. Another aspect of interest of the analysis is to consider the fact that the data come from different studies based on different grounds and that we have several assessments on the same patient. In order to handle this extra variability, we frame the problem into a mixed effects Cox model that considers a random intercept per subject as well as correlated random intercept and slope for pre-cART VL per study. Our procedure has been implemented in R using two packages: truncdist and coxme, and can be applied to any data set that presents both interval-censored survival times and a grouped data structure that could be treated as a random effect in a regression model. The properties of the parameter estimators obtained with our proposed method are addressed through a simulation study.Peer ReviewedPostprint (author's final draft

Transverse charge and current densities in the nucleon from dispersively improved chiral effective field theory

Background: The transverse densities $\rho_{1, 2}(b)$ describe the distributions of electric charge and magnetic moment at fixed light-front time and connect the nucleon's elastic form factors with its partonic structure. The dispersive representation of the form factors $F_{1, 2}(t)$ expresses the densities in terms of exchanges of hadronic states in the $t$-channel and permits their analysis using hadronic physics methods. Purpose: Compute the densities at peripheral distances $b = \mathcal{O}(M_\pi^{-1})$, where they are generated predominantly by the two-pion states in the dispersive representation. Quantify the uncertainties. Methods: Dispersively improved chiral effective field theory (DI$\chi$EFT) is used to calculate the isovector spectral functions $\textrm{Im}\, F_{1, 2}(t)$ on the two-pion cut. The method includes $\pi\pi$ interactions ($\rho$ resonance) through elastic unitarity and provides realistic spectral functions up to $t \approx$ 1 GeV$^2$. Higher-mass states are parametrized by effective poles and constrained by sum rules (charges, radii, superconvergence relations). The densities $\rho_{1, 2}(b)$ are obtained from their dispersive representation. Uncertainties are quantified by varying the spectral functions. The method respects analyticity and ensures the correct $b \rightarrow \infty$ asymptotic behavior of the densities. Results: Accurate densities are obtained at all distances $b \gtrsim 0.5$ fm, with correct behavior down to $b \rightarrow 0$. The region of distances is quantified where transverse nucleon structure is governed by the two-pion state. The light-front current distributions in the polarized nucleon are computed and discussed. Conclusions: Peripheral nucleon structure can be computed from first principles using DI$\chi$EFT. The method can be extended to generalized parton distributions and other nucleon form factors.Comment: 20 pages, 12 figures, 1 tabl

Consolidation problems

The analysis of deformation in soils is of paramount importance in geotechnical engineering. For a long time the complex behaviour of natural deposits defied the ingenuity of engineers. The time has come that, with the aid of computers, numerical methods will allow the solution of every problem if the material law can be specified with a certain accuracy. Boundary Techniques (B.E.) have recently exploded in a splendid flowering of methods and applications that compare advantegeously with other well-established procedures like the finite element method (F.E.). Its application to soil mechanics problems (Brebbia 1981) has started and will grow in the future. This paper tries to present a simple formulation to a classical problem. In fact, there is already a large amount of application of B.E. to diffusion problems (Rizzo et al, Shaw, Chang et al, Combescure et al, Wrobel et al, Roures et al, Onishi et al) and very recently the first specific application to consolidation problems has been published by Bnishi et al. Here we develop an alternative formulation to that presented in the last reference. Fundamentally the idea is to introduce a finite difference discretization in the time domain in order to use the fundamental solution of a Helmholtz type equation governing the neutral pressure distribution. Although this procedure seems to have been unappreciated in the previous technical literature it is nevertheless effective and straightforward to implement. Indeed for the special problem in study it is perfectly suited, because a step by step interaction between the elastic and flow problems is needed. It allows also the introduction of non-linear elastic properties and time dependent conditions very easily as will be shown and compares well with performances of other approaches