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X. N. Wang

Publications and source records attributed to X. N. Wang.

14 recordsLinked to original sources

Dijet Acoplanarity in CUJET3 as a Probe of the Nonperturbative Color Structure of QCD Perfect Fluids

Using the CUJET3=DGLV+VISHNU jet-medium interaction framework, we show that dijet azimuthal acoplanarity in high energy $A+A$ collisions is sensitive to possible non-perturbative enhancement of the jet transport coefficient, $\hat{q}(T,E)$, in the QCD crossover temperature $T\sim 150-300$ MeV range. With jet-medium couplings constrained by global RHIC\& LHC $χ^2$ fits to nuclear modification data on $R_{AA}$, we compare predictions of the medium induced dijet transverse momentum squared, $Q_s^2\sim \langle \hat{q} L \rangle \sim Δϕ^2 E^2$, in two models of the temperature, $T$, and jet energy $E$ dependence of the jet medium transport coefficient, $\hat{q}(T,E)$. In one model, wQGP, only perturbative quark and gluon dof are assumed. In the second model, sQGMP, nonperturbative three component semi-Quark-Gluon and Magnetic Monopole dof are asssumed. We show that the dijet path averaged medium induced azimuthal acoplanarity, $Δϕ^2$, in sQGMP is robustly a factor of $\sim 2$ larger than in perturbative wQGP while the radiative energy loss in both models is nearly identical as required to fit the same single jet nuclear modification $R_{AA}$ data. Future A+A dijet acoplanarity measurements correlated with $R_{AA}$ and azimuthal asymmetry $v_n$ measurements therefore appears to be a promising strategy to search for possible signatures of critical opalescence like phenomena in the QCD confinement temperature range.

hep-ph

Precision Dijet Acoplanarity Tomography of the Chromo Structure of Perfect QCD Fluids

Dijet acoplanarity is dominated by vacuum (Sudakov) pQCD radiation even in Pb+Pb collisions, but future higher precision measurements of the tails of the acoplanarity distributions can help to resolve {\em separately} the medium opacity, $χ=L/λ$, and the color screening scale $μ^2$ from the path averaged BDMS saturation scale $Q_s^2[χ,μ]=\int dL\; \hat{q}(E,T(L))\propto μ^2 L/λ$, that is already well constrained by nuclear modification factor data on $R_{AA}(p_T)$. We compare Gaussian (BDMS) and non-Gaussian (GLV) models of medium broadening of vacuum (Sudakov) induced acoplanarity distributions in A+A. With few percent accuracy on the ratio of A+A to p+p distributions, experiments can easily identify non-Gaussian Landau and Rutherford tails due to multiple collisions. However, we find that sub-percent precision will be required to constrain $χ$ and $μ$ separately from $Q_s$.

hep-ph

Baryon Junction Loops in HIJING/BBv2.0 and the Baryon/Meson Anomaly at RHIC

A new version, v2.0, of the HIJING/BB Monte Carlo nuclear collision event generator is introduced in order to explore further the possible role of baryon junctions loops in the baryon/meson anomaly (2 $< p_{T} <$ 5 GeV/c) observed in 200A GeV Au+Au reactions at RHIC. We show that junction loops with an enhanced intrinsic $k_T\approx 1$ GeV/c transverse momentum kick may provide a partial explanation of the anomaly as well as other important baryon stopping observables.

nucl-th

The Mini-Jet Scale and Energy Loss at RHIC in the HIJING and RQMD Models

The original default mini-jet scale p_0=2 GeV/c and energy loss, dE/dx=2 GeV/fm in HIJING lead to a too rapid growth of the multiplicity with energy. RQMD model without mini-jet leads on the other hand to a too slowly increasing multiplicity with energy. We show that a slight increase of p_0 from the default 2.0 GeV/c at sqrt(s)=130A GeV to 2.2 GeV/c at sqrt{s}=200A GeV is sufficient to account for the bulk observables. Jet quenching of pi^0 above p_T>4 GeV/c at 200A GeV is found to be well reproduced in central Au+Au by the original HIJING default prediction. However, the effective surface emission in the HIJING model formulation of energy loss over-predicts the quenching in more peripheral collisions. Neither HIJING nor HIJING/B{bar}B can account for the observed anomalous excess of moderate p_T<4 GeV/c baryons. The agreement of RQMD with data in this p_T region is shown to be fortuitous because, without mini-jets, it fails to reproduce the p+p spectrum.

nucl-th

Last Call for RHIC Predictions

This paper contains the individual contributions of all speakers of the session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary by the convenor.

nucl-th

Charmonium production from the hadronic phase

Charmonium production from the hadron gas formed in ultra-relativistic heavy-ion collisions is studied. Using the $J/ψ$-hadron absorption cross section determined from the nonperturbative quark-exchange model, which has a peak value similar to that used in the comover model for $J/ψ$ suppression and a thermally averaged value consistent from that extracted from $J/ψ$ data in heavy-ion collisions, we find that $J/ψ$ production from the hadron gas is negligible in heavy-ion collisions at RHIC energies but is important at LHC energies as a result of the large number of charm mesons produced at higher energy collisions. The number of $J/ψ$ produced from these secondary collisions at LHC may be comparable to that of primary $J/ψ$'s, which are expected to be dissociated in the quark-gluon plasma created in the collisions, leading thus to a possible absence of $J/ψ$ suppression. Similar results are obtained for $ψ^\prime$ production in ultra-relativistic heavy-ion collisions.

nucl-th

Dilepton from Disoriented Chiral Condensates

Disoriented chiral condensates are manifested as long wavelength pionic oscillations and their interaction with the thermal environment can be a significant source of dileptons. We calculate the yield of such dilepton production within the linear sigma model and illustrate the basic features of the dilepton spectrum in a schematic model. We find that the dilepton yield with invariant mass near and below $2m_π$ due to the soft pion modes can be up to two orders of magnitude larger than the corresponding equilibrium yield. We conclude with a discussion on how this enhancement can be detected by present dilepton experiments.

nucl-th

Dileptons from Disoriented Chiral Condensates

Disoriented chiral condensates or long wavelength pionic oscillations and their interaction with the thermal environment can be a significant source of dileptons. We calculate the yield of such dilepton production within the linear sigma model, both in a quantal mean-field treatment and in a semi-classical approximation. We then illustrate the basic features of the dilepton spectrum in a schematic model. We find that dilepton yield with invariant mass near and below $2m_π$ due to the soft pion modes can be up to two orders of magnitude larger than the corresponding equilibrium yield.

nucl-th

Reply to Comment of Gazdzicki and Heinz on Strangeness Enhancement in $p+A$ and $S+A$

The Comment of Gazdzicki and Heinz is flawed because their assumed baryon stopping power in $pA$ is inconsistent with data and because they ignored half the analysis based on the VENUS model. The Comment continues the misleading presentation of strangeness enhancement by focusing on ratios of integrated yields. Those ratios discard essential experimental information on the rapidity dependence of produced $Λ$ and obscure discrepancies between different data sets. Our conclusion remains that the NA35 minimum bias data on $p+S\rightarrowΛ+X $ indicate an anomalous enhancement of central rapidity strangeness in few nucleon reactions that points to non-equilibrium dynamics as responsible for strangeness enhancement in nuclear reactions.

nucl-th

Expansion, Thermalization and Entropy Production in High-Energy Nuclear Collisions

The thermalization process is studied in an expanding parton gas using the Boltzmann equation with two types of collision terms. In the relaxation time approximation we determine the criteria under which a time-dependent relaxation time leads to thermalization of the partons. We calculate the entropy production due to collisions for the general time-dependent relaxation time. In a perturbative QCD approach on the other hand, we can estimate the parton collision time and its dependence on expansion time. The effective `out of equilibrium' collision time differs from the standard transport relaxation time, $τ_{\rm tr}\simeq(α_s^2\ln(1/α_s)T)^{-1}$, by a weak time dependence. It is in both cases Debye screening and Landau damping that regulate the singular forward scattering processes. We find that the parton gas does thermalize eventually but only after having undergone a phase of free streaming and gradual equilibration where considerable entropy is produced (``after-burning"). The final entropy and thus particle density depends on the collision time as well as the initial conditions (a ``memory effect"). Results for entropy production are presented based upon various model estimates of early parton production.

hep-ph

Strangeness Enhancement in $p+A$ and $S+A$ Interactions at SPS Energies

The systematics of strangeness enhancement is calculated using the HIJING and VENUS models and compared to recent data on $\,pp\,$, $\,pA\,$ and $\,AA\,$ collisions at CERN/SPS energies ($200A\,\, GeV\,$). The HIJING model is used to perform a {\em linear} extrapolation from $pp$ to $AA$. VENUS is used to estimate the effects of final state cascading and possible non-conventional production mechanisms. This comparison shows that the large enhancement of strangeness observed in $S+Au$ collisions, interpreted previously as possible evidence for quark-gluon plasma formation, has its origins in non-equilibrium dynamics of few nucleon systems. % Strangeness enhancement %is therefore traced back to the change in the production dynamics %from $pp$ to minimum bias $pS$ and central $SS$ collisions. A factor of two enhancement of $Λ^{0}$ at mid-rapidity is indicated by recent $pS$ data, where on the average {\em one} projectile nucleon interacts with only {\em two} target nucleons. There appears to be another factor of two enhancement in the light ion reaction $SS$ relative to $pS$, when on the average only two projectile nucleons interact with two target ones.

nucl-th

Strangeness Production in Chemically Non-Equilibrated Parton Plasma

Strangeness production was investigated during the equilibration of a gluon dominated parton plasma produced at RHIC and LHC energies. The time evolution of parton densities are followed by a set of rate equations in a 1-dimensional expanding system. The strangeness production will depend on the initial chemical equilibration level and in our case the parton densities will remain far from the full equilibrium. We investigate the influence of gluon fragmentation on final strangeness content.

hep-ph

Parton Equilibration in Relativistic Heavy Ion Collisions

We investigate the processes leading to phase-space equilibration of parton distributions in nuclear interactions at collider energies. We derive a set of rate equations describing the chemical equilibration of gluons and quarks including medium effects on the relevant QCD transport coefficients, and discuss their consequences for parton equilibration in heavy ion collisions.

nucl-th