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Qun Wang

Publications and source records attributed to Qun Wang.

At least 181 records · Page 10Linked to original sources
arXiv2011

How Perfect a Gluon Plasma Can Be in Perturbative QCD?

The shear viscosity to entropy density ratio, η/s, characterizes how perfect a fluid is. We calculate the leading order η/s of a gluon plasma in perturbation using the kinetic theory. The leading order contribution only involves the elastic gg -> gg (22) process and the inelastic gg<->ggg (23) process. The Hard-Thermal-Loop (HTL) treatment is used for the 22 matrix element, while the exact matrix element in vacuum is supplemented by the gluon Debye mass insertion for the 23 process. Also, the asymptotic mass is used for the external gluons in the kinetic theory. The errors from not implementing HTL and the Landau-Pomeranchuk-Migdal effect in the 23 process, and from the uncalculated higher order corrections, are estimated. Our result for η/s lies between that of Arnold, Moore and Yaffe (AMY) and Xu and Greiner (XG). Our result shows that although the finite angle contributions are important at intermediate α_s (α_s \sim 0.01-0.1), the 22 process is still more important than 23 when α_s < 0.1. This is in qualitative agreement with AMY's result. We find no indication that the proposed perfect fluid limit η/s \simeq 1/(4π) can be achieved by perturbative QCD alone.

hep-ph↗
arXiv2011

A consistent description of kinetic equation with triangle anomaly

We provide a consistent description of the kinetic equation with triangle anomaly which is compatible with the entropy principle of the second law of thermodynamics and the charge/energy-momentum conservation equations. In general an anomalous source term is necessary to ensure that the equations for the charge and energy-momentum conservation are satisfied and that the correction terms of distribution functions are compatible to these equations. The constraining equations from the entropy principle are derived for the anomaly-induced leading order corrections to the particle distribution functions. The correction terms can be determined for minimum number of unknown coefficients in one charge and two charge cases by solving the constraining equations.

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arXiv2010

Spin-one color superconductors: collective modes and effective Lagrangian

We investigate the collective excitations in spin-one color superconductors. We classify the Nambu--Goldstone modes by the pattern of spontaneous symmetry breaking, and then use the Ginzburg--Landau theory to derive their dispersion relations. These soft modes play an important role for the low-energy dynamics of the system such as the transport phenomena and hence are relevant for late-stage evolution of neutron stars. In the case of the color-spin-locking phase, we use a functional technique to obtain the low-energy effective action for the physical Nambu--Goldstone bosons that survive after gauging the color symmetry.

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arXiv2010

The entropy puzzle and the quark combination model

We use two available methods, the Duhem-Gibbs relation and the entropy formula in terms of particle phase space distributions, to calculate the entropy in a quark combination model. The entropy of the system extracted from the Duhem-Gibbs relation is found to increase in hadronization if the average temperature of the hadronic phase is lower than that of the quark phase. The increase of the entropy can also be confirmed from the entropy formula if the volume of the hadronic phase is larger than 2.5-3.0 times that of the quark phase. So whether the entropy increases or decreases during combination depends on the temperature before and after combination and on how much expansion the system undergoes during combination. The current study provides an example to shed light on the entropy issue in the quark combination model.

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arXiv2009

Some aspects of color superconductivity: an introduction

A pedagogical introduction to color superconductivity in the weak coupling limit is given. The focus is on the basic tools of thermal field theory necessary to compute observables of color superconductivity. The rich symmetry structure and symmetry breaking patterns are analyzed on the basis of the Anderson-Higgs mechanism. Some techniques can also be applied for computing neutrino processes in compact stars. As an example, we show how to obtain the neutrino emissivity for Urca processes in neutron stars by computing the polarization tensor of the W-boson. We also illustrate how a spin-1 color superconducting phase generates an anisotropic neutrino emissions in compact stars.

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arXiv2009

Shear Viscosity of a Gluon Plasma in Perturbative QCD

We calculate the shear viscosity $η$ to entropy density $s$ ratio $η/s$ of a gluon plasma in kinetic theory including the gg->gg and gg<->ggg processes. Due to the suppressed contribution to $η$ in the gg->gg forward scattering, it is known that the gluon bremsstrahlung gg<->ggg process also contributes at the same order $O(α_{s}^{-2})$ in perturbative QCD. Using the Gunion-Bertsch formula for the gg<->ggg matrix element which is valid for the limit of soft bremsstrahlung, we find that the result is sensitive to whether the same limit is taken for the phase space. Using the exact phase space, the gg<->ggg contribution becomes more important to $η$ than gg->gg for $α_{s}\gtrsim 2\times 10^{-3}$. Therefore, at $α_{s}=0.1$, $η/s\simeq 1.0$, between 2.7 obtained by Arnold, Moore and Yaffe (AMY) and 0.5 obtained by Xu and Greiner. If the soft bremsstrahlung limit is imposed on the phase space such that the recoil effect from the bremsstrahlung gluon is neglected, then the correction from the gg<->ggg process is about 10-30% of the total which is close to AMY's prediction. This shows that the soft bremsstrahlung approximation is not as good as previously expected.

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arXiv2009

Mini-jet thermalization and diffusion of transverse momentum correlation in high-energy heavy-ion collisions

Transverse momentum correlation in azimuthal angle of produced hadrons due to mini-jets are studied first within the HIJING Monte Carlo model in high-energy heavy-ion collisions. Jet quenching in the early stage of thermalization is shown to lead to significant diffusion (broadening) of the correlation. Evolution of the transverse momentum density fluctuation that gives rise to such correlation in azimuthal angle in the later stage of heavy-ion collisions is further investigated within a linearized diffusion-like equation and is shown to be determined by the shear viscosity of the evolving dense matter. Such a diffusion equation for the transverse momentum fluctuation is solved with initial values given by HIJING and together with the hydrodynamic equation for the bulk medium. The final transverse momentum correlation in azimuthal angle is calculated along the freeze-out hyper-surface and is found further diffused for larger values of shear viscosity to entropy density ratio $η/s \sim 0.2-0.4$. Therefore the final transverse momentum correlation in azimuthal angle can be used to study the thermalization of mini-jets in the early stage of heavy-ion collisions and the viscous effect in the hydrodynamic evolution of the strongly coupled quark gluon plasma.

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arXiv2009

Neutrino Emission From Inhomogeneous Pion Condensed Quark Matter

It is believed that quark matter can exist in neutron star interior if the baryon density is high enough. When there is a large isospin density, quark matter could be in a pion condensed phase. We compute neutrino emission from direct Urca processes in such a phase, particularly in the inhomogeneous Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states. The neutrino emissivity and specific heat are obtained, from which the cooling rate is estimated.

hep-ph↗
arXiv2009

Symmetry breaking patterns and collective modes of spin-one color superconductors

Spin-one color superconductor is a viable candidate phase of dense matter in the interiors of compact stars. Its low-energy excitations will influence the transport properties of such matter and thus have impact on late-stage evolution of neutron stars. It also provides a good example of spontaneous symmetry breaking with rich breaking patterns. In this contribution, we reanalyze the phase diagram of a spin-one color superconductor and point out that a part of it is occupied by noninert states, which have been neglected in literature so far. We classify the collective Nambu--Goldstone modes, which are essential to the transport phenomena.

hep-ph↗
arXiv2009

A new approach to bulk viscosity in strange quark matter at high densities

A new method is proposed to compute the bulk viscosity in strange quark matter at high densities. Using the method it is straightforward to prove that the bulk viscosity is positive definite, which is not so easy to accomplish in other approaches especially for multi-component fluids like strange quark matter with light up and down quarks and massive strange quarks.

astro-ph↗
arXiv2008

Shear viscosity in late time of hydrodynamic evolution in AdS/CFT duality

We investigate the shear viscosity $η$ and the entropy density $s$ of strongly coupled $\mathcal{N}=4$ super Yang-Mills (SYM) plasma in late time of hydrodynamic evolution with AdS/CFT duality and Bjorken scaling. We use correlation function method proposed by Kovtun, Son and Starinets. We obtain the metric $g_{μν}$ in a proper time dependent $AdS_{5}$ space through holographic renormalization, whose boundary condition is given by energy-momentum tensor of the plasma in 2+1 dimension with transverse expansion or radial flow. With the metric we compute $η$ and $s$ of fluids in 1+1 and 2+1 dimension without and with radial flow. We find the ratio $η/s=1/(4π)$ in 1+1 dimension consistent with the Kovtun-Son-Starinets bound if next-to-leading terms in proper time are included in the equation of motion for metric perturbations. For 2+1 dimension the result is unchanged in the leading order of transverse rapidity.

hep-ph↗
arXiv2008

BCS-BEC crossover in a relativistic boson-fermion model beyond mean field approximation

We investigate the fluctuation effect of the di-fermion field in the crossover from Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) in a relativistic superfluid. We work within the boson-fermion model obeying a global U(1) symmetry. To go beyond the mean field approximation we use Cornwall-Jackiw-Tomboulis (CJT) formalism to include higher order contributions. The quantum fluctuations of the pairing condensate is provided by bosons in non-zero modes, whose interaction with fermions gives the two-particle-irreducible (2PI) effective potential. It changes the crossover property in the BEC regime. With the fluctuations the superfluid phase transition becomes the first order in grand canonical ensemble. We calculate the condensate, the critical temperature $T_{c}$ and particle abundances as functions of crossover parameter the boson mass.

hep-ph↗
arXiv2007

Neutrino Emission From Direct Urca Processes in Pion Condensed Quark Matter

We study neutrino emission from direct Urca processes in pion condensed quark matter. In compact stars with high baryon density, the emission is dominated by the gapless modes of the pion condensation which leads to an enhanced emissivity. While for massless quarks the enhancement is not remarkable, the emissivity is significantly larger and the cooling of the condensed matter is considerably faster than that in normal quark matter when the mass difference between $u$- and $d$-quarks is sizable.

hep-ph↗
arXiv2007

Global quark polarization in non-central $A+A$ collisions

Partons produced in the early stage of non-central heavy-ion collisions can develop a longitudinal fluid shear because of unequal local number densities of participant target and projectile nucleons. Under such fluid shear, local parton pairs with non-vanishing impact parameter have finite local relative orbital angular momentum along the direction opposite to the reaction plane. Such finite relative orbital angular momentum among locally interacting quark pairs can lead to global quark polarization along the same direction due to spin-orbital coupling. Local longitudinal fluid shear is estimated within both Landau fireball and Bjorken scaling model of initial parton production. Quark polarization through quark-quark scatterings with the exchange of a thermal gluon is calculated beyond small-angle scattering approximation in a quark-gluon plasma. The polarization is shown to have a non-monotonic dependence on the local relative orbital angular momentum dictated by the interplay between electric and magnetic interaction. It peaks at a value of relative orbital angular momentum which scales with the magnetic mass of the exchanged gluons. With the estimated small longitudinal fluid shear in semi-peripheral $Au+Au$ collisions at the RHIC energy, the final quark polarization is found to be small $|P_q|<0.04$ in the weak coupling limit. Possible behavior of the quark polarization in the strong coupling limit and implications on the experimental detection of such global quark polarization at RHIC and LHC are also discussed.

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arXiv2007

Relativistic BCS-BEC crossover in a boson-fermion model

We investigate the crossover from Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) in a relativistic superfluid within a boson-fermion model. The model includes, besides the fermions, separate bosonic degrees of freedom, accounting for the bosonic nature of the Cooper pairs. The crossover is realized by tuning the difference between the boson mass and boson chemical potential as a free parameter. The model yields populations of condensed and uncondensed bosons as well as gapped and ungapped fermions throughout the crossover region for arbitrary temperatures. Moreover, we observe the appearance of antiparticles for sufficiently large values of the crossover parameter. As an application, we study pairing of fermions with imbalanced populations. The model can potentially be applied to color superconductivity in dense quark matter at strong couplings.

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arXiv2007

Baryon number conservation and enforced electric charge neutrality for bulk viscosity in quark matter

General constraints on fluid velocity divergences for particles in quark matter are derived from baryon number conservation and enforced electric charge neutrality. A new oscillation pattern in three-flavor normal quark matter satisfying these conditions is found and its bulk viscosity is calculated. The result may have astrophysical implication for maximum rotation frequencies of compact stars.

astro-ph↗