arXiv subjects

A. Salam

Publications and source records attributed to A. Salam.

9 recordsLinked to original sources

arXiv2025

Sparse learning with concave regularization: relaxation of the irrepresentable condition

Learning sparse models from data is an important task in all those frameworks where relevant information should be identified within a large dataset. This can be achieved by formulating and solving suitable sparsity promoting optimization problems. As to linear regression models, Lasso is the most popular convex approach, based on an $\ell_1$-norm regularization. In contrast, in this paper, we analyse a concave regularized approach, and we prove that it relaxes the irrepresentable condition, which is sufficient and essentially necessary for Lasso to select the right significant parameters. In practice, this has the benefit of reducing the number of necessary measurements with respect to Lasso. Since the proposed problem is non-convex, we also discuss different algorithms to solve it, and we illustrate the obtained enhancement via numerical experiments.

math.OC↗

arXiv2022

Macroscopic quantum electrodynamics theory of resonance energy transfer involving chiral molecules

Resonance energy transfer between chiral molecules can be used to discriminate between different enantiomers. The transfer rate between chiral molecules consists of a non-discriminatory and discriminatory parts. We derive these two rate contributions in the framework of macroscopic quantum electrodynamics. We show that their ratio is usually larger in the retarded regime or far-zone of large separation distances and that the degree of discrimination can be modified when considering a surrounding medium. We highlight the importance of local field effects onto the degree of discrimination and predict for general identical chiral molecules the optimum dielectric medium for discrimination. We apply our results on to 3-methylcyclopentanone and show that exotic media can even invert the discriminatory effect.

quant-ph↗

arXiv2017

Enhanced chiral discriminatory van der Waals interactions mediated by chiral surfaces

We predict a discriminatory interaction between a chiral molecule and an achiral molecule which is mediated by a chiral body. To achieve this, we generalize the van der Waals interaction potential between two ground-state molecules with electric, magnetic and chiral response to non-trivial environments. The force is evaluated using second-order perturbation theory with an effective Hamiltonian. Chiral media enhance or reduce the free interaction via many-body interactions, making it possible to measure the chiral contributions to the van der Waals force with current technology. The van der Waals interaction is discriminatory with respect to enantiomers of different handedness and could be used to separate enantiomers. We also suggest a specific geometric configuration where the electric contribution to the van der Waals interaction is zero, making the chiral component the dominant effect.

quant-ph↗

arXiv2012

Body-assisted dispersion potentials of diamagnetic atoms

We derive Casimir-Polder and van der Waals potentials of one or two atoms with diamagnetic properties in an arbitrary environment of magnetoelectric bodies. The calculations are based on macroscopic quantum electrodynamics and leading-order perturbation theory. For the examples of an atom and a perfect mirror and two atoms in free space we show that diamagnetic dispersion potentials have the same sign as their electric counterparts, but can exhibit quite different distance dependences.

quant-ph↗

arXiv2011

Role of the $K_1$ meson in $K^0$ photoproduction off the deuteron

Neutral kaon photoproduction off the nucleon and deuteron has been reinvestigated by utilizing the new experimental data on both targets. An isobar model for elementary operator and impulse approximation for the reaction on the deuteron have been used. The available free parameters in the elementary model have been extracted from both elementary and deuteron data. In contrast to the elementary reaction, fitting the deuteron data requires an inclusion of weighting factor. The result indicates that the angular distribution of the elementary $K^0Λ$ process does not show backward peaking behavior.

nucl-th↗

arXiv2009

Neutral kaon photoproduction on the deuteron

Neutral kaon photoproduction on the deuteron has been investigated by including the final state effects and compared with the experimental data. Comparison shows that the models used in this calculation can reproduce the data in the $Σ$ channel regions fairly well but still give over predictions in the $Λ$ channel. It seems that the tensor target asymmetries are more suitable for studying the final state effects. The extractions of the elementary photoproduction amplitude are also demonstrated.

nucl-th↗

arXiv2006

K0 photoproduction on the deuteron and the extraction of the elementary amplitude

The photoproduction of K0 on the deuteron has been investigated with the inclusion of YN and KN final-state interaction (FSI), as well as the pion-mediated process gamma d --> pi NN --> KYN. The YN rescattering effects for the inclusive cross section are found to be large in the threshold regions. Polarization observables show sizable FSI effects at larger kaon and hyperon angles. It is shown that the extraction of the elementary gamma N --> KY amplitude is possible in the quasi-free scattering region where FSI effects are negligible. Furthermore, the cross sections in this region are large, indicating that measurements in this kinematical region are favored.

nucl-th↗

arXiv2004

Photoproduction of Theta+ on the nucleon and deuteron

Photoproduction of the pentaquark particle Theta^+ on the nucleon has been studied by using an isobar and a Regge model. Using the isobar model, total cross sections around 100 nb for the gamma n --> K- Theta+ channel and 400 nb for the gamma p --> {\bar K}0 Theta+ process are obtained. The inclusion of the K* intermediate state yields a substantially large effect, especially in the gamma p --> {\bar K}0 Theta+ process. The Regge approach predicts smaller cross sections, i.e., less than 100 nb (20 nb) for the process on the neutron (proton). By using an elementary operator from the isobar model, cross sections for the process on a deuteron are predicted.

nucl-th↗

arXiv2004

Rescattering effects and two-step process in kaon photoproduction on the deuteron

Kaon photoproduction on the deuteron is investigated by considering YN and KN rescattering and the two-step process gamma d --> pi NN --> KYN. A strong enhancement in the total cross section is found from the two-step process. YN rescattering has remarkable effects in the inclusive and exclusive cross section, while the effect of KN rescattering is much less important.

nucl-th↗