arXiv subjects
Publications and source records attributed to Xin-Nian Wang.
We numerically study the effects of high gluon density at small x on the evolution of gluon distribution function in both hadrons and nuclei. Using a newly derived, Wilson renormalization group-based evolution equation which includes n to 1 gluon ladder fusion, we find significant reduction in nuclear gluon distribution function for large nuclei at zero impact parameter at energies relevant for RHIC and LHC experiments.
A novel non-perturbative gluon junction mechanism is introduced within the HIJING/B nuclear collision event generator to calculate baryon number transport and hyperon production in pA and AA collisions. This gluonic mechanism can account for the observed large mid-rapidity valence baryon yield in Pb+Pb at 160 AGeV and predicts high initial baryon densities at RHIC. However, the highly enhanced Lambda-Lambdabar yield and the baryon transverse momentum flow observed in this reaction can only be partially described.
We study the dimuon yield from open charm decays in the intermediate mass region(IMR) in heavy-ion collisions at the CERN-SPS energy. We find that final state rescatterings which broaden the $m_T$ spectrum of charmed mesons can enrich the part of the phase space covered by the NA50 experiment, thus leading to an apparent enhancement of IMR dileptons within the acceptance of the experiment. Such an enhancement increases with the dimuon invariant mass and the single muon energy cut-off.
Since large $p_T$ particles in high-energy hadronic or nuclear collisions come from jet fragmentation, jet quenching due to parton energy loss in dense matter will cause the suppression of large $p_T$ hadron spectra in high-energy heavy-ion collisions. Assuming an effective energy loss $dE/dx$ for the high $E_T$ partons, effective jet fragmentation functions are constructed in which leading hadrons will be suppressed. Using such effective fragmentation functions, high $p_T$ hadron spectra and particle suppression factors relative to $pp$ collisions are estimated in central high-energy nuclear collisions with a given range of the assumed $dE/dx$. It is found that the suppression factors are very sensitive to the value of the effective energy loss. Systematic nuclear and flavor dependence of the hadron spectra are also studied.
Because of the rapidly falling particle spectrum at large $p_T$ from jet fragmentation at the CERN SPS energy, the high-$p_T$ hadron distribution should be highly sensitive to parton energy loss inside a dense medium as predicted by recent perturbative QCD (pQCD) studies. A careful analysis of recent data from CERN SPS experiments via pQCD calculation shows little evidence of energy loss. This implies that either the life-time of the dense partonic matter is very short or one has to re-think about the problem of parton energy loss in dense matter. The hadronic matter does not seem to cause jet quenching in $Pb+Pb$ collisions at the CERN SPS. High-$p_T$ two particle correlation in the azimuthal angle is proposed to further clarify this issue.
Baryon stopping at the SPS and RHIC energies is calculated by introducing a new baryon junction mechanism into HIJING. The exchange of a baryon junction, according to Regge phenomenology, leads to a cosh(y/2) rapidity dependence and an s^(-1/4) energy dependence of the inclusive baryon cross section. This baryon junction dynamics also leads naturally to enhanced p_T broadening in pA and AA together with enhanced mid-rapidity hyperon production.
I will give a review with some comments of the parallel session on QCD and the related talks in the Quark Matter'97 meeting.
Hard QCD processes in ultrarelativistic heavy-ion collisions become increasingly relevant and they can be used as probes of the dense matter formed during the violent scatterings. We will discuss how one can use these hard probes to study the properties of the dense matter and the associated phenomenologies. In particular, we study the effect of jet quenching due to medium-induced energy loss on inclusive particle $p_T$ distributions and investigate how one can improve the measurement of parton energy loss in direct photon events
We study the effect of energy loss on charm and bottom quarks in high-energy heavy-ion collisions including longitudinal expansion and partial thermalization. We find that high $p_\perp$ heavy quarks are greatly suppressed, and consequently, high-mass dileptons from heavy quark decays are also suppressed. We consider in detail the detector geometry and single lepton energy cuts of the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC). Because of the longitudinal expansion, the suppressions of $ee, eμ$ and $μμ$ pairs resulting from the heavy quark energy loss are very different due to the different rapidity coverages and energy cuts. The assumed energy loss rate, on the order of 1 GeV/fm, results in a large suppression on dielectrons, and dielectrons from heavy quark decays become comparable or even lower than the Drell-Yan yield. It is thus possible to probe the energy loss rate of the medium using dileptons from heavy quark decays.
The effect of medium-induced parton energy loss on jet fragmentation is studied in high-energy heavy-ion collisions. It is shown that an effective jet fragmentation function can be extracted from the inclusive $p_T$ spectrum of charged particles in the opposite direction of a tagged direct photon with a fixed transverse energy. We study the modification of the effective jet fragmentation function due to parton energy loss in AA as compared to pp collisions, including $E_T$ smearing from initial state radiations for the photon-tagged jets. The effective fragmentation function at $z=p_T/E_T^γ\sim 1$ in pA collisions is shown to be sensitive to the additional $E_T$ smearing due to initial multiple parton scatterings whose effect must be subtracted out in AA collisions in order to extract the effective parton energy loss. Jet quenching in deeply inelastic lepton-nucleus scatterings as a measure of the parton energy loss in cold nuclear matter is also discussed. We also comment on the experimental feasibilities of the proposed study at the RHIC and LHC energies and some alternative measurements such as using $Z^0$ as a tag at the LHC energy.
The high density of scattered partons predicted in nuclear collisions at very high energy makes color screening effects significant. We explain how these screening mechanisms may suppress nonperturbative, soft QCD processes, permitting a consistent calculation of quark-gluon plasma formation within the framework of perturbative QCD. We present results of a model calculation of these effects including predictions for the initial thermalized state for heavy nuclei colliding at RHIC and LHC.
A pQCD-based model for parton production and equilibration in ultrarelativistic heavy-ion collisions is reviewed. The model combines pQCD processes including initial and final state radiations together with string phenomenology for nonperturbative soft processes. Nuclear effects on the initial parton production, such as multiple parton scattering and nuclear shadowing of parton distribution functions are considered. Comparisons with existing data are made and further tests of the model to constrain model parameters are proposed. With the obtained space-time history of the parton production, evolution of the minijet gas toward a fully equilibrated parton plasma is studied. Direct probes of the early parton dynamics, such as pre-equilibrium photon and dilepton production, open charm production, $J/ψ$ suppressionand jet quenching are also reviewed.
We point out that events associated with large $E_T$ direct photons in high-energy heavy-ion collisions can be used to study jet energy loss in dense matter. In such events, the $p_T$ spectrum of charged hadrons from jet fragmentation in the opposite direction of the tagged photon is estimated to be well above the background which can be reliably subtracted at moderately large $p_T$. We demonstrate that comparison between the extracted fragmentation function in $AA$ and $pp$ collisions can be used to determine the jet energy loss and the interaction mean-free-path in the dense matter produced in high-energy heavy-ion collisions.
Since the electromagnetic current for a pion system coincides with the third component of the isovector current, the isospin angular oscillation of a coherent field can be a significant source for the electromagnetic emissions. We study the characteristic dilepton and photon emissions from the classical pion field oscillation in the QCD vacuum. The general analytical solution obtained in the nonlinear sigma model is used to calculate the electromagnetic current density, which exhibits a light-front singularity and decreases rapidly as inverse square of the proper time due to a longitudinal expansion. The momentum and invariant mass spectra of the direct photon and dilepton are found to be a sensitive probe of the space-time evolution of the chiral condensate field.
Thermalization in an expanding parton plasma is studied within the framework of Boltzmann equation in the absence of any mean fields. In particular, we study the time-dependence of the relaxation time to the lowest order in finite temperature QCD and how such time-dependence affects the thermalization of an expanding parton plasma. Because of Debye screening and Landau damping at finite temperature, the relaxation time (or transport rates) is free of infrared divergencies in both longitudinal and transverse interactions. The resultant relaxation time decreases with time in an expanding plasma like $1/τ^β$, with $β<1$. We prove in this case that thermal equilibrium will eventually be established given a long life-time of the system. However, a fixed momentum cut-off in the calculation of the relaxation time gives rise to a much stronger time dependence which will slow down thermal equilibrium. It is also demonstrated that the ``memory effect'' of the initial condition affects the approach to thermal equilibrium and the final entropy production.
We present a novel method for studying the formation of a disoriented chiral condensate (DCC) in high-energy hadronic and heavy-ion collisions utilizing a discrete wavelet transformation. Due to its salient feature of space-scale locality, the discrete wavelet proves to be very effective in probing physics simultaneously at different locations in phase space and at different scales. We show that the probability distributions of the neutral pion fraction for various rapidity-bin sizes have distinctive shapes in the case of a DCC and exhibit a delay in approaching the Gaussian distribution required by the Central Limit Theorem. We find the wavelet power spectrum for a DCC to exhibit a strong dependence on the scale while an equilibrium system and the standard dynamical models such as HIJING have a flat spectrum.
Short-distance QCD is employed to calculate the $J/ψ$ survival probability in an equilibrating parton gas, whose evolution is governed by a set of master rate equations. Partons in the early stage of high-energy nuclear collisions may initially not be in equilibrium, but their average transverse momentum is sufficiently high to break up a $Q\bar{Q}$ bound state. Such a break-up during the evolution of the parton gas is shown to cause a substantial $J/ψ$ suppression at both RHIC and LHC energies, using realistic estimates of the initial parton densities. The transverse momentum dependence of the suppression is also shown to be sensitive to the initial conditions and the evolution history of the parton plasma.
Screening of initial parton production due to the presence of on-shell partons in high-energy heavy-ion collisions is discussed. It is shown that the divergent cross sections in the calculation of parton production can be regulated self-consistently without an {\it ad hoc} cut-off, and that the resultant parton production and transverse energy production rate are finite. Consequences on the energy density estimates are discussed.