Transport Approach Research Articles

Role of proton-antiproton regeneration in the late stages of heavy-ion collisions

We investigate the long-standing question of the effect of proton-antiproton annihilation on the (anti-)proton yield, while respecting detailed balance for the five-body back-reaction for the first time in a full microscopic description of the late stages of heavy-ion collisions. This is achieved by employing a stochastic collision criterion in a hadronic transport approach (SMASH), which is used to account for the regeneration of (anti-)protons via $5\ensuremath{\pi}\ensuremath{\rightarrow}\mathrm{p}\overline{\mathrm{p}}$. We investigate $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Pb}+\mathrm{Pb}$ collisions from $\sqrt{{s}_{NN}}=17.3\phantom{\rule{4pt}{0ex}}\mathrm{GeV}\ensuremath{-}5.02$ TeV in a viscous hybrid approach. Our results show that back-reactions happen for a fraction of 15%--20% of all annihilations, independent of the beam energy or centrality of the system. The inclusion of the back-reaction results in the regeneration of half of the (anti-)proton yield lost to annihilations at midrapidity. We also find that, concerning the multiplicities, treating the back-reaction as a chain of two-body reactions is equivalent to a single 5-to-2 reaction.

An Optimal Transport Approach for Selecting a Representative Subsample with Application in Efficient Kernel Density Estimation

Subsampling methods aim to select a subsample as a surrogate for the observed sample. Such methods have been used pervasively in large-scale data analytics, active learning, and privacy-preserving analysis in recent decades. Instead of model-based methods, in this article, we study model-free subsampling methods, which aim to identify a subsample, that is, not confined by model assumptions. Existing model-free subsampling methods are usually built upon clustering techniques or kernel tricks. Most of these methods suffer from either a large computational burden or a theoretical weakness. In particular, the theoretical weakness is that the empirical distribution of the selected subsample may not necessarily converge to the population distribution. Such computational and theoretical limitations hinder the broad applicability of model-free subsampling methods in practice. We propose a novel model-free subsampling method by using optimal transport techniques. Moreover, we develop an efficient subsampling algorithm, that is, adaptive to the unknown probability density function. Theoretically, we show the selected subsample can be used for efficient density estimation by deriving the convergence rate for the proposed subsample kernel density estimator. We also provide the optimal bandwidth for the proposed estimator. Numerical studies on synthetic and real-world datasets demonstrate the performance of the proposed method is superior.

An Optimal Transport Approach to Personalized Federated Learning

Federated learning is a distributed machine learning paradigm, which aims to train a model using the local data of many distributed clients. A key challenge in federated learning is that the data samples across the clients may not be identically distributed. To address this challenge, personalized federated learning with the goal of tailoring the learned model to the data distribution of every individual client has been proposed. In this paper, we focus on this problem and propose a novel personalized Federated Learning scheme based on Optimal Transport (FedOT) as a learning algorithm that learns the optimal transport maps for transferring data points to a common distribution as well as the prediction model under the applied transport map. To formulate the FedOT problem, we extend the standard optimal transport task between two probability distributions to multi-marginal optimal transport problems with the goal of transporting samples from multiple distributions to a common probability domain. We then leverage the results on multi-marginal optimal transport problems to formulate FedOT as a min-max optimization problem and analyze its generalization and optimization properties. We discuss the results of several numerical experiments to evaluate the performance of FedOT under heterogeneous data distributions in federated learning problems.

Harmonic flow correlations in Au+Au reactions at 1.23 AGeV: a new testing ground for the equation-of-state and expansion geometry

Correlations between the harmonic flow coefficients v_1, v_2, v_3 and v_4 of nucleons in semi-peripheral Au+Au collisions at a beam energy of 1.23 AGeV are investigated within the hadronic transport approach ultra-relativistic quantum molecular dynamics (UrQMD). In contrast to ultra-relativistic collision energies (where the flow coefficients are evaluated with respect to the respective event plane), we predict strong correlations between the flow harmonics with respect to the reaction plane. Based on an event-by-event selection of the midrapidity final state elliptic flow of nucleons we show that as a function of rapidity, (I) the sign of the triangular flow changes, (II) that the shape of v_4 changes from convex to concave, and (III) that v_3propto v_1v_2 and v_4propto v_2^2 for all different event classes, indicating strong correlations between all investigated harmonic flow coefficients.

Modeling Approaches for Fluidic Mass Transport in Next Generation Micro and Nano Biomedical Sensors

This review discusses on current methodologies and trends in modeling fluidic mass transport phenomena in micro and nano scale biomedical devices. We have presented the governing equations for species transport in micro and nano scales and provided analytical as well as computational approaches that can aid in obtaining solutions for complex flow problems. We have also reviewed novel methodologies that modern research community utilized for simulating species transport in micro and nano biomedical sensing devices.

Modeling Approaches for Fluidic Mass Transport in Next Generation Micro and Nano Biomedical Sensors

Modeling Approaches for Fluidic Mass Transport in Next Generation Micro and Nano Biomedical Sensors

Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: a first-principles Floquet-nonequilibrium Green function study

Motivated by the recent experiment (Vaidya et al 2020 Science 368 160) on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF2/Cu and Pt/MnF2/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use a more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green’s functions in order to take into account: SO-proximitized AFI as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; strong SO coupling at the interface; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting rather than insulating as in the isolated AFI. The DC component of pumped spin current vs. precession cone angle of the Néel vector l of AFI does not follow putative , except for very small angles for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. In addition, the angular dependence differs for opposite directions of precession of the Néel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.

Electrochemical On-Site Switching of the Directional Liquid Transport on a Conical Fiber.

Directional liquid transport (DLT), especially that proceeding on a conical fiber (DLT-CF), is an important mass-transfer process widely used both by natural organisms and in practical applications. However, on-site switching of the DLT-CF remains a challenge due to the nontunable driving force imparted by the structural gradient, which greatly limits its application. Here, unprecedently, a facile electrochemical strategy is developed for reaching the on-site switchable DLT-CF, featuring in situ control and fast response. Depending on the poised electric potential, the droplet can either move directionally or be pinned at any position for a tunable duration time, exhibiting completely different moving characteristics from the traditional DLT-CF with no control. It is proposed that the surface hysteresis resistance, closely related to both the surface hydrogen-bonding network and the droplet topology on the fiber, can be largely altered electrochemically. The tunable hysteresis resistance works synergistically with the conical-structure-induced Laplace pressure to on-site tune the forces acting on the droplet, leading to various controllable DLTs-CF, including those with tunable distance and direction, array manipulation, and assembly line processing of droplets. The strategy is applicable for versatile liquids, offering a general approach for controllable liquid transport in fibrous systems.

Gradual sustainability approach for urban transport through subtle measures

Gradual sustainability approach for urban transport through subtle measures

Quantitative observability for the Schrödinger and Heisenberg equations: An optimal transport approach

We establish a quantitative observation inequality for the Schrödinger and the Heisenberg equations on [Formula: see text], uniform in the Planck constant [Formula: see text]. The proof is based on the pseudometric introduced in F. Golse and T. Paul, The Schrödinger equation in the mean-field and semiclassical regime, Arch. Ration. Mech. Anal. 223 (2017) 57–94. This inequality involves only effective constants which are computed explicitly in their dependence in [Formula: see text] and all parameters involved.

Avalanche breakdown and quenching in Ge SPAD using 3D Monte Carlo simulation

A Ge-based single-photon-avalanche-diode (SPAD) is investigated by using self-consistent 3-D Monte Carlo simulation including the presence of a passive quenching circuit. This approach of transport allows us to capture all stochastic features of carrier transport and SPAD operation. We analyze particularly the probabilistic character of the quenching mechanism and its dependence on the parameters (resistance and capacitance) of the passive quenching circuit.

Deuteron production in ultrarelativistic heavy-ion collisions: A comparison of the coalescence and the minimum spanning tree procedure

The formation of deuterons in heavy-ion collisions at relativistic energies is investigated by employing two recently advanced models---the minimum spanning tree (MST) method and the coalescence model---by embedding them in the parton-hadron quantum molecular dynamics (PHQMD) and the ultrarelativistic quantum molecular dynamics (UrQMD) transport approaches. While the coalescence mechanism combines nucleons into deuterons at the kinetic freeze-out hypersurface, the MST identifies the clusters during the different stages of time evolution. We find that both clustering procedures give very similar results for the deuteron observables in the UrQMD as well as in the PHQMD environment. Moreover, the results agree well with the experimental data on deuteron production in Pb$+$Pb collisions at $\sqrt{{s}_{NN}}=8.8$ GeV (selected for the comparison of the methods and models in this study). A detailed investigation shows that the coordinate space distribution of the produced deuterons differs from that of the free nucleons and other hadrons. Thus, deuterons are not destroyed by additional rescattering.

Out-of-equilibrium photon production in the late stages of relativistic heavy-ion collisions

In this work, we assess the importance of nonequilibrium dynamics in the production of photons from the late stages of relativistic heavy-ion collisions. The ${p}_{\mathrm{T}}$-differential spectra and ${v}_{2}$ of photons from the late hadronic stage are computed within a nonequilibrium hadron transport approach, and compared to the results of a local equilibrium evolution using ideal relativistic hydrodynamics. It is found that nonequilibrium dynamics enhance the late-stage photon production at low ${p}_{\mathrm{T}}$ and decreases it at higher ${p}_{\mathrm{T}}$ compared to the estimate from hydrodynamics. This same comparison points to a significant increase in the momentum anisotropies of these photons due to nonequilibrium dynamics. Once combined with photons produced above the particlization temperature in the hydrodynamics evolution, the differences between the two approaches appear modest for the ${p}_{\mathrm{T}}$ differential spectra, but are clearly noticeable at low ${p}_{\mathrm{T}}$ for the elliptic flow: nonequilibrium dynamics enhance the photon ${v}_{2}$ below ${p}_{\mathrm{T}} \ensuremath{\approx}1.4$ GeV.

Estimating the performance of SOI FinFET transistor using electro-thermal formulation in conjunction with the quasi-ballistic effective electron mobility approach

Modeling and simulating FinFET based circuits, disentangled as Physics-based FinFET models, constitute predictive technology models for device and circuit simulations. The electrostatic properties as well as the electron and phonon transport are very responsive to the thermal effects in diffusive as well as in quasi-ballistic regimes suggesting thereby, the necessity of their consideration for effective device modeling and design particularly in nano FinFET devices. In this paper, we describe the calibration of the drift-diffusion (D-D) transport approach for simulating the quasi-ballistic effect in short channel occurring in nanoscale devices. A strong coupling of the calibrated D-D model with the enhanced Ballistic-Diffusive Equation (BDE), susceptive to prescribe the phonon and electron transports is introduced. The Finite Element Method (FEM) is used to produce results by numerical simulation of the electro-thermal model offered in Bulk and SOI FinFET devices. We have found that the proposed mobility model performs quite well and mainly infuences the drain current and the heat source by Joule effect. Our results extracted from the calibrated D-D model are compared and contrasted with those obtained experimentally and with those obtained with the numerically exact configuration. A decent agreement between the proposed model dealing with SOI FinFET as well as Bulk FinFET and the experimental data is obtained. Results obtained by using the enhanced electro-thermal model hold promise for thermal simulations of heat transport in Bulk and SOI FinFET devices, suggesting that the dopant concentration have a crucial role for the increasing of the temperature inside the Bulk/SOI FinFET.

Computational optimal transport for molecular spectra: The semi-discrete case.

Comparing a discrete molecular spectrum to a continuous molecular spectrum in a quantitative manner is a challenging problem, for example, when attempting to fit a theoretical stick spectrum to a continuous spectrum. In this paper, the use of computational optimal transport is investigated for such a problem. In the optimal transport literature, the comparison of a discrete and a continuous spectrum is referred to as semi-discrete optimal transport and is a situation where a metric such as least-squares may be difficult to define except under special conditions. The merits of an optimal transport approach for this problem are investigated using the transport distance defined for the semi-discrete case. A tutorial on semi-discrete optimal transport for molecular spectra is included in this paper, and several well-chosen synthetic spectra are investigated to demonstrate the utility of computational optimal transport for the semi-discrete case. Among several types of investigations, we include calculations showing how the frequency resolution of the continuous spectrum affects the transport distance between a discrete and a continuous spectrum. We also use the transport distance to measure the distance between a continuous experimental electronic absorption spectrum of SO2 and a theoretical stick spectrum for the same system. The comparison of the theoretical and experimental SO2 spectra also allows us to suggest a theoretical value for the band origin that is closer to the observed band origin than previous theoretical values.

Modelling approach to predict the fire-related heat transfer in porous gypsum based on multi-phase simulations including water vapour transport, phase change and radiative heat transfer

The heat transfer in gypsum exposed to fire is significantly affected by heat conduction, mass transfer and condensation/evaporation effects of water vapour in the porous structure. In the past, numerical models to predict the heat transfer in gypsum were mainly limited to heat conduction and mass transfer of water vapour in thin gypsum boards used in wall assemblies. Thus, in the present study a multi-phase approach is proposed to predict the heat transfer within gypsum under fire exposure including conduction, mass transfer and condensation/evaporation. The consideration of water vapour transport and its condensation in the porous structure was leading to a good prediction of the heating process of gypsum up to approx. 100 °C. Furthermore, the calculated temperatures above 100 °C were adequate up to 2 cm from the fire side. However, at a higher distance from the fire the additional implementation of a thermal radiation model was crucial to improve the heat transfer in gypsum. Including the thermal radiation, the proposed numerical model is able to calculate the temperatures in the gypsum blocks in close accordance to the measurement. • Fire resistance test of gypsum and measurement of the temperature profile. • Lee model to predict evaporation/condensation of water in porous gypsum. • Multi-phase approach for the water vapour transport through gypsum in fire tests. • Thermal radiation modelling in porous gypsum with discrete ordinates model (DOM). • Porosity-corrected DO modelling was successfully used for radiation in gypsum.

Comparison of heavy ion transport simulations: Ag + Ag collisions at E lab = 1.58A GeV

Abstract We compare the microscopic transport models UrQMD, PHSD, PHQMD, and SMASH to make predictions for the upcoming Ag + Ag data at E lab = 1.58A GeV ( s NN = 2.55 GeV) by the HADES collaboration. We study multiplicities, spectra and effective source temperatures of protons, π ±,0, K ±, the η, Λ + Σ0 and the Ξ− within these models. Despite variations in the detailed implementation of the dynamics in the different models, the employed transport approaches all show consistent multiplicities of the bulk of investigated hadrons. The main differences are in the Ξ− production, which is treated differently between UrQMD/SMASH on one side employing high mass resonance states with explicit decays to Resonance → Ξ + K + K in contrast to PHSD/PHQMD which account only non-resonant Ξ production channels. A comparison of the spectra, summarized by effective source temperatures, shows that all models provide similar source temperatures around T source = 80–95 MeV, and show substantial radial flow on the order of ⟨v T ⟩ = 0.18c − 0.24c even for such a small system.

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

Feynman-Vernon influence functional approach to quantum transport in interacting nanojunctions: An analytical hierarchical study

We present a nonperturbative and formally exact approach for the charge transport in interacting nanojunctions based on a real-time path-integral formulation of the reduced system dynamics. For reservoirs of noninteracting fermions, the exact trace over the leads' degrees of freedom results in the time-nonlocal Feynman-Vernon influence functional, a functional of the Grassmann-valued paths of the nanojunction, which induces correlations among the tunneling transitions in and out of the nanojunction. An expansion of the influence functional in terms of the number of tunneling transitions, and integration of the Grassmann variables between the tunneling times, allows us to obtain a still exact generalized master equation for the populations of the reduced density matrix in the occupation-number representation, as well as a formally exact expression for the current. By borrowing the nomenclature of the famous spin-boson model, we parametrize the two-state dynamics of each single-particle fermionic degree of freedom, in the occupation-number representation, in terms of blips and sojourns. We apply our formalism to the exactly solvable resonant level model (RLM) and to the single-impurity Anderson model (SIAM), the latter being a prototype system for studying strong correlations. For both systems, we demonstrate a hierarchical diagrammatic structure. While the hierarchy closes at the second tier for the RLM, this is not the case for the interacting SIAM. Upon inspection of the current kernel, known results from various perturbative and nonperturbative approximation schemes to quantum transport in the SIAM are recovered. Finally, a noncrossing approximation for the hierarchical kernel is developed, which enables us to systematically decrease temperature at each next level of the approximation. Analytical results for a simplified fourth-tier scheme are presented both in equilibrium and nonequilibrium and with an applied magnetic field.

Collective flow at SIS energies within a hadronic transport approach: Influence of light nuclei formation and equation of state

Collective flow observables are known to be a sensitive tool to gain insights on the equation of state of nuclear matter from heavy-ion collision observations. Towards more quantitative constraints one has to carefully assess other influences on the collective behavior. In this work a hadronic transport approach smash (simulating many accelerated strongly interacting hadrons) is applied to study the first four anisotropic flow coefficients in $\text{Au}+\text{Au}$ collisions at ${E}_{\mathrm{lab}}=1.23A$ GeV in the context of the recently measured data by the HADES Collaboration. In particular, the formation of light nuclei is important in this energy regime. Two different approaches are contrasted to each other: A clustering algorithm inspired by coalescence as well as microscopic formation of deuterons via explicit cross sections. The sensitivity of directed and elliptic flow observables to the strength of the Skyrme mean field is explored. In addition, it is demonstrated that the rapidity-odd ${v}_{3}$ coefficient is practically zero in this energy regime and the ratio of ${v}_{4}/{v}_{2}^{2}$ is close to the value of 0.5 expected from hydrodynamic behavior. This study establishes the current understanding of collective behavior within the smash approach and lays the ground for future more quantitative constraints on the equation of state of nuclear matter within improved mean-field calculations.