String Melting Version Research Articles

Exploring light nuclei production at RHIC and LHC energies with A Multi-Phase Transport model and a coalescence afterburner

In heavy-ion collisions, understanding how light nuclei species are produced can provide insight into the nature of hadronic interactions in extreme conditions. It can also shed light on understanding the matter-antimatter asymmetry and dark matter searches in astrophysical processes. To investigate the production mechanism of light nuclei such as deuteron, triton, and helium-3, we use a naive coalescence afterburner coupled to the well-known “A Multi-Phase Transport model” (AMPT). We focus on studying the production of light nuclei in central Au+Au collisions at different center of mass energies (sNN = 19.6, 39, and 200 GeV) and in Pb+Pb collisions at sNN = 2.76 TeV, at mid-rapidity. We generate events with the string melting version of AMPT, and feed the information of the nucleons with spatial and momentum conditions into the coalescence afterburner. Our study reports differential and integrated yields in transverse momentum (pT) of the light nuclei in different center of mass energies. We also estimate the coalescence parameters (BA) as a function of pT and collision energy for (anti-)deuterons, tritons and helium-3s for Au+Au and Pb+Pb collisions, which are compared to other light nuclei production studies. All results are compared with measurements from the STAR and ALICE experiments.

Study of strange quark density fluctuations in Au + Au Collisions at $$\sqrt{\textrm{s}_{_{\textrm{NN}}}}$$ = 7.7–200 GeV from AMPT Model

The strangeness production is an important observable to study the QCD phase diagram. The yield ratios of strange quark can be helpful to search for the QCD critical end point (CEP) and/or first-order phase transition. In this work, we studied the production of $$K^{\pm }$$ , $$\Xi ^-({\bar{\Xi }}^{+})$$ , $$\phi $$ and $$\Lambda ({\bar{\Lambda }})$$ in Au + Au collisions at $$\sqrt{\textrm{s}_{_{\textrm{NN}}}}$$ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV from A Multi-Phase Transport model with string melting version (AMPT-SM). We calculated the invariant yield of these strange hadrons using a different set of parameters compared to those reported in earlier studies and also by varying the hadronic cascade time ( $$t_{max}$$ ) in the AMPT-SM model. We also calculated the yield ratios, $${\mathcal {O}}_{K^{\pm }-\Xi ^{-}({\bar{\Xi }}^{+})-\phi -\Lambda (\bar{\Lambda })}$$ which are reported as sensitive to the strange quark density fluctuations and found that the AMPT-SM model fails to describe the non-monotonic trend observed by the experimental data. The negative particle ratio are found to be higher than the ratio of positive particles which is consistent with the experimental data. A significant effect is also seen on these ratios by varying the $$t_{max}$$ . For a crossover transition between the Quark-Gluon Plasma (QGP) and hadronic matter, the double yield ratios considered in the present study based on AMPT-SM model do not show any non-monotonic behaviors and thus providing a baseline for the search of CEP, because there is no first-order or second-order phase transition in the AMPT model. The more realistic equation of state based dynamical modeling is still required for the heavy-ion collisions in order to extract the definite physics conclusion about the non-monotonic energy dependence behavior.

First Order Harmonic Flow of Heavy Quarks using a Hybrid Transport Model

We report the investigation of the first order flow of charmed quarks in gold-to-gold collision at the energy near to the speed of the light using A Multi-Phase Transport (AMPT) model. We have used the AMPT string melting version known as partonic interactions option. In this study, we investigated the directed flow v1 of the charmed quark D0 as a function of rapidity. The model predicts the correct sign for D0v1, but the size of the predicted directed flow signal is too small by about an order of magnitude comparing to the real data from the STAR collaboration analysis. The AMPT model shows that the charm quark v1 magnitude is larger than that of the light quarks at large rapidity. This indicates that the charm quarks can retain more information from initial condition than the light quarks.

Transverse momentum spectra and suppression of charged hadrons in deformed Xe–Xe collisions at $$\sqrt{s_{NN}}$$ = 5.44 TeV using HYDJET++ model

In this study, we systematically investigate deformed Xe-Xe collisions at 5.44 TeV center of mass energy. We exploit the Monte Carlo HYDJET++ model to compute transverse momentum ($p_{T}$) distribution, nuclear modification factor $R_{AA}$ and relative suppression in terms of $R_{CP}$ as a function of transverse momentum and centrality of collision of charged hadrons in body-body and tip-tip geometrical configurations, respectively. We have compared HYDJET++ model results to those from ALICE experimental data and AMPT model (String-Melting version) results. Minimum bias Xe-Xe collisions show a suitable match with ALICE experimental data at midrapidity. Average transverse momentum ($\langle p_{T} \rangle$) show strong centrality dependence. The contribution of hard parton scatterings to the total charged hadron yield is discussed. Both nuclear modification factor $R_{AA}$ and relative suppression $R_{CP}$ show a clear dependence on centrality as well as transverse momentum. The charged hadron observables show a clear dependence on the geometrical configuration of the collisions. HYDJET++ model justifies experimental data more closely than the AMPT model.

Single-particle space-momentum angle distribution effect on two-pion HBT correlation with Coulomb interaction

We calculate the HBT radius for with Coulomb interaction using the string melting version of a multiphase transport (AMPT) model. We study the relationship between the single-particle space-momentum angle and the particle sources and discuss HBT radii without single-particle space-momentum correlation. Additionally, we study the Coulomb interaction effect on the numerical connection between the single-particle space-momentum angle distribution and the transverse momentum dependence of .

Thermodynamics of partonic matter in relativistic heavy-ion collisions from a multiphase transport model

Using the string melting version of a multiphase transport model, we focus on the evolution of thermodynamic properties of the central cell of parton matter produced in Au$+$Au collisions ranging from 200 GeV down to 2.7 GeV. The temperature and chemical potentials have been calculated based on both Boltzmann and quantum statistics in order to locate their evolution trajectories in the QCD phase diagram. We demonstrate that the trajectories can depend on many physical factors, especially the finite nuclear thickness at lower energies. However, from the evolution of pressure anisotropy, only partial thermalization can be achieved when the partonic systems reach the predicted QCD phase boundary. It provides some helpful insights to studying the QCD phase structure through relativistic heavy-ion collisions.

Model investigations of the correlation between the mean transverse momentum and anisotropic flow in shape-engineered events

The correlation between the event mean-transverse momentum [pT], and the anisotropic flow magnitude vn, ρ(vn2,[pT]), has been argued to be sensitive to the initial conditions in heavy-ion collisions. We use simulated events generated with the AMPT and EPOS models for Au+Au at sNN = 200 GeV, to investigate the model dependence and the response and sensitivity of the ρ(v22,[pT]) correlator to collision-system size and shape, and the viscosity of the matter produced in the collisions. We find good qualitative agreement between the correlators for the string melting version of the AMPT model and the EPOS model. The model investigations for shape-engineered events as well as events with different viscosity (η/s), indicate that ρ(v22,[pT]) is sensitive to the initial-state geometry of the collision system but is insensitive to sizable changes in η/s for the medium produced in the collisions. These findings suggest that precise differential measurements of ρ(v22,[pT]) as a function of system size, shape, and beam-energy could provide more stringent constraints to discern between initial-state models and hence, more reliable extractions of η/s.

The effect of single-particle space-momentum angle distribution on two-pion HBT correlation in relativistic heavy-ion collisions using a multiphase transport model

Using the string melting version of a multiphase transport (AMPT) model, we analyze the transverse momentum dependence of the HBT radius and the single-pion angle distribution on the transverse plane in central Au+Au collisions at =19.6, 27, 39, 62.4, and 200 GeV. Additionally, a numerical connection between these two phenomena is established with a series of functions. We can estimate the single-pion angle distribution on the transverse plane from HBT analysis.

Estimation of initial-state structures in high-energy heavy-ion collisions using principal component analysis

In high-energy heavy-ion collisions, structures in the initial collision zone are a matter of intense investigation, both from theory and experimental points of views. A large number of models have been developed to represent the initial state of the collision including the Glauber model and color glass condensate (CGC), among others. Another important aspect of the study is to investigate proper observables that will be sensitive to the initial collision zone. In this work, we discuss a formalism to implement the spatial clusters at the partonic level in the string melting version of the ampt model for PbPb collisions at $\sqrt{{s}_{NN}}=200\phantom{\rule{4pt}{0ex}}\mathrm{GeV}$. These clusters are then propagated through the ampt hadronization scheme. Principal component analysis (PCA) has been used on the $\ensuremath{\eta}$, $\ensuremath{\phi}$, and ${p}_{T}$ distributions of the produced charged particles and the eigenvalues have been compared before and after the implementation of the clustering. It is found that for all these three different distributions, all the prominent PCA modes show sensitivity to the clustering. A centrality dependent study has also been performed on those eigenvalues.

Pseudorapidity density, transverse momentum spectra, and elliptic flow studies in Xe-Xe collision systems at sNN=5.44TeV using the HYDJET++ model

In this paper, we present a systematic study of Xe-Xe collisions at 5.44-TeV center-of-mass energy. We employ the Monte Carlo (hydrodynamics plus jets) $\mathrm{HYDJET}++$ model to calculate the pseudorapidity distribution, transverse momentum $({p}_{T})$ spectra and the elliptic flow $({v}_{2})$ of charged hadrons with different parameters in two geometrical configurations: body-body and tip-tip type of Xe-Xe collisions. The kinematic ranges $0<{p}_{T}<50\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}/c$ and $|\ensuremath{\eta}|<0.8$ are considered for our paper. Results are obtained for seven classes of centrality. For comparison, we have shown results from the AMPT model with the string-melting version. The results obtained for Xe-Xe collision systems for minimum bias at midrapidity match well with the experimental data of the ALICE/CMS Collaborations. We observe that the pseudorapidity density depends on the size and geometry of the colliding system. The centrality dependence of average transverse momentum ($\ensuremath{\langle}{p}_{T}\ensuremath{\rangle}$) and average elliptic flow ($\ensuremath{\langle}{v}_{2}\ensuremath{\rangle}$) is also observed. The charged hadron properties also show clear dependence on the geometrical configuration of the collisions. Our model results have been compared to results obtained in the AMPT model. The $\mathrm{HYDJET}++$ model justifies data more closely than the AMPT model, the latter mostly overpredicts the experimental data.

Particle productions and anisotropic flows from the AMPT model for Cu+Au collisions at $$\sqrt{s_{NN}}=200$$ GeV

We use the string melting version of a multi-phase transport (AMPT) model to study Cu+Au collisions at $\sqrt{s_{NN}}=200$ GeV. The rapidity distributions of identified hadrons show asymmetric dependences on rapidity. In addition, elliptic and triangular flows at mid-rapidity from the AMPT model for pions, kaons, and protons agree reasonably with the experimental data up to $p_{T}\sim1$ GeV$/c$. We then investigate the forward/backward asymmetry of $v_2$ and $v_3$. We find that these anisotropic flows are larger on the Au-going side than the Cu-going side, while the asymmetry tends to go away in very peripheral collisions. We also make predictions on transverse momentum spectra of identified hadrons and longitudinal decorrelations of charged particles, where the average decorrelation of elliptic flow in asymmetric Cu+Au collisions is found to be stronger than that in Au+Au collisions.

Hadronization using the Wigner function approach for a multiphase transport model

In the string melting version of a multiphase transport model, the hadronization algorithm has been improved by favoring parton combinations close in not only coordinate space but also momentum space. Formation probabilities of mesons, baryons, and antibaryons during hadronization are determined by their corresponding Wigner functions for the valence parton combinations with no free parameters, and the net baryon, electric, and strangeness charges are conserved during quark coalescence. Effects of the hadronization on anisotropic flows, collision energy dependence of the proton directed flow, relative particle yield ratios, as well as di-hadron correlations in relativistic heavy-ion collisions at the energies ranging from RHIC beam energy scan to LHC have been extensively discussed.

Centrality and transverse momentum dependencies of hadrons in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV and Xe+Xe collisions at $\sqrt{s_{NN}} = 5.44$ TeV from a multi-phase transport model

In this paper, we study and predict the charged-particle pseudorapidity multiplicity density ($\frac{dN_{ch}}{d\eta}$), transverse momentum spectra of identified particles and their ratios in relativistic heavy ion collisions at the Large Hadron Collider (LHC), using the string-melting version of a multi-phase transport (AMPT) model with improved quark coalescence method. Results of the charged-particle pseudorapidity multiplicity density from AMPT model calculations for Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV are compared with the experimental data. Good agreements are generally found between theoretical calculations and experimental data. We predict $\frac{dN_{ch}}{d\eta}$ for Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV at different centralities, and $p_T$ spectra of charged pions, kaons and protons, and their ratios $K/\pi$ and $p/\pi$ in Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV and Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV that are being studied at LHC. The $p_T$ spectra of identified particles in Pb+Pb collisions from the improved AMPT model are compared and found to be consistent with results from the iEBE-VISHNU hybrid model with TRENTo initial condition.

Study of charged particle multiplicity, average transverse momentum and azimuthal anisotropy in Xe+Xe collisions at $\sqrt{s_{NN}} = 5.44$ TeV using AMPT model

We have studied the average charged particle density ($\langle \mathrm{d} \mathrm{N}_{ch}/\mathrm{d} \eta\rangle$), transverse momentum ($ p_{\mathrm{T}}$) spectra, average transverse momentum ($ \langle p_{\mathrm{T}}\rangle$) and azimuthal anisotropies of inclusive charged particles produced in Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV using A Multiphase Transport Model (AMPT), which includes the deformation of 129Xe nucleus. Calculations have been performed with the string melting version of AMPT model and compared with the recent measurements from the ALICE experiment. The model result overestimates the measured $ \langle \mathrm{d} \mathrm{N}_{ch}/\mathrm{d} \eta\rangle$ for central collisions, agrees with the data for mid-central collisions and underestimates the measurements for the peripheral collisions. The centrality dependence of $ \langle p_{\mathrm{T}}\rangle$ of charged particles measured in ALICE is not reproduced by the model results. The calculated elliptic flow ($v_{2}$) from AMPT model is larger than the ALICE measurements in central collisions but is consistent with the data for 30-50% collisions centrality. We find that the model shows a mild centrality dependence of triangular flow and overestimates the ALICE measurements. Within the model framework, we have also studied various collision configurations of Xe nuclei such as body-body, tip-tip, side-side and random. We find a strong dependence of the above observable on the collision configurations.

Two-particle angular correlations in heavy ion collisions from a multiphase transport model

We extend our earlier study on two-particle angular correlations in $pp$ collisions at low transverse momentum ($p_T$) to $p$-Pb, Pb-Pb and Au-Au collisions at RHIC and LHC energies. We mainly use the string melting version of a multiphase transport model with improved quark coalescence for this study. We start from the analysis of $\pi^{\pm}$, $K^{\pm}$ and $p$($\bar{p}$) $p_T$ and rapidity distributions at different centralities. We then focus on two-particle angular correlations in $p$-Pb collisions at $\mathrm{\sqrt{s_{NN}} = 5.02}$ TeV and Pb-Pb collisions at $\mathrm{\sqrt{s_{NN}}= 2.76}$ TeV. For $p$-Pb collisions, a near side depression in the angular correlation is observed for low $p_T$ proton pairs and $\Lambda$ pairs but not for pion pairs or kaon pairs, similar to our earlier finding for $pp$ collisions at $\mathrm{\sqrt{s}= 7}$ TeV. This is also the case for very low multiplicity Pb-Pb and Au-Au collisions. We also find that parton interactions and the improved quark coalescence are mainly responsible for the depression feature in baryon pair angular correlations. However, no such baryon-baryon anti-correlations are observed in Pb-Pb and Au-Au collisions at higher multiplicities. Therefore our results suggest that low $p_T$ baryon-baryon angular anti-correlations have a strong multiplicity dependence.

Directed flow of open charm in Au+Au collisions atsNN=200GeV using a quark coalescence model

The directed flow ($v_{1}$) of open charm meson ($D^{0}$) is studied in Au+Au collisions at $\sqrt{s_{NN}}$ = 200~GeV using A Multi-Phase Transport (AMPT) model framework with partonic interactions (string melting version). Within this framework, it is found that although the initial spatial eccentricity ($\epsilon_{1}$) of charm quark is smaller than light quarks, the charm quark $v_{1}$ magnitude is found to be approximately 7 times larger than that of the light u quark at large rapidity. This indicates that the charm quarks can retain more information from initial condition than the light quarks. We have studied the directed flow of $D^{0}$ as a function of rapidity and transverse momentum using quark coalescence as the mechanism for hadron production. Like charm quark, the $D^{0}$ $v_{1}$ magnitude is found to be about 7 times larger than that of the light ($\pi$) hadrons at large rapidity.

Predictions for isobaric collisions at sNN=200 GeV from a multiphase transport model

The isobaric collisions of $^{96}_{44}$Ru + $^{96}_{44}$Ru and $^{96}_{40}$Zr + $^{96}_{40}$Zr have recently been proposed to discern the charge separation signal of the chiral magnetic effect (CME). In this article, we employ the string melting version of a multiphase transport model to predict various charged-particle observables, including $dN/d\eta$, $p_T$ spectra, elliptic flow ($v_2$), and particularly possible CME signals in Ru + Ru and Zr + Zr collisions at $\sqrt{s_{_{\rm NN}}}$ = 200 GeV. Two sets of the nuclear structure parametrization have been explored, and the difference between the two isobaric collisions appears to be small, in terms of $dN/d\eta$, $p_T$ spectra, and $v_2$ for charged particles. We mimic the CME by introducing an initial charge separation that is proportional to the magnetic field produced in the collision, and study how the final-state interactions affect the CME observables. The relative difference in the CME signal between the two isobaric collisions is found to be robust, insensitive to the final-state interactions.

Improved quark coalescence for a multi-phase transport model

The string melting version of a multi-phase transport model is often applied to high-energy heavy-ion collisions since the dense matter thus formed is expected to be in parton degrees of freedom. In this work we improve its quark coalescence component, which describes the hadronization of the partonic matter to a hadronic matter. We removed the previous constraint that forced the numbers of mesons, baryons, and antibaryons in an event to be separately conserved through the quark coalescence process. A quark now could form either a meson or a baryon depending on the distance to its coalescence partner(s). We then compare results from the improved model with the experimental data on hadron $dN/dy$, $p_{_{\rm T}}$ spectra, and $v_2$ in heavy-ion collisions from $\sqrt{s_{_{\rm NN}}}=62.4$ GeV to $5.02$ TeV. We show that, besides being able to describe these observables for low-$p_{_{\rm T}}$ pions and kaons, the improved model also better describes the low-$p_{_{\rm T}}$ baryon observables in general, especially the baryon $p_{_{\rm T}}$ spectra and antibaryon-to-baryon ratios for multistrange baryons.

Investigating the role of partonic and hadronic dynamics in mass splitting of elliptic anisotropy in p -Pb collisions at sNN=5.02 TeV

The mass ordering of $v_{2}^{hadron}$ is regarded as one of the key signatures of collective behaviour in ultra relativistic heavy ion collisions. This observation has been found to be in compliance with the hydrodynamical response of a strongly interacting system to the initial spatial anisotropy. Flow co-efficients measured with identified particles in p-Pb/d-Au collisions have shown similar mass-splitting of $v_{2}^{hadron}$ indicating towards the presence of collective dynamics in small collision systems. Arguably, small size in the overlap geometry of such colliding systems may not be suitable for hydrodynamical treatment that demands an early thermalization. Studies based on a multi phase transport model suggests that elliptic or triangular anisotropy is primarily due to escape mechanism of partons rather than hydro like collectivity and mass ordering of $v_{2}^{hadron}$ can be generated from coalescence dynamics as implemented in string melting version of AMPT even when parton azimuthal directions are randomized. In this work, studies have been performed on p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV using AMPT model which has been found to explain the elliptic and traingular flow in such a system where escape mechanism is the dominant source of flow generation. We report that the mass splitting of $v_{2}^{hadron}$ can originate independently both at the partonic and hadronic level in the string melting version of the AMPT model.

Two-particle correlations with identified triggers in p -Pb collisions at sNN=5.02 TeV using a multiphase transport model

We report calculations regarding recent measurements of near-side jetlike yields obtained from the angular correlations with pions and protons as trigger particles and unindentified charged hadrons as associated at intermediate transverse momentum (${p}_{T}$). Events generated from the string melting version of a multiphase transport model (AMPT) and HIJING are analyzed to calculate the near-side jetlike yields associated with pion and proton triggers in $p$-Pb collisions at 5.02 TeV. Multiplicity dependence of the near-side jetlike yield is studied by subtracting the so-called long-range correlations. In $A\ensuremath{-}A$ collisions, at intermediate ${p}_{T}$, jetlike yield associated with baryon trigger shows a gradual suppression from peripheral to central events following an initial rise. This suppression was interpreted as a combined effect of parton energy loss due to jet-quenching and particle production via quark recombination generating anomalous baryon to meson enhancement at intermediate ${p}_{T}$. Similar enhancement was also observed in small collision systems like $p$-Pb at CERN Large Hadron Collider (LHC) energy. In absence of any significant jet-medium interplay in small systems, trigger species dependence of the associated jetlike yield may be attributed dominantly to the dynamics of quark coalescence and/or radial flow. The multiplicity evolution of the trigger species dependence of jetlike yields, obtained from AMPT, found to be completely different from the same when extracted from a hydrodynamical model, EPOS. With the data available at LHC, these results may be tested to achieve further insight on the underlying dynamics in small collision systems, and the mechanism of hadronization at intermediate ${p}_{T}$ as well.