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Qun Wang

Publications and source records attributed to Qun Wang.

At least 127 records · Page 7Linked to original sources
arXiv2019

Kinetic theory for massive spin-1/2 particles from the Wigner-function formalism

We calculate the Wigner function for massive spin-1/2 particles in an inhomogeneous electromagnetic field to leading order in the Planck constant $\hbar$. Going beyond leading order in $\hbar$ we then derive a generalized Boltzmann equation in which the force exerted by an inhomogeneous electromagnetic field on the particle dipole moment arises naturally. Carefully taking the massless limit we find agreement with previous results. The case of global equilibrium with rotation is also studied. Finally, we outline the derivation of fluid-dynamical equations from the components of the Wigner function. The conservation of total angular momentum is promoted as an additional fluid-dynamical equation of motion. Our framework can be used to study polarization effects induced by vorticity and magnetic field in relativistic heavy-ion collisions.

hep-ph↗
arXiv2019

The chiral vortical effect in Wigner function approach

It is more subtle to obtain the chiral vortical effect (CVE) than chiral magnetic effect (CME) in quantum transport approach. To investigate the subtlty of the CVE we present two different derivation in the Wigner function approach. The first one is based on the method in our previous work \cite{Gao:2012ix} in which the CVE was derived under static-equilibrium conditions without details. We provide a detailed derivation using a more transparent and powerful method, which can be easily generalized to higher order calculation. In this derivation of the CVE current, there is an explicit Lorentz covariance. The second derivation is based on a more general chiral kinetic theory in a semi-classical expansion of the Wigner function without assuming static-equilibrium conditions \cite{Gao:2018wmr}. In this derivation, there is a freedom to choose a reference frame for the CVE current, so the explicit Lorentz covariance seems to be lost. Howerver, under static-equilibrium conditions, we show that the CVE current in this derivation can be decomposed into two parts, identified as the normal and magnetization current. Each part depends on the reference frame, but the sum of two parts does give the total CVE current which is independent of the reference frame. In the comoving frame of the fluid, it can be shown that the normal and magnetization current give one-third and two-thirds of the total CVE current respectively. This gives a natural solution to the 'one-third' puzzle in the CVE current in three-dimensional version of the chiral kinetic theory in the literature.

nucl-th↗
arXiv2019

A microscopic description for polarization in particle scatterings

We propose a microscopic description for the polarization from the first principle through the spin-orbit coupling in particle collisions. The model is different from previous ones based on local equilibrium assumptions for the spin degree of freedom. It is based on scatterings of particles as wave packets, an effective method to deal with particle scatterings at specified impact parameters. The polarization is then the consequence of particle collisions in a non-equilibrium state of spins. The spin-vorticity coupling naturally emerges from the spin-orbit one encoded in polarized scattering amplitudes of collisional integrals when one assumes local equilibrium in momentum but not in spin.

nucl-th↗
arXiv2019

Anomalous magnetohydrodynamics with longitudinal boost invariance and chiral magnetic effect

We study relativistic magnetohydrodynamics with longitudinal boost invariance in the presence of chiral magnetic effects and finite electric conductivity. With initial magnetic fields parallel or anti-parallel to electric fields, we derive the analytic solutions of electromagnetic fields and the chiral number and energy density in an expansion of several parameters determined by initial conditions. The numerical solutions show that such analytic solutions work well in weak fields or large chiral fluctuations. We also discuss the properties of electromagnetic fields in the laboratory frame.

hep-ph↗
arXiv2019

Wigner function and pair production in parallel electric and magnetic fields

We derive analytical formulas for the equal-time Wigner function in an electromagnetic field of arbitrary strength. While the magnetic field is assumed to be constant, the electric field is assumed to be space-independent and oriented parallel to the magnetic field. The Wigner function is first decomposed in terms of the so-called Dirac-Heisenberg-Wigner (DHW) functions and then the transverse-momentum dependence is separated using a new set of basis functions which depend on the quantum number $n$ of the Landau levels. Equations for the coefficients are derived and then solved for the case of a constant electric field. The pair-production rate for each Landau level is calculated. In the case of finite temperature and chemical potential, the pair-production rate is suppressed by Pauli's exclusion principle.

hep-ph↗
arXiv2018

Choreographic Holomorphic Spheres with Application to Hamiltonian Systems of $N$-Vortex Type

We study the existence of (relative) simple choreographies for a class of Hamiltonian systems describing the interaction of particles in the plane motivated mainly by the n-vortex type problem. In particular, by constructing choreographic pseudo-holomorphic spheres, we prove that there exist infinitely many non-trivial relative choreographies for the identical n-vortex problem arising from both the Euler equation and the Gross-Pitaevskii equation.

math.DS↗
arXiv2018

Alternative methods to measure global polarization of $Λ$ hyperons

We propose alternative methods to measure the global polarization of $Λ$ hyperons. These methods involve event averages of proton's and $Λ$'s momenta in the lab frame. We carry out simulations using these methods and show that all of them work equivalently well in obtaining the global polarization of $Λ$ hyperons.

nucl-th↗
arXiv2018

Non-resistive dissipative magnetohydrodynamics from the Boltzmann equation in the 14-moment approximation

We derive the equations of motion of relativistic, non-resistive, second-order dissipative magnetohydrodynamics from the Boltzmann equation using the method of moments. We assume the fluid to be composed of a single type of point-like particles with vanishing dipole moment or spin, so that the fluid has vanishing magnetization and polarization. In a first approximation, we assume the fluid to be non-resistive, which allows to express the electric field in terms of the magnetic field. We derive equations of motion for the irreducible moments of the deviation of the single-particle distribution function from local thermodynamical equilibrium. We analyze the Navier-Stokes limit of these equations, reproducing previous results for the structure of the first-order transport coefficients. Finally, we truncate the system of equations for the irreducible moments using the 14-moment approximation, deriving the equations of motion of relativistic, non-resistive, second-order dissipative magnetohydrodynamics. We also give expressions for the new transport coefficients appearing due to the coupling of the magnetic field to the dissipative quantities.

nucl-th↗
arXiv2018

Probing vorticity structure in heavy-ion collisions by local $Λ$ polarization

We study the local structure of the vorticity field and the $Λ$ polarization in Au+Au collisions in the energy range $\sqrt{s_{\mathrm{NN}}}=7.7$--$200$ GeV and Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2760$ GeV using A Multi-Phase Transport (AMPT) model. We focus on the vorticity field arising from the non-uniform expansion of the fireball, which gives the circular structure of the transverse vorticity $\boldsymbolω_{\perp}=(ω_{x},ω_{y})$ around the $z$ direction as well as the quadrupole pattern of the longitudinal vorticity $ω_{z}$ in the transverse plane. As a consequence, the three components of the polarization vector $\mathbf{P}=(P_{x},P_{y},P_{z})$ for $Λ$ hyperons show harmonic behaviors as $\mathrm{sgn}(Y)\sinϕ_{p}$, $-\mathrm{sgn}(Y)\cosϕ_{p}$, and $-\sin(2ϕ_{p})$, where $ϕ_{p}$ and $Y$ are the azimuthal angle and rapidity in momentum space. These patterns of the local $Λ$ polarization are expected to be tested in future experiments.

nucl-th↗
arXiv2018

Disentangling covariant Wigner functions for chiral fermions

We develop a general formalism for the quantum kinetics of chiral fermions in a background electromagnetic field based on a semiclassical expansion of covariant Wigner functions in the Planck constant $\hbar$. We demonstrate to any order of $\hbar$ that only the time-component of the Wigner function is independent while other components are explicit derivative. We further demonstrate to any order of $\hbar$ that a system of quantum kinetic equations for multiple-components of Wigner functions can be reduced to one chiral kinetic equation involving only the single-component distribution function. These are remarkable properties of the quantum kinetics of chiral fermions and will significantly simplify the description and simulation of chiral effects in heavy ion collisions and Dirac/Weyl semimetals. We present the unintegrated chiral kinetic equations in four-momenta up to $O(\hbar ^2)$ and the integrated ones in three-momenta up to $O(\hbar)$. We find that some singular terms emerge in the integration over the time component of the four-momentum, which result in a new source term contributing to the chiral anomaly, in contrast to the well-known scenario of the Berry phase term. Finally we rewrite our results in any Lorentz frame with a reference four-velocity and show how the non-trivial transformation of the distribution function in different frames emerges in a natural way.

hep-ph↗
arXiv2018

Anharmonicity-Induced Criticality of Collective Excitation in a Trapped Bose-Einstein Condensate

We investigate the low energy excitations of a dilute atomic Bose gas confined in a anharmonic trap interacting with repulsive forces. The dispersion law of both surface and compression modes are derived and analyzed for large numbers of atoms in the trap, which show two branches of excitation and appear a two critical value. For a upper limit, BEC can be unstable with respect to some specific collective excitation, while for the lower limit, the frequency of collective excitation under anharmonic influence can be effectively lower than that without anharmonicity. Our work reveals the key role played by the anharmonicity and interatomic forces which introduce a rich structure in the dynamic behavior of these new many-body systems.

cond-mat.quant-gas↗
arXiv2018

Off-equilibrium infrared structure of self-interacting scalar fields: Universal scaling, Vortex-antivortex superfluid dynamics and Bose-Einstein condensation

We map the infrared dynamics of a relativistic single component ($N=1$) interacting scalar field theory to that of nonrelativistic complex scalar fields. The Gross-Pitaevskii (GP) equation, describing the real time dynamics of single component ultracold Bose gases, is obtained at first nontrivial order in an expansion proportional to the powers of $λϕ^2/m^2$ where $λ$, $ϕ$ and $m$ are the coupling constant, the scalar field and the particle mass respectively. Our analytical studies are corroborated by numerical simulations of the spatial and momentum structure of overoccupied scalar fields in (2+1)-dimensions. Universal scaling of infrared modes, vortex-antivortex superfluid dynamics and the off-equilibrium formation of a Bose-Einstein condensate are observed. Our results for the universal scaling exponents are in agreement with those extracted in the numerical simulations of the GP equation. As in these simulations, we observe coarsening phase kinetics in the Bose superfluid with strongly anomalous scaling exponents relative to that of vertex resummed kinetic theory. Our relativistic field theory framework further allows one to study more closely the coupling between superfluid and normal fluid modes, specifically the turbulent momentum and spatial structure of the coupling between a quasi-particle cascade to the infrared and an energy cascade to the ultraviolet. We outline possible applications of the formalism to the dynamics of vortex-antivortex formation and to the off-equilibrium dynamics of the strongly interacting matter formed in heavy-ion collisions.

hep-th↗
arXiv2018

Relative Periodic Solutions Of The N-Vortex Problem Via The Variational Method

This article studies the N-vortex problem in the plane with positive vorticities. After an investigation of some properties for normalised relative equilibria of the system, we use symplectic capacity theory to show that, there exist infinitely many normalised relative periodic orbits on a dense subset of all energy levels, which are neither fixed points nor relative equilibria.

math.DS↗
arXiv2018

A quark coalescence model for polarized vector mesons and baryons

A non-relativistic quark coalescence model is formulated for polarized vector mesons and baryons of spin-1/2 octet and spin-3/2 decuplet. With the spin density matrix, one can compute in a uniform way the polarizations of vector mesons and baryons from those of quarks and antiquarks with explicit momentum dependence. The results are compared to that obtained from kinetic and statistical models for hadrons.

nucl-th↗
arXiv2017

Some aspects of global Lambda polarization in heavy-ion collisions

Large orbital angular momentum can be generated in non-central heavy-ion collisions, and part of it is expected to be converted into final particle's polarization due to the spin-orbit coupling. Within the framework of A Multi-Phase Transport (AMPT) model, we studied the vorticity-induced polarization of $Λ$ hyperons at the midrapidity region $|η|<1$ in Au-Au collisions at energies $\sqrt{s_{NN}}=7.7\sim200$ GeV. Our results show that the global polarization decreases with the collisional energies and is consistent with the recent STAR measurements. This behavior can be understood by less asymmetry of participant matter in the midrapidity region due to faster expansion of fireball at higher energies. As another evidence, we discuss how much the angular momentum is deposited in different rapidity region. The result supports our asymmetry argument.

nucl-th↗
arXiv2017

Global $Λ$ polarization in heavy-ion collisions from a transport model

The polarizations of $Λ$ and $\barΛ$ hyperons are important quantities in extracting the fluid vorticity of the strongly coupled quark gluon plasma and the magnitude of the magnetic field created in off-central heavy-ion collisions, through the spin-vorticity and spin-magnetic coupling. We computed the energy dependence of the global $Λ$ polarization in off-central Au+Au collisions in the energy range $\sqrt{s_{NN}}=7.7-200$ GeV using a multiphase transport model. The observed polarizations with two different impact parameters agree quantitatively with recent STAR measurements. The energy dependence of the global $Λ$ polarization is decomposed as energy dependence of the $Λ$ distribution at hadronization and the space-time distribution of the fluid-vorticity field. The visualization of both the $Λ$ distribution and the fluid-vorticity field show a smaller tilt at higher collisional energies, which indicates that the smaller global polarization at higher collisional energies is caused by a smaller angular momentum deposition at midrapidity.

nucl-th↗
arXiv2017

Wigner functions of massive fermions in strong magnetic fields

We compute the covariant Wigner function for spin-1/2 fermions in an arbitrarily strong magnetic field by exactly solving the Dirac equation at non-zero fermion-number and chiral-charge densities. The Landau energy levels as well as a set of orthonormal eigenfunctions are found as solutions of the Dirac equation. With these orthonormal eigenfunctions we construct the fermion field operators and the corresponding Wigner-function operator. The Wigner function is obtained by taking the ensemble average of the Wigner-function operator in global thermodynamical equilibrium, i.e., at constant temperature $T$ and non-zero fermion-number and chiral-charge chemical potentials $μ$ and $μ_5$, respectively. Extracting the vector and axial-vector components of the Wigner function, we reproduce the currents of the chiral magnetic and separation effect in an arbitrarily strong magnetic field.

hep-ph↗