arXiv subjects

Zi-Wei Lin

Publications and source records attributed to Zi-Wei Lin.

66 records · Page 4Linked to original sources

arXiv2016

Origin of the mass splitting of elliptic anisotropy in a multiphase transport model

The mass splitting of elliptic anisotropy ($v_2$) at low transverse momentum is considered as a hallmark of hydrodynamic collective flow. We investigate a multiphase transport (AMPT) model where the $v_2$ is mainly generated by an anisotropic escape mechanism, not of the hydrodynamic flow nature, and where mass splitting is also observed. We demonstrate that the $v_2$ mass splitting in AMPT is small right after hadronization (especially when resonance decays are included); the mass splitting mainly comes from hadronic rescatterings, even though their contribution to the overall charged hadron $v_2$ is small. These findings are qualitatively the same as those from hybrid models that combine hydrodynamics with a hadron cascade. We further show that there is no qualitative difference between heavy ion collisions and small system collisions. Our results indicate that the $v_2$ mass splitting is not a unique signature of hydrodynamic collective flow and thus cannot distinguish whether the elliptic flow is generated mainly from hydrodynamics or the anisotropic parton escape.

nucl-th↗

arXiv2016

Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models

We trace the development of elliptic anisotropy ($v_2$) via parton-parton collision history in two transport models. The parton $v_2$ is studied as a function of the number of collisions of each parton in Au+Au and $d$+Au collisions at $\sqrt{s_{_{\rm NN}}}=200$ GeV. It is found that the majority of $v_2$ comes from the anisotropic escape probability of partons, with no fundamental difference at low and high transverse momenta. The contribution to $v_2$ from hydrodynamic-type collective flow is found to be small. Only when the parton-parton cross-section is set unrealistically large does this contribution start to take over. Our findings challenge the current paradigm emerged from hydrodynamic comparisons to anisotropy data.

nucl-th↗

arXiv2015

Elliptic Anisotropy $v_2$ May Be Dominated by Particle Escape instead of Hydrodynamic Flow

It is commonly believed that azimuthal anisotropies in relativistic heavy ion collisions are generated by hydrodynamic evolution of the strongly interacting quark-gluon plasma. Here we use transport models to study how azimuthal anisotropies depend on the number of collisions that each parton suffers. We find that the majority of $v_2$ comes from the anisotropic escape of partons, not from the parton collective flow, for semi-central Au+Au collisions at 200A GeV. As expected, the fraction of $v_2$ from the anisotropic particle escape is even higher for smaller systems such as d+Au. Our transport model results also confirm that azimuthal anisotropies would be dominated by hydrodynamic flow at unrealistically-high parton cross sections. Our finding thus naturally explains the similarity of azimuthal anisotropies in small and large systems; however, it presents a challenge to the paradigm of anisotropic flow.

nucl-th↗

arXiv2015

Studying re-scattering effect in heavy-ion collision through $K^*$ Production

We have studied the $K^*$ production within A Multi-Phase Transport model (AMPT) for Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV to understand the hadronic re-scattering effect on the measured yields of the resonance. The hadronic re-scattering of the $K^*$ decay daughter particles ($π$ and K) will alter their momentum distribution thereby making it difficult to reconstruct the $K^*$ signal through the invariant mass method. An increased hadronic re-scattering effect thus leads to a decrease in the reconstructed yield of $K^*$ in heavy-ion collisions. Through this simulation study we argue that a decrease in $K^*$/K ratio with increase in collision centrality necessarily reflects the hadronic re-scattering effect. Since the re-scattering occurs in the hadronic phase and $K^*$ has a lifetime of 4 fm/c, we present a toy model based discussion on using measured $K^*$/K to put a lower limit on the hadronic phase lifetime in high energy nuclear collisions.

nucl-ex↗

arXiv2014

Evolution of transverse flow and effective temperatures in the parton phase from a multi-phase transport model

I study the space-time evolution of transverse flow and effective temperatures in the dense parton phase with the string melting version of a multi-phase transport model. Parameters of the model are first constrained to reproduce the bulk data on the rapidity density, $p_{\rm T}$ spectrum and elliptic flow at low $p_{\rm T}$ for central and mid-central Au+Au collisions at $200A$ GeV and Pb+Pb collisions at $2760A$ GeV. I then calculate the transverse flow and effective temperatures in volume cells within mid-spacetime-rapidity $|η|<1/2$. I find that the effective temperatures extracted from different variables, which are all evaluated in the rest frame of a volume cell, can be very different; this indicates that the parton system in the model is not in full chemical or thermal equilibrium locally, even after averaging over many events. In particular, the effective temperatures extracted from the parton energy density or number density are often quite different than those extracted from the parton mean $p_{\rm T}$ or mean energy. For these collisions in general, effective temperatures extracted from the parton energy density or number density are higher than those extracted from the parton mean $p_{\rm T}$ in the inner part of the overlap volume, while the opposite occurs in the outer part of the overlap volume. I argue that this indicates that the dense parton matter in the inner part of the overlap volume is over-populated; I also find that all cells with energy density above 1 GeV/fm$^3$ are over-populated after a couple of fm/$c$.

nucl-th↗

arXiv2012

Multiplicity, average transverse momentum and azimuthal anisotropy in U+U collisions at $\sqrt{s_{NN}}$ = 200 GeV using AMPT model

Using a multi-phase transport (AMPT) model that includes the implementation of deformed Uranium nuclei, we have studied the centrality dependence of the charged particle multiplicity, , eccentricity (e2), triangularity (e3), their fluctuations, elliptic flow (v2) and triangular flow (v3) for different configurations of U+U collisions at midrapidity for \sqrt(s_NN)=200 GeV. The results are compared to the corresponding observations from Au+Au collisions. We find that for the U+U collisions the dNch/dηat midrapidity is enhanced by about 15-40% depending on the collision and model configuration chosen, compared to Au+Au collisions. The tip-to-tip collisions leads to the largest values of Nch,transverse energy (ET) and . The and its fluctuation shows a rich centrality dependence, whereas not much variations are observed for and its fluctuations. The U+U side-on-side collision configuration provides maximum values of and minimum values of eccentricity fluctuations, whereas for peripheral collisions and mid-central collisions minimum values of and maximum value of eccentricity fluctuations are observed for body-to-body configuration and the tip-to-tip configuration has minimum value of and maximum value of eccentricity fluctuations for central collisions. The calculated v2 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 tip-to-tip configuration in central collisions. For peripheral collisions the v2 in U+U can be about 40% larger than in Au+Au whereas for central collisions it can be a factor 2 higher depending on the collision configuration. It is also observed that the v3(pT) is higher for tip-to-tip and body-to-body configurations compared to other systems for the collision centrality studied.

nucl-ex↗

arXiv2012

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, providing thus a plausible explanation of the different elliptic flows between $p$ and ${\bar p}$, $K^+$ and $K^-$, and $π^+$ and $π^-$ observed in recent Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC).

nucl-th↗

arXiv2011

Quark Coalescence with Quark Number Conservation and the Effect on Quark-Hadron Scaling

We develop a new formulation of the quark coalescence model by including the quark number conservation in order to describe the hadronization of the bulk of the quark-gluon plasma. The scalings between hadron and quark phase space distributions are shown to depend on the transverse momentum. For hard quarks, our general scalings reproduce the usual quadratic scaling relation for mesons and the cubic scaling relation for baryons. For softer quarks, however, the inclusion of the quark number conservation leads to a linear scaling for the hadron species that dominates the quark number of each flavor, while the scalings of non-dominant hadrons depend on the coalescence dynamics. For charm mesons, we find that the distribution of soft $D$ mesons does not depend on the light quark distribution but the distribution of soft $J/ψ$ mesons is inversely correlated to the light quark distribution.

nucl-th↗

arXiv2009

Deuteron production and elliptic flow in relativistic heavy ion collisions

The hadronic transport model \textsc{art} is extended to include the production and annihilation of deuterons via the reactions $BB \leftrightarrow dM$, where $B$ and $M$ stand for baryons and mesons, respectively, as well as their elastic scattering with mesons and baryons in the hadronic matter. This new hadronic transport model is then used to study the transverse momentum spectrum and elliptic flow of deuterons in relativistic heavy ion collisions, with the initial hadron distributions after hadronization of produced quark-gluon plasma taken from a blast wave model. The results are compared with those measured by the PHENIX and STAR Collaborations for Au+Au collisions at $\sqrt{s_{NN}^{}} = 200$ GeV, and also with those obtained from the coalescence model based on freeze-out nucleons in the transport model.

nucl-th↗

arXiv2005

A Multi-Phase Transport Model for Relativistic Heavy Ion Collisions

We describe in detail how the different components of a multi-phase transport (AMPT) model, that uses the Heavy Ion Jet Interaction Generator (HIJING) for generating the initial conditions, Zhang's Parton Cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and A Relativistic Transport (ART) model for treating hadronic scatterings, are improved and combined to give a coherent description of the dynamics of relativistic heavy ion collisions. We also explain the way parameters in the model are determined, and discuss the sensitivity of predicted results to physical input in the model. Comparisons of these results to experimental data, mainly from heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC), are then made in order to extract information on the properties of the hot dense matter formed in these collisions.

nucl-th↗

arXiv2004

Partonic effects on higher-order anisotropic flows in relativistic heavy-ion collisions

Higher-order anisotropic flows $v_{4}$ and $v_{6}$ in heavy ion collisions at the Relativistic Heavy Ion Collider are studied in a multiphase transport model that has previously been used successfully for describing the elliptic flow $v_2$ in these collisions. We find that the same parton scattering cross section of about 10 \textrm{mb} used in explaining the measured $v_2$ can also reproduce the recent data on $v_{4}$ and $v_{6}$ from Au + Au collisions at $\sqrt{s}=200$ \textrm{AGeV}. It is further found that the $% v_{4}$ is a more sensitive probe of the initial partonic dynamics in these collisions than $v_{2}$. Moreover, higher-order parton anisotropic flows are nonnegligible and satisfy the scaling relation $v_{n,q}(p_{T})\sim v_{2,q}^{n/2}(p_{T})$, which leads naturally to the observed similar scaling relation among hadron anisotropic flows when the coalescence model is used to describe hadron production from the partonic matter.

nucl-th↗

arXiv2002

J/psi production in relativistic heavy ion collisions from a multi-phase transport model

Using A Multi-Phase Transport (AMPT) model, we study J/psi production from interactions between charm and anti-charm quarks in initial parton phase and between D and Dbar mesons in final hadron phase of relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC). Including also the inverse reactions of J/psi absorption by gluons and light mesons, we find that the net number of J/psi from the parton and hadron phases is smaller than that expected from the superposition of initial nucleon-nucleon collisions, contrary to the J/psi enhancement predicted by the kinetic formation model. The production of J/psi is further suppressed if one includes the color screening effect in the parton phase. We have also studied the dependence of J/psi production on the charm quark mass and the effective charm meson mass.

nucl-th↗