arXiv subjects
Publications and source records attributed to Qun Wang.
The history and recent progresses in the study of bulk viscosity in nuclear and quark matter are reviewed. The constraints from baryon number conservation and electric neutrality in quark matter on particle densities and fluid velocity divergences are discussed.
Several relevant issues in computing neutrino emissivity in Urca processes in color superconducting quark matter are addressed. These include: (1) The constraint on $u$ quark abundance is given from electric neutrality and the triangle relation among Fermi momenta for participants. (2) The phase space defined by Fermi momentum reduction of quarks is discussed in QCD and NJL model. (3) Fermi effective model of weak interaction is reviewed with special focus on its form in Nambu-Gorkov basis.
We study the phase space for neutrino emissions with massive quarks in direct Urca processes in normal and color superconducting quark matter. We derive in QCD and the NJL model the Fermi momentum reduction resulting from Fermi liquid properties which opens up the phase space for neutrino emissions. The relation between the Fermi momentum and chemical potential is found to be $p_{F}\approxμ(1-κ)$ with $κ$ depending on coupling constants. We find in the weak coupling regime that $κ$ is a monotonously increasing function of the chemical potential. This implies quenched phase space for neutrino emissions at low baryon densities. We calculate neutrino emissivities with massive quarks in a spin-one color superconductor. The quark mass corrections are found to be of the same order as the contributions in the massless case, which will bring sizable effects on the cooling behavior of compact stars.
Neutrino emissivities due to direct Urca processes of several spin-one color-superconducting phases of dense quark matter are calculated. In particular, the role of anisotropies and nodes of the gap functions is analyzed. Results for the specific heat as well as for the cooling rates of the color-spin-locked, planar, polar, and {\it A} phases are presented and consequences for the physics of neutron stars are briefly discussed. Furthermore, it is shown that the {\em A} phase exhibits a helicity order, giving rise to a reflection asymmetry in the neutrino emissivity.
We propose a new neutrino propulsion mechanism for neutron stars which can lead to strong velocity kicks, needed to explain the observed bimodal velocity distribution of pulsars. The spatial asymmetry in the neutrino emission is naturally provided by a stellar core containing spin-1 color-superconducting quark matter in the A phase. The neutrino propulsion mechanism switches on when the stellar core temperature drops below the transition temperature of this phase.
The hadron production in relativistic heavy ion collisions is well described by the quark combination model. The mixed ratios for various hadrons and the transverse momentum spectra for long-life hadrons are predicted and agree with recent RHIC data. The production rates for the pentaquarks $Θ^+$, $Θ^{*++}$ and the di-baryon $(ΩΩ)_{0^{+}}$ are estimated, neglecting the effect from the transition amplitude for constituent quarks to form an exotic state.
One of the non-Abelian features of parton energy loss is the ratio $ΔE_g/ΔE_q=9/4$ between gluon and quark jets. Since jet production rate is dominated by quark jets at high $x_T=2p_T/\sqrt{s}$ and by gluon jets at low $x_T$, high $p_T$ hadron suppression in high-energy heavy-ion collisions should reflect such a non-Abelian feature. Within a leading order perturbative QCD parton model that incorporates transverse expansion and Woods-Saxon nuclear distribution, the energy dependence of large $p_T\sim 5-20$ GeV/$c$ hadron suppression is found to be sensitive to the non-Abelian feasture of parton energy loss and could be tested by data from low energy runs at RHIC or data from LHC.
We derive a general effective action for quark matter at nonzero temperature and/or nonzero density. For this purpose, we distinguish irrelevant from relevant quark modes, as well as hard from soft gluon modes by introducing two separate cut-offs in momentum space, one for quarks, $Λ_q$, and one for gluons, $Λ_g$. We exactly integrate out irrelevant quark modes and hard gluon modes in the functional integral representation of the QCD partition function. Depending on the specific choice for $Λ_q$ and $Λ_g$, the resulting effective action contains well-known effective actions for hot and/or dense quark matter, for instance the ``Hard Thermal Loop'' or the ``Hard Dense Loop'' action, as well as the high-density effective theory proposed by Hong and others. We then apply our effective action to review the calculation of the color-superconducting gap parameter to subleading order in weak coupling, where the strong coupling constant $g \ll 1$. In this situation, relevant quark modes are those within a layer of thickness $2 Λ_q$ around the Fermi surface. The non-perturbative nature of the gap equation invalidates naive attempts to estimate the importance of the various contributions via power counting on the level of the effective action. Nevertheless, once the gap equation has been derived within a particular many-body approximation scheme, the cut-offs $Λ_q, Λ_g$ provide the means to rigorously power count different contributions to the gap equation. We recover the previous result for the QCD gap parameter for the choice $Λ_q \alt g μ\ll Λ_g \alt μ$, where $μ$ is the quark chemical potential. We also point out how to improve this result beyond subleading order in weak coupling.
Recent developments in weak-coupling color superconductivity are reviewed. These developments are as follows. The mean field gap equation is solved for most common superconducting phases up to subleading order; BCS relation is found to be violated in color-flavor-locking and color-spin-locking phases due to the two-gaps structure of the order parameter; The Debye and Meissner masses of gluons and photon with their rotated partners are calculated for these phases; We found that there is no electromagnetic Meissner effect in spin-one color superconductor; A proof of gauge parameter independence at subleading order is given in covariant gauge.
We derive a generalized Ward identity for color-superconducting quark matter via the functional integral approach. The identity implies the gauge independence of the color-superconducting gap parameter on the quasi-particle mass shell to subleading order in covariant gauge.
It is shown that color-superconducting quark matter, where quarks of the same flavor form Cooper pairs with spin one, exhibits an electromagnetic Meissner effect. This is in contrast to spin-zero color superconductors where Cooper pairs consist of quarks with different flavors.
We replace in the event generator JETSET the color singlet chain connection with the color separate state one as the interface between the hard and soft sectors of hadronic processes. The modified generator is applied to produce the hadronic events in $e^+ e^-$ annihilation. It describes the experimental data at the same level as the original JETSET with default parameters. This should be understood as a demonstration that color singlet chain is not the unique color connection. We also search for the difference in special sets of three-jet events arising from different color connections, which could subject to further experimental test.
We study color superconductivity with $N_f=1,2,$ and 3 massless flavors of quarks. We present a general formalism to derive and solve the gap equations for condensation in the even-parity channel. This formalism shows that the leading-order contribution to the gap equation is unique for all color superconductors studied here, and that differences arise solely at the subleading order. We discuss a simple method to compute subleading contributions from the integration over gluon momenta in the gap equation. Subleading contributions enter the prefactor of the color-superconducting gap parameter. In the case of color-flavor and color-spin locking we identify further corrections to this prefactor arising from the two-gap structure of the quasiparticle excitations. Computing the transition temperature, $T_c$, where the color-superconducting condensate melts, we find that these contributions lead to deviations from the BCS behavior $T_c\simeq 0.57 ϕ_0$, where $ϕ_0$ is the magnitude of the zero-temperature gap at the Fermi surface.
Hadronic events in $e^+ e^- \to Z^0$ are generated by a Monte-Carlo model including the production of colour separate states. These events are compared with those produced by the JETSET with default set of parameters where the colour connections are colour singlet chains. By selecting two-jet-like events and a sensitive observable, we find that these two kinds of colour connections lead to significant differences in the hadronic states.
We derive the kinetic equation for pure gluon QCD plasma in a general way, applying the background field method. We show that the quantum kinetic equation contains a term as in the classical case, that describes a color charge precession of partons moving in the gauge field. We emphasize that this new term is necessary for the gauge covariance of the resulting equation.
We consider color superconductivity with two flavors of massless quarks which form Cooper pairs with total spin zero. We solve the gap equation for the color-superconducting gap parameter to subleading order in the QCD coupling constant $g$ at zero temperature. At this order in $g$, there is also a previously neglected contribution from the real part of the quark self-energy to the gap equation. Including this contribution leads to a reduction of the color-superconducting gap parameter $\f_0$ by a factor $b_0'=\exp \big[ -(\p ^2+4)/8 \big]\simeq 0.177$. On the other hand, the BCS relation $T_c\simeq 0.57\f_0$ between $\f_0$ and the transition temperature $T_c$ is shown to remain valid after taking into account corrections from the quark self-energy. The resulting value for $T_c$ confirms a result obtained previously with a different method.
We use the method of the color effective Hamiltonian to study the structure of color singlet chain states in $N_c=3$ and in the large $N_c$ limit. In order to obtain their total fraction when $N_c$ is finite, we illustrate how to orthogonalize these non-orthogonal states. We give numerical results for the fraction of orthogonalized states in $e^+e^-\to q\bar{q}gg$. With the help of a diagram technique, we derive their fraction up to $O(1/N_c^2)$ for the general multigluon process. For large $N_c$ the singlet chain states correspond to well-defined color topologies. Therefore we may expect that the fraction of non-color-singlet-chain states is an estimate of the fraction of events where color reconnection is possible. In the case of soft gluon bremsstrahlung, we give an explicit form for the color effective Hamiltonian which leads to the dipole cascade formulation for parton showering in leading order in $N_c$. The next-to-leading order corrections are also given for $e^+e^-\to q\bar{q}g_1g_2$ and $e^+e^-\to q\bar{q}g_1g_2g_3$.
By using the background field method of QCD in a path integral approach, we derive the equation of motion for the classical chromofield and for the gluon in a system containing the gluon and the classical chromofield simultaneously. This inhomogeneous field equation contains a current term, which is the expectation value of a composite operator including linear, square and cubic terms of the gluon field. We also derive identities which the current should obey from the gauge invariance. We calculate the current at the leading order where the current induced by the gluon is opposite in sign to that induced by the quark. This is just the feature of the non-Abelian gauge field theory which has asymptotic freedom. Physically, the induced current can be treated as the 'displacement' current in the polarized vacuum, and its effect is equivalent to redefining the field and the coupling constant.