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Nu Xu

Publications and source records attributed to Nu Xu.

At least 55 records · Page 3Linked to original sources
arXiv2013

Quarkonium Production and Medium Effects in High Energy Nuclear Collisions

Color screening and regeneration are both hot medium effects on quarkonium production in high energy nuclear collisions. However, they affect in an opposite way the finally observed quarkonium spectra. Due to the competition of the two dynamical effects, the ratio of the integrated quarkonium yield between nuclear and elementary nucleon collisions loses its sensitivity. Once the information of quarkonium transverse motion is included, on the other hand, the ratio of averaged transverse momentum square reveals the nature of the QCD medium created in high energy nuclear collisions.

nucl-th↗
arXiv2013

Volume Fluctuation and Autocorrelation Effects in the Moment Analysis of Net-proton Multiplicity Distributions in Heavy-Ion Collisions

Moments (Variance ($σ^2$), Skewness($S$), Kurtosis($κ$)) of multiplicity distributions of conserved quantities, such as net-baryon,net-charge and net-strangeness, are predicted to be sensitive to the correlation length of the system and connected to the thermodynamic susceptibilities computed in Lattice QCD and Hadron Resonance Gas (HRG) model. In this paper, we present several measurement artifacts that could lead to volume fluctuation and auto-correlation effects in the moment analysis of net-proton multiplicity distributions in heavy-ion collisions using the UrQMD model. We discuss methods to overcome these artifacts so that the extracted moments could be used to obtain physical conclusions. In addition we present methods to properly estimate the statistical errors in moment analysis.

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arXiv2013

Velocity Dependence of Charmonium Dissociation Temperature in High-Energy Nuclear Collisions

In high-energy nuclear collisions, heavy quark potential at finite temperature controls the quarkonium production. Including the relaxation of the medium induced by the relative velocity between quarkonia and the deconfined expanding matter, the Debye screening is reduced and the quarkonium dissociation takes place at a higher temperature. As a consequence of the velocity dependent dissociation temperature, the quarkonium suppression at high transverse momentum is significantly weakened in high energy nuclear collisions at RHIC and LHC.

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arXiv2013

Elliptic flow of ϕ-meson a sensitive probe for onset of de-confinement transition in high energy heavy-ion collisions

Elliptic flow (v_2) of ϕ-meson is shown to be a sensitive probe of the partonic collectivity using A Multi Phase Transport (AMPT) model. Within the ambit of the AMPT model with partonic interactions (string melting version), the ϕ-meson v_2 at midrapidity is found to have negligible contribution from hadronic interactions for Au+Au collisions at 200 GeV per nucleon. Changing the hadron cascade time in the model calculations does not change the ϕ-meson v_2, while it reduces the v_2 of proton at low transverse momentum (pT). These observations indicate that a substantial reduction in ϕ-mesons v_2 as a function of colliding beam energy would suggest the dominance of hadronic interactions over partonic interactions.

nucl-ex↗
arXiv2011

Dilepton flow and deconfinement phase transition in heavy ion collisions

The dilepton radial flow in Au+Au collisions at \sqrt{s_{NN}}=200 GeV is investigated. The space-time evolution of the fireball is described by a 2+1 dimensional ideal hydrodynamics with a variety of equations of state. The slope parameters of the transverse momentum spectra from the partonic and hadronic phases show distinct features and are sensitive to equation of state parameters. The elliptic flow and breaking of M_T scaling are also studied and have distinct features for the two phases. These features can serve as clean signals for the formation of a quark-gluon plasma in ultra-relativistic heavy ion collisions.

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arXiv2011

Search for the QCD Critical Point: Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions have been applied to search for the QCD critical point. Model results are used to provide a baseline for this search. The measured moment products, $κσ^2$ and $Sσ$ of net-proton distributions, which are directly connected to the thermodynamical baryon number susceptibility ratio in Lattice QCD and Hadron Resonance Gas (HRG) model, are compared to the transport and thermal model results. We argue that a non-monotonic dependence of $κσ^2$ and $S σ$ as a function of beam energy can be used to search for the QCD critical point.

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arXiv2011

Scale for the Phase Diagram of Quantum Chromodynamics

Matter described by Quantum Chromodynamics (QCD), the theory of strong interactions, may undergo phase transitions when its temperature and the chemical potentials are varied. QCD at finite temperature is studied in the laboratory by colliding heavy-ions at varying beam energies. We present a test of QCD in the non-perturbative domain through a comparison of thermodynamic fluctuations predicted in lattice computations with the experimental data of baryon number distributions in high-energy heavy-ion collisions. This study provides evidence for thermalization in these collisions, and allows us to find the crossover temperature between normal nuclear matter and a deconfined phase called the quark gluon plasma. This value allows us to set a scale for the phase diagram of QCD.

hep-ph↗
arXiv2011

Upsilon Production as a Probe for Early State Dynamics in High Energy Nuclear Collisions at RHIC

Upsilon production in heavy ion collisions at RHIC energy is investigated. While the transverse momentum spectra of the ground state Upsilon(1s) are controlled by the initial state Cronin effect, the excited $b\bar b$ states are characterized by the competition between the cold and hot nuclear matter effects and sensitive to the dissociation temperatures determined by the heavy quark potential. We emphasize that it is necessary to measure the excited heavy quark states in order to extract the early stage information in high energy nuclear collisions at RHIC.

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arXiv2009

J/psi elliptic flow in relativistic heavy ion collisions

The J/psi elliptic flow in high energy nuclear collisions is calculated in a transport model. While the flow is very small at SPS and RHIC energies, it is strongly enhanced at LHC energy due to the dominance of the regeneration mechanism.

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arXiv2009

J/psi production at mid and forward rapidity at RHIC

We calculate the rapidity dependence of $J/ψ$ nuclear modification factor and averaged transverse momentum square in heavy ion collisions at RHIC in a 3-dimensional transport approach with regeneration mechanism.

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arXiv2009

Predictions of elliptic flow and nuclear modification factor from 200 GeV U + U collisions at RHIC

Predictions of elliptic flow ($v_2$) and nuclear modification factor ($R_{AA}$) are provided as a function of centrality in U + U collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. Since the $^{238}$U nucleus is naturally deformed, one could adjust the properties of the fireball, density and duration of the hot and dense system, for example, in high energy nuclear collisions by carefully selecting the colliding geometry. Within our Monte Carlo Glauber based approach, the $v_2$ with respect to the reaction plane $v_2^{RP}$ in U + U collisions is consistent with that in Au + Au collisions, while the $v_2$ with respect to the participant plane $v_2^{PP}$ increases $\sim$30-60% at top 10% centrality which is attributed to the larger participant eccentricity at most central U + U collisions. The suppression of $R_{AA}$ increases and reaches $\sim$0.1 at most central U + U collisions that is by a factor of 2 more suppression compared to the central Au + Au collisions due to large size and deformation of Uranium nucleus.

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arXiv2009

Hadron Production in Heavy Ion Collisions

We review hadron production in heavy ion collisions with emphasis on pion and kaon production at energies below 2 AGeV and on partonic collectivity at RHIC energies.

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arXiv2009

Rapidity Dependence of $J/ψ$ Production at RHIC and LHC

The motion of charmonium in heavy ion collisions is described by a three dimensional transport equation with initial production and continuous regeneration in hot medium. The observation of apparently stronger $J/ψ$ suppression at forward rapidity compared to that at midrapidity, so called $J/ψ$ puzzle at RHIC, can well be explained by the competition between the two production mechanisms. At LHC, however, the rapidity dependence of the $J/ψ$ production is dominated by the regeneration process.

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arXiv2009

J/psi Transverse Momentum Distribution in High Energy Nuclear Collisions

The transverse momentum (pt) dependence of J/psi production in heavy ion collisions is investigated in a transport model with both initial production and continuous regeneration of charmonia. The competition between the two production mechanisms results in a pt suppression in central collisions, the gluon multi-scattering in the initial stage leads to a high pt enhancement, and the regeneration populates J/psis at low pt region and induces a minimum in R_AA(pt). These three phenomena are indeed observed in both 200 GeV Cu+Cu and Au+Au collisions at RHIC energy.

nucl-th↗
arXiv2006

J/psi Production in Quark-Gluon Plasma

We study J/psi production at RHIC and LHC energies with both initial production and regeneration. We solve the coupled set of transport equation for the J/psi distribution in phase space and the hydrodynamic equation for evolution of quark-gluon plasma. At RHIC, continuous regeneration is crucial for the J/psi momentum distribution while the elliptic flow is still dominated by initial production. At LHC energy, almost all the initially created J\psis are dissociated in the medium and regeneration dominates the J/psi properties.

nucl-th↗
arXiv2004

J/psi transport in QGP and Pt distribution at SPS and RHIC

Combining the hydrodynamic equations for the QGP evolution and the transport equation for the primordially produced $J/ψ$ in the QGP, we investigate the $J/ψ$ transverse momentum distribution as well as its suppression in the $\sqrt{s}$=17.3A GeV Pb-Pb collisions at SPS and $\sqrt{s}$=200A GeV Au-Au collisions at RHIC. The two sets of equations are connected by the $J/ψ$ anomalous suppression induced by its inelastic scattering with gluons in the QGP. The calculated centrality dependence of $J/ψ$ suppression and average transverse momentum square agree well with the SPS data. From the comparison with the coalescence model where charm quark is fully thermalized, our calculated elliptical flow of the primordially produced $J/ψ$ is much smaller. This may be helpful to differentiate the $J/ψ$ production mechanisms in relativistic heavy ion collisions.

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