arXiv subjects

Qun Wang

Publications and source records attributed to Qun Wang.

At least 73 records · Page 4Linked to original sources

arXiv2023

Optimal distributions for randomized unbiased estimators with an infinite horizon and an adaptive algorithm

The randomized unbiased estimators of Rhee and Glynn (Operations Research:63(5), 1026-1043, 2015) can be highly efficient at approximating expectations of path functionals associated with stochastic differential equations (SDEs). However, there is a lack of algorithms for calculating the optimal distributions with an infinite horizon. In this article, based on the method of Cui et.al. (Operations Research Letters: 477-484, 2021), we prove that, under mild assumptions, there is a simple representation of the optimal distributions. Then, we develop an adaptive algorithm to compute the optimal distributions with an infinite horizon, which requires only a small amount of computational time in prior estimation. Finally, we provide numerical results to illustrate the efficiency of our adaptive algorithm.

math.ST↗

arXiv2023

Electric and magnetic conductivities in magnetized fermion systems

In Wigner function approach with relaxation time approximation, we calculate electric and magnetic conductivities of a fermion system in the strong magnetic field. The linear response has been calculated to the perturbation of electromagnetic fields on the background constant magnetic field. The Wigner function is separated into an equilibrium part in the background magnetic field and an off-equilibrium part induced by perturbative fields. The analytical expression for the equilibrium part and the corresponding equilibrium conditions are given. For the off-equilibrium part, we obtain the kinetic equation at the leading order in $\hbar$ from the master equation of the Wigner function. When perturbative fields only depend on the proper time, the off-equilibrium part can be analytically solved from which the vector and axial vector currents are obtained. We obtain the longitudinal and transverse Ohm conductivities as well as Hall conductivity as the linear response of the vector current to the perturbative electric field. The behaviors of these conductivities as functions of the evolving time, relaxation time, particle mass, and strength of the background magnetic field are investigated both analytically and numerically.

nucl-th↗

arXiv2023

Production of Strange and Charm Hadrons in Pb+Pb Collisions at $\sqrt{s_{NN}}=$ 5.02 TeV

Using a quark combination model with the equal-velocity combination approximation, we study the production of hadrons with strangeness and charm flavor quantum numbers in Pb+Pb collisions at $\sqrt{s_{NN}}=$5.02 TeV. We present analytical expressions and numerical results for these hadrons' transverse momentum spectra and yield ratios. Our numerical results agree well with the experimental data available. The features of strange and charm hadron production in the quark--gluon plasma at the early stage of heavy ion collisions are also discussed.

nucl-th↗

arXiv2023

Spin alignment of vector mesons in heavy-ion collisions

Polarized quarks and antiquarks in high-energy heavy-ion collisions can lead to the spin alignment of vector mesons formed by quark coalescence. Using the relativistic spin Boltzmann equation for vector mesons derived from Kadanoff-Baym equations with an effective quark-meson model for strong interaction and quark coalescence model for hadronizaton, we calculate the spin density matrix element $ρ_{00}$ for $ϕ$ mesons and show that anisotropies of local field correlations or fluctuations with respect to the spin quantization direction lead to $ϕ$ meson's spin alignment. We propose that the local correlation or fluctuation of $ϕ$ fields is the dominant mechanism for the observed the $ϕ$ meson's spin alignment and its strength can be extracted from experimental data as functions of collision energies. The calculated transverse momentum dependence of $ρ_{00}$ agrees with STAR's data. We further predict the azimuthal angle dependence of $ρ_{00}$ which can be tested in future experiments.

nucl-th↗

arXiv2023

Minkowski's lost legacy and hadron electromagnetism

We revisit Minkowski's lost legacy on relativistic electromagnetism in order to resolve long-standing puzzles over the charge distribution of relativistic systems like hadrons. Hadrons are unique relativistic electromagnetic systems characterized by their comparable size and Compton wavelength $r_h \sim λ_C$. As such, it was recently realized that the traditional Sachs definition of the charge distribution based on a non-relativistic formula is invalid. We explain that this is the same problem pursued by Lorentz, Einstein and others, on the electromagnetism of a moving body. We show how various charge distributions proposed in hadronic physics naturally emerge as the multipole moment densities in the macroscopic theory of relativistic electromagnetism.

hep-ph↗

arXiv2023

Relativistic spin dynamics for vector mesons

We propose a relativistic theory for spin density matrices of vector mesons based on Kadanoff-Baym equations in the closed-time-path formalism. The theory puts the calculation of spin observables such as the spin density matrix element $ρ_{00}$ for vector mesons on a solid ground. Within the theory we formulate $ρ_{00}$ for $ϕ$ mesons into a factorization form in separation of momentum and space-time variables. We argue that the main contribution to $ρ_{00}$ at lower energies should be from the $ϕ$ fields that can polarize the strange quark and antiquark in the same way as electromagnetic fields. The key observation is that there is correlation inside the $ϕ$ meson wave function between the $ϕ$ field that polarizes the strange quark and that polarizes the strange antiquark. This is reflected by the fact that the contributions to $ρ_{00}$ are all in squares of fields which are nonvanishing even if the fields may strongly fluctuate in space-time. The fluctuation of strong force fields can be extracted from $ρ_{00}$ of quarkonium vector mesons as links to fundamental properties of quantum chromodynamics.

hep-ph↗

arXiv2022

Lorentz-covariant kinetic theory for massive spin-1/2 particles

We construct a matrix-valued spin-dependent distribution function (MVSD) for massive spin-1/2 fermions and study its properties under Lorentz transformations. Such transformations result in a Wigner rotation in spin space and in a nontrivial matrix-valued shift in space-time, which corresponds to the side jump in the massless case. We express the vector and axial-vector components of the Wigner function in terms of the MVSD and show that they transform in a Lorentz-covariant manner. We then construct a manifestly Lorentz-covariant Boltzmann equation which contains a nonlocal collision term encoding spin-orbit coupling. Finally, we obtain the spin-dependent distribution function in local equilibrium by demanding detailed balance.

nucl-th↗

arXiv2022

Gevrey regularity for the formally linearizable billiard of Treschev

Treschev made the remarkable discovery that there exists formal power series describing a billiard with locally linearizable dynamics. We show that if the frequency for the linear dynamics is Diophanine, the Treschev example is $(1+ α)$-Gevrey for some $α> 0$. Our proof is based on an iterative scheme that further clarifies the structure and symmetries underlying the original Treschev construction. Hopefully, Our result sheds a light on the more important question of whether this example is convergent.

math.DS↗

arXiv2022

Photoproduction of $e^{+}e^{-}$ in peripheral isobar collisions

We investigate the photoproduction of di-electrons in peripheral collisions of $_{44}^{96}\textrm{Ru}+_{44}^{96}\textrm{Ru}$ and $_{40}^{96}\textrm{Zr}+_{40}^{96}\textrm{Zr}$ at 200 GeV. With the charge and mass density distributions given by the calculation of the density functional theory, we calculate the spectra of transverse momentum, invariant mass and azimuthal angle for di-electrons at 40-80\% centrality. The ratios of these spectra in Ru+Ru collisions over to Zr+Zr collisions are shown to be smaller than $(44/40)^{4}$ (the ratio of $Z^{4}$ for Ru and Zr) at low transverse momentum. The deviation arises from the different mass and charge density distributions in Ru and Zr. So the photoproduction of di-leptons in isobar collisions may provide a new way to probe the nuclear structure.

hep-ph↗

arXiv2022

Adaptive Leading Cruise Control in Mixed Traffic Considering Human Behavioral Diversity

This paper presents an adaptive leading cruise control strategy for the connected and automated vehicle (CAV) and first considers its impact on the following human-driven vehicle (HDV) with diverse driving characteristics in the unified optimization framework for improved holistic energy efficiency. The car-following behaviors of HDV are statistically calibrated using the Next Generation Simulation dataset. In a typical single-lane car-following scenario where CAVs and HDVs share the road, the longitudinal speed control of CAVs can substantially reduce the energy consumption of the following HDV by avoiding unnecessary acceleration and braking. Moreover, apart from the objectives including car-following safety and traffic efficiency, the energy efficiencies of both CAV and HDV are incorporated into the reward function of reinforcement learning. The specific driving pattern of the following HDV is learned in real-time from historical speed information to predict its acceleration and power consumption in the optimization horizon. A comprehensive simulation is conducted to statistically verify the positive impacts of CAV on the holistic energy efficiency of the mixed traffic flow with uncertain and diverse human driving behaviors. Simulation results indicate that the holistic energy efficiency is improved by 4.38% on average.

eess.SY↗

arXiv2022

Spin Boltzmann equation for non-relativistic spin-1/2 fermions

We derive the spin Boltzmann equations for spin-1/2 fermions in a non-relativistic model with four-fermion contact interaction which conserves spin degrees of freedom. A great advantage of the model is that the spin matrix elements in collision terms can be completely worked out and be put into such a compact form that one can clearly see how spins are coupled in particle scatterings. A semi-classical expansion in the Planck constant has been made and the on-shell part of the spin Boltzmann equation up to the next-to-leading order is derived. At the leading order the equilibrium spin distribution can be obtained from the vanishing of the collision term for the spin density. The spin chemical potential emerges as a natural consequence of spin conservation. The off-shell part of the spin Boltzmann equation is also discussed. The work can be extended to more sophisticated interaction such as nuclear force in order to apply to spin polarization phenomena in heavy-ion collisions at low energies.

nucl-th↗

arXiv2022

A topological realization of spin polarization through vortex formation in collisions of Bose-Einstein condensates

The global spin polarization of hadrons in heavy ion collisions has been measured in STAR (the Solenoidal Tracker At Relativistic heavy ion collider) experiments, which opens up a new window in the study of the hottest, least viscous and most vortical fluid that has ever been produced in the laboratory. We present a different approach to spin polarization from conventional ones: a topological realization of spin polarization through quantum vortex formation in collisions of Bose-Einstein condensates (BEC). This approach is based on the observation that the vortex is a topological excitation in a superfluid in presence of local orbital angular momentum and is an analogue of spin degrees of freedom. The formation processes of vortices and vortex-antivortex pairs are investigated by solving the Gross-Pitaevskii Equation with a large-scale parallel algorithm on Graphics Processing Unit (GPU) to very high precision. In a rotating environment, the primary vortex with winding number one is stable against perturbation, which has a minimal energy and fixed orbital angular momentum (OAM), but the vortices with larger winding numbers are unstable and will decay into primary vortices through a redistribution of the energy and vorticity. The injection of OAM can also be realized in non-central collisions of self-interacting condensates, part of the OAM of the initial state will induce the formation of vortices through concentration of energy and vorticity density around topological defects. Different from a hydrodynamical description, the interference of the wave function plays an important role in the transport of energy and vorticity, reflecting the quantum nature of the vortex formation process. The study of the vortex formation may shed light on the nature of particle spin and spin-orbit couplings in strong interaction matter produced in heavy-ion collisions.

nucl-th↗

arXiv2022

Charge-dependent directed flows in heavy-ion collisions by Boltzmann-Maxwell equations

We have calculated the directed flow $v_{1}$ and charge-dependent directed flow $Δv_{1}$ for pions and protons in Au+Au collisions at $\sqrt{s_{NN}}=200$GeV by solving the coupled Boltzmann-Maxwell equations self-consistently. Our numerical results show that $v_{1}$ for pions and protons are all negative in the positive mid rapidity region and have similar behavior and magnitude. In contrast we find a quite different behavior in $Δv_{1}$ for pions and protons. The difference lies in that $Δv_{1}$ for protons mainly comes from pressure gradients of the medium, while the dominant contribution to $Δv_{1}$ for pions is from electromagnetic fields. Our results indicate that the effect of the electric field will slightly exceed that of the magnetic and lead to a small negative slope of $Δv_{1}$ for pions

hep-ph↗

arXiv2022

Lepton pair photoproduction in peripheral relativistic heavy-ion collisions

We study the lepton pair photoproduction in peripheral heavy-ion collisions based on the formalism in our previous work [Phys. Rev. D 104, 056011 (2021)]. We present the numerical results for the distributions of the transverse momentum, azimuthal angle and invariant mass for $e^{+}e^{-}$ and $μ^{+}μ^{-}$ pairs as functions of the impact parameter and other kinematic variables in Au+Au collisions. Our calculation incorporates the information on the transverse momentum and polarization of photons which is essential to describe the experimental data. We observe a broadening effect in the transverse momentum for lepton pairs with and without smear effects. We also observe a significant enhancement in the distribution of $\cos(2φ)$ for $μ^{+}μ^{-}$ pairs. Our results provide a baseline for future studies of other higher order corrections beyond Born approximation and medium effects in the lepton pair production.

hep-ph↗

arXiv2022

High precision solutions to quantized vortices within Gross-Pitaevskii equation

The dynamics of vortices in Bose-Einstein condensates of dilute cold atoms can be well formulated by Gross-Pitaevskii equation. To better understand the properties of vortices, a systematic method to solve the nonlinear differential equation for the vortex to a very high precision is proposed. Through two-point Pad$\acute{\text{e}}$ approximants, these solutions are presented in terms of simple rational functions, which can be used in the simulation of vortex dynamics. The precision of the solutions is sensitive to the connecting parameter and the truncation orders. It can be improved significantly with a reasonable extension in the order of rational functions. The errors of the solutions and the limitation of two-point Pad$\acute{\text{e}}$ approximants are discussed. This investigation may shed light on the exact solution to the nonlinear vortex equation.

nlin.PS↗

arXiv2022

Triangle-oriented Community Detection considering Node Features and Network Topology

The joint use of node features and network topology to detect communities is called community detection in attributed networks. Most of the existing work along this line has been carried out through objective function optimization and has proposed numerous approaches. However, they tend to focus only on lower-order details, i.e., capture node features and network topology from node and edge views, and purely seek a higher degree of optimization to guarantee the quality of the found communities, which exacerbates unbalanced communities and free-rider effect. To further clarify and reveal the intrinsic nature of networks, we conduct triangle-oriented community detection considering node features and network topology. Specifically, we first introduce a triangle-based quality metric to preserve higher-order details of node features and network topology, and then formulate so-called two-level constraints to encode lower-order details of node features and network topology. Finally, we develop a local search framework based on optimizing our objective function consisting of the proposed quality metric and two-level constraints to achieve both non-overlapping and overlapping community detection in attributed networks. Extensive experiments demonstrate the effectiveness and efficiency of our framework and its potential in alleviating unbalanced communities and free-rider effect.

cs.SI↗

arXiv2022

Foundations and Applications of Quantum Kinetic Theory

Many novel quantum phenomena emerge in non-equilibrium relativistic quantum matter under extreme conditions such as strong magnetic fields and rotations. The quantum kinetic theory based on Wigner functions in quantum field theory provides a powerful and effective microscopic description of these quantum phenomena. In this article we review some of recent advances in the quantum kinetic theory and its applications in describing these quantum phenomena.

hep-ph↗

arXiv2022

What can we learn from global spin alignment of $ϕ$ meson in heavy-ion collisions?

We propose that a significant positive deviation from 1/3 for the spin density matrix element $ρ_{00}$ of the $ϕ$ meson may indicate the existence of a mean field of the $ϕ$ meson generated in heavy-ion collisions. This explains why STAR preliminary data for the $ϕ$ meson's $ρ_{00}$ are much larger than 1/3 while the data of $Λ$ and $\barΛ$ polarization seem not to allow such a significant and positive deviation. The contribution may be from the polarization of the strange quark and antiquark through the $ϕ$ field, an effective mode of the gluon field in strong interaction. We show that $ρ_{00}$ for the $ϕ$ meson is a good analyzer for fields even if they may strongly fluctuate in space-time.

nucl-th↗