A Multiphase Transport Research Articles

Search for Long-Range Correlations in Relativistic Heavy-Ion Collisions at SPS Energies

Long-range correlations are searched for by analysing the experimental data on16O-AgBr and32S-AgBr collisions at 200 A GeV/c and the results are compared with the predictions of a multi phase transport (AMPT) model. The findings reveal that the observed forward-backward (F-B) multiplicity correlations are mainly of short range in nature. The range of F-B correlations are observed to extend with increasing projectile mass. The observed extended range of F-B correlations might be due to overall multiplicity fluctuations arising because of nuclear geometry. The findings are not sufficient for making any definite conclusions regarding the presence of long-range correlations.

Properties of Full Jet in High-Energy Heavy-Ion Collisions from Parton Scatterings

The properties of fully reconstructed jet are investigated in p + p and Pb + Pb collisions atsNN= 2.76 TeV within a multiphase transport (AMPT) model with both partonic scatterings and hadronic rescatterings. A large transverse momentum (pT) asymmetry of dijet or photon-jet arises from the strong interactions between jet and partonic matter. Theξ-dependent jet fragmentation function in Pb + Pb collisions is decomposed into two contributions from different jet hadronization mechanisms, that is, fragmentation versus coalescence. The medium modification of differential jet shape displays that the jet energy is redistributed towards a larger radius owing to jet-medium interactions in heavy-ion collisions. Jet triangular azimuthal anisotropy coefficient,v3jet, which shows a smaller magnitude than the elliptic coefficientv2jet, decreases more quickly with increasing jetpT, which can be attributed to a path-length effect of jet energy loss. All of these properties of full jet are consistent with the jet energy loss mechanism in a stronglyinteracting partonic matter in high-energy heavy-ion collisions.

Effect of partonic cascade on two-pion HBT parameters of the core-halo source in relativistic heavy-ion collisions

Using a multi-phase transport (AMPT) model, we investigate the effect of partonic cascade on two-pion HBT parameters of the core–halo source in relativistic heavy-ion collisions. The evolution time as a function of the partonic cross section for Au+Au central collisions at GeV shows that the evolution time of the pion emission source is decreased with an increasing partonic cross section, and the drop is due to the evolution time of the core. The partonic cascade with a larger cross section leads to a shorter lifetime of the source.

Energy and centrality dependence of chemical freeze-out thermodynamics parameters

Driven by the Beam Energy Scan (BES) program at the RHIC, researches and discussions on the QCD phase diagram have flourished recently. In order to provide a reference from microscopic transport models, we performed a systematic analysis, using a multiphase transport (AMPT) model for the particle yields and a statistical model (THERMUS) for the thermal fit, for Au+Au collisions at $\sqrt{s_{\text{NN}}}$=7.7-200 GeV. It is found that at a fixed collision centrality the chemical freeze-out parameter, temperature $T_{\text{ch}}$, increases with collision energy and somehow saturates at certain values of $T_{\text{ch}}$ in collisions near $\sqrt{s_{\text{NN}}}$=10 GeV, indicating the limiting temperature in hadronic interactions; meanwhile the baryon chemical potential $\mu_B$ decrease with the collision energy. The saturation temperature is also found to be dependent on partonic interaction. At a given collision energy, it is found that both $T_{\text{ch}}$ and $\mu_B$ decrease towards more peripheral collisions in the grand canonical approach. The energy and centrality dependence of other chemical freeze-out parameters, strangeness chemical potential $\mu_S$, strangeness undersaturation factor $\gamma_S$, and the volume of the fireball $V$ are also presented in this paper. The chemical potential ratio $\mu_s/\mu_B$ is also compared with lattice QCD calculation. The AMPT default model gives better descriptions on both the particle yields and the chemical freeze-out parameters than those from the AMPT string-melting model.

Azimuthal anisotropies of reconstructed jets in Pb + Pb collisions atsNN=2.76TeV in a multiphase transport model

Azimuthal anisotropies of reconstructed jets [${v}_{n}^{\mathrm{jet}}\phantom{\rule{4pt}{0ex}}(n=2,3)$] have been investigated in Pb + Pb collisions at the center of mass energy $\sqrt{{s}_{NN}}=2.76$ TeV within a framework of a multiphase transport (AMPT) model. The ${v}_{2}^{\mathrm{jet}}$ is in good agreement with the recent ATLAS data. However, the ${v}_{3}^{\mathrm{jet}}$ shows a smaller magnitude than ${v}_{2}^{\mathrm{jet}}$, and approaches zero at a larger transverse momentum. It is attributed to the path-length dependence in which the jet energy loss fraction depends on the azimuthal angles with respect to different orders of event planes. The ratio ${v}_{n}^{\mathrm{jet}}/{\ensuremath{\varepsilon}}_{n}$ increases from peripheral to noncentral collisions, and ${v}_{n}^{\mathrm{jet}}$ increases with the initial spatial asymmetry $({\ensuremath{\varepsilon}}_{n})$ for a given centrality bin. These behaviors indicate that the ${v}_{n}^{\mathrm{jet}}$ is produced by the strong interactions between jet and the partonic medium with different initial geometry shapes. Therefore, azimuthal anisotropies of reconstructed jet are proposed as a good probe to study the initial spatial fluctuations, which are expected to provide constraints on the path-length dependence of jet quenching models.

Final state effects on charge asymmetry of pion elliptic flow in high-energy heavy-ion collisions

Within a multi-phase transport (AMPT) model with the string melting mechanism and imported initial electric quadrupole moment, the difference between the elliptic flow of positive and negative pions is calculated. The slope parameter r of the linear dependence of Δv2=v2(π−)−v2(π+) on Ach=(N+−N−)/(N++N−) is yielded owing to the parton cascade, which converts the initial electric quadrupole distribution into the final charge-dependent elliptic flow. The slope parameter r is found to be increased by the hadronization given by the coalescence, and decreased by the resonance decays. The slope parameter r is very sensitive to both the initial electric quadrupole percentage and centrality bin and consequently a helpful constraint on the quadrupole moment of the chiral magnetic wave is obtained for Au + Au collisions at the top RHIC energy.

Tomography of QGP with jet asymmetries

Within a multi-phase transport (AMPT) model, the transverse momentum asymmetries for γ-jet and dijet are studied in Pb+Pb collisions at = 2.76 TeV. A large asymmetry is produced by strong interactions between jets and partonic matter. It is demonstrated that hadronization and hadronic rescatterings have little effects on the formed asymmetries. The asymmetry evolution function discloses that final asymmetry is driven by both initial asymmetry and partonic jet energy loss. The γ-jet imbalance ratio, Xjγ, is sensitive to both production position and passing direction of γ-jet, which could enable a detail tomographic study on the formed partonic matter by selecting xjγ.

Free streaming and sudden equilibration approximation for the early stages of relativistic heavy-ion collisions

Using a multi-phase transport (AMPT) model, we study the equilibration process of the early collision stages with free streaming and sudden equilibration approximation. In addition, the results of the transverse momentum spectra, the elliptic flow, the Hanbury-Brown twiss (HBT) parameters and the freeze-out eccentricity from the azimuthal angle dependence of the HBT radii are described within our approach. The results of the transverse momentum spectra and the elliptic flow are close to the experimental data. The HBT radii result is similar to that from the AMPT model. The eccentricity results indicate that the approximation leads to a slow drop of the eccentricity with increasing time. The slow decrease can be reflected in the freeze-out eccentricity obtained by HBT radii.

Charm quarks as a probe of matter produced in relativistic nucleus-nucleus collisions

Direct measurement of hadrons containing charm quark carries important in- formation about the initial stage of the nucleus-nucleus collision at relativistic energies. The study of open charm in Pb-Pb collisions at SPS energies will be a powerful tool to investigate the production of heavy flavours and their interaction with the medium pro- duced in such collisions. A feasibility study was initiated for the measurement of the D 0 mesons (open charm) by its two-body decay into pion and kaon in central Pb-Pb col- lision at SPS energies in NA61/SHINE experiment. To generate the physical input we used AMPT (A Multi Phase Transport Model) event generator and employed GEANT4 application to describe particle transport through the NA61/SHINE experimental setup supplemented by a future vertex detector (VD) that will allow for precise vertex recon- struction close to the primary interaction point. The results of the simulation shows that this measurement is feasible with a dedicated VD which allows the precise tracking close to the target.

Recent Developments of A Multi-phase Transport Model

After the public release of A Multi-Phase Transport (AMPT) model in 2004 and detailed descriptions of its physics in a 2005 paper, the model has been constantly updated and developed to make it more versatile and to include more physical processes. This an overview of recent developments of the AMPT model. Ongoing work to fix the violation of charge conservation in the code as well as possible directions for future work are also discussed.

Simulation of energy scan of pion interferometry in central Au+Au collisions at relativistic energies

We present a systematic analysis of two-pion interferometry for the central Au+Au collisions at √sNN = 3, 5, 7, 11, 17, 27, 39, 62, 130 and 200 GeV/c with the help of a multiphase transport (AMPT) model. Emission source-size radius parameters Rlong, Rout, Rside and the chaotic parameter λ are extracted and compared with the experimental data. Transverse momentum and azimuthal angle dependencies of the HBT radii are also discussed for central Au+Au collisions at 200 GeV/c. The results show that the HBT radii in central collisions do not change much above 7 GeV/c. For central collisions at 200 GeV/c, the radii decrease with the increasing of transverse momentum pT but are not sensitive to the azimuthal angle. These results provide a theoretical reference for the energy scan program of the RHIC-STAR experiment.

Towards detailed tomography of high energy heavy-ion collisions by γ-jet

Within a multi-phase transport (AMPT) model with string melting scenario, the transverse momentum imbalance between prompt photon and jet is studied in Pb+Pb collisions at sNN=2.76 TeV. Jet loses more energy in more central collisions due to strong partonic interactions between jet parton shower and partonic matter, which is more significant than due to hadronic interactions only. The hadronization and final-state hadronic interactions have little influences on the imbalance. The imbalance ratio xjγ is sensitive to both production position and passing direction of γ-jet, which provides an opportunity to do detail γ-jet tomography on the formed partonic matter by selecting different xjγ ranges. It is also proposed that γ-hadron azimuthal correlation associated with photon+jet is a probe to see the medium responses to different γ-jet production configurations.

Dijet asymmetry in Pb+Pb collisions atsNN=2.76TeV within a multiphase transport model

Within a multi-phase transport (AMPT) model, dijet asymmetry is studied in Pb+Pb collisions at $\sqrt{s_{_{\rm NN}}}$ = 2.76 TeV. It is found that a large dijet asymmetry ($A_{J}$) is produced by strong interactions between jets and partonic matter. It is demonstrated that hadronization and final-state hadronic rescatterings have little effects on $A_{J}$. The final $A_{J}$ is found to be driven by both initial $A_{J}$ and partonic jet energy loss, which is consistent with an increasing jet energy loss in a hot and strongly interacting partonic medium in more central Pb+Pb collisions.

Medium information from anisotropic flow and jet quenching in relativistic heavy ion collisions

Within a multiphase transport (AMPT) model, where the initial conditions are obtained from the recently updated HIJING 2.0 model, the recent anisotropic flow and suppression data for charged hadrons in Pb+Pb collisions at the LHC energy of TeV are explored to constrain the properties of the partonic medium formed. In contrast to RHIC, the measured centrality dependence of charged hadron multiplicity dNch/dη at LHC provides severe constraint to the largely uncertain gluon shadowing parameter sg. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller sg to be consistent with dNch/dη data at LHC. With the parton shadowing so constrained, hadron production and flow over a wide transverse momenta range are investigated in AMPT. The model calculations for the elliptic and triangular flow are found to be in excellent agreement with the RHIC data, and predictions for the flow coefficients vn(PT, cent) at LHC are given. The magnitude and pattern of suppression of the hadrons in AMPT are found consistent with the measurements at RHIC. However, the suppression is distinctly overpredicted in Pb+Pb collisions at the LHC energy. Reduction of the QCD coupling constant αs by ~ 30% in the higher temperature plasma formed at LHC reproduces the measured hadron suppression.

Elliptic flow ofϕmesons as a sensitive probe for the onset of the deconfinement transition in high energy heavy-ion collisions

Elliptic flow (v_2) of \phi-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 \phi-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 \phi-meson v_2, while it reduces the v_2 of proton at low transverse momentum (pT). These observations indicate that a substantial reduction in \phi-mesons v_2 as a function of colliding beam energy would suggest the dominance of hadronic interactions over partonic interactions.

Mean-field effects on matter and antimatter elliptic flow

We report our recent work on mean-field potential effects on the elliptic flows of matters and antimatters in heavy ion collisions leading to the production of a baryon-rich matter. Within the framework of a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we have found that including mean-field potentials in the hadronic phase leads to a splitting of the elliptic flows of particles and their antiparticles, providing thus a plausible explanation of the different elliptic flows between p and anti-p, K+ and K-, and pi(+) and pi(-) observed by the STAR Collaboration in the Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider (RHIC). Using a partonic transport model based on the Nambu-Jona-Lasinio (NJL) model, we have also studied the effect of scalar and vector mean fields on the elliptic flows of quarks and antiquarks in these collisions. Converting quarks and antiquarks at hadronization to hadrons via the quark coalescence model, we have found that the elliptic flow differences between particles and antiparticles also depend on the strength of the quark vector coupling in baryon-rich quark-gluon plasma, providing thus the possibility of extracting information on the latter's properties from the BES program at RHIC.

Effects of hadronic potentials on elliptic flows in relativistic heavy ion collisions

Within the framework of a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we show that including mean-field potentials in the hadronic phase leads to a splitting of the elliptic flows of particles and their antiparticles, thus providing a plausible explanation of the different elliptic flows between $p$ and $\overline{p}$, ${K}^{+}$ and ${K}^{\ensuremath{-}}$, and ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ observed in the recent Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC).

Multiplicity, average transverse momentum, and azimuthal anisotropy in U+U collisions atSNN=200GeV using a multiphase transport model

Using a multiphase transport (AMPT) model that includes the implementation of deformed uranium nuclei, we have studied the centrality dependence of the charged particle multiplicity (${N}_{\mathrm{ch}}$, $d{N}_{\mathrm{ch}}/d\ensuremath{\eta}$), average transverse momentum ($\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$), eccentricity (${\ensuremath{\varepsilon}}_{2}$), triangularity (${\ensuremath{\varepsilon}}_{3}$), their fluctuations, elliptic flow (${v}_{2}$), and triangular flow (${v}_{3}$) for different configurations of $\mathrm{U}+\mathrm{U}$ collisions at midrapidity for $\sqrt{{s}_{NN}}$ $=$ 200 GeV. The calculations have been done for both the default and string-melting versions of the AMPT model. The results are compared to the corresponding observations from $\mathrm{Au}+\mathrm{Au}$ collisions. We find that for the $\mathrm{U}+\mathrm{U}$ collisions $d{N}_{\mathrm{ch}}/d\ensuremath{\eta}$ at midrapidity is enhanced by about 15$%$--40$%$ depending on the collision and model configuration chosen, compared to $\mathrm{Au}+\mathrm{Au}$ collisions. Within the several configurations studied, the tip-to-tip collisions lead to the largest values of ${N}_{\mathrm{ch}}$, transverse energy (${E}_{\mathrm{T}}$), and $\ensuremath{\langle}{p}_{\mathrm{T}}\ensuremath{\rangle}$. Both $\ensuremath{\langle}{\ensuremath{\varepsilon}}_{2}\ensuremath{\rangle}$ and its fluctuation show a rich centrality dependence, whereas little variation is observed for $\ensuremath{\langle}{\ensuremath{\varepsilon}}_{3}\ensuremath{\rangle}$ and its fluctuations. The $\mathrm{U}+\mathrm{U}$ side-on-side collision configuration provides maximum values of $\ensuremath{\langle}{\ensuremath{\varepsilon}}_{2}\ensuremath{\rangle}$ and minimum values of eccentricity fluctuations, whereas for peripheral collisions and mid-central collisions minimum values of $\ensuremath{\langle}{\ensuremath{\varepsilon}}_{2}\ensuremath{\rangle}$ and the maximum value of eccentricity fluctuations are observed for the body-to-body configuration and the tip-to-tip configuration has a minimum value of $\ensuremath{\langle}{\ensuremath{\varepsilon}}_{2}\ensuremath{\rangle}$ and a maximum value of eccentricity fluctuations for central collisions. The calculated ${v}_{2}$ value closely correlates with the eccentricity in the model. It is smallest for the body-to-body configuration in peripheral and mid-central collisions while it is minimum for the tip-to-tip configuration in central collisions. For peripheral collisions ${v}_{2}$ in $\mathrm{U}+\mathrm{U}$ can be about 40$%$ larger than in $\mathrm{Au}+\mathrm{Au}$ whereas for central collisions it can be a factor of 2 higher depending on the collision configuration. It is also observed that ${v}_{3}$(${p}_{\mathrm{T}}$) is higher for tip-to-tip and body-to-body configurations compared to other systems for the collision centrality studied.

Suppression of high [formula omitted] hadrons in Pb + Pb collisions at LHC

Hadron production and their suppression in Pb+Pb collisions at LHC at a center-of-mass energy of sNN=2.76 TeV are studied within a multiphase transport (AMPT) model whose initial conditions are obtained from the recently updated HIJING 2.0 model. The centrality dependence of charged hadron multiplicity dNch/dη at midrapidity was found quite sensitive to the largely uncertain gluon shadowing parameter sg that determines the nuclear modification of the gluon distribution. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller gluon shadowing to be consistent with dNch/dη data at LHC. With such a constrained parton shadowing, charged hadron and neutral pion production over a wide transverse momenta range are investigated in AMPT. Relative to nucleon–nucleon collisions, the particle yield in central heavy ion collisions is suppressed due to parton energy loss. While the calculated magnitude and pattern of suppression is found consistent with that measured in Au+Au collisions at sNN=0.2 TeV at RHIC, at the LHC energy the suppression is overpredicted which may imply the medium formed at LHC is less opaque than expected from simple RHIC extrapolations. Reduction of the QCD coupling constant αs by ∼30% in the higher temperature plasma formed at LHC as compared to that at RHIC was found to reproduce the measured suppression at LHC.

Low-mass dilepton production in a relativistic Hagedorn-resonance gas model

Dilepton production in the low-invariant-mass region in $p+p$ and Au + Au collisions at $\sqrt{{s}_{NN}}=200$ GeV and in Pb + Au collisions at 158 $A$GeV is studied in the relativistic hadron resonance gas model with Hagedorn mass spectrum that grows exponentially. The model provides good agreement with the observed yield and spectra of stable hadrons and resonances produced in both $p+p$ and nucleus-nucleus collisions. While the measured dilepton yields in $p+p$ collisions are well understood in the model with Dalitz and direct decays of conventional neutral meson sources, a sizable amount of the mesons produced from the Hagedorn states still cannot explain the large enhancement observed in the invariant mass region 200--700 MeV/c${}^{2}$ in Pb + Au and especially in Au + Au collisions. In a multiphase transport (AMPT) model, the dilepton produced from the standard hadronic sources in absence of Hagedorn states underpredicts the data to a larger extent. The study suggests that Hagedorn states in conjugation with in-medium modification of the properties of $\ensuremath{\rho}$ mesons may be essential for the enhanced dilepton production.