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In this work, we investigate the collective flow development in high energy proton proton (pp) collisions with a multiphase transport model (AMPT) based on PYTHIA8 initial conditions with a sub-nucleon structure. It is found that the PYTHIA8 based AMPT model can reasonably describe both the charged hadron productions and elliptic flow experimental data measured in pp collisions at $\sqrt{s}=13$ TeV. By turning on the parton and hadron rescatterings in AMPT separately, we find that the observed collective flow in pp collisions is largely developed during the parton evolutions, while no significant flow effect can be generated with the pure hadronic rescatterings. It is also shown that the parton escape mechanism is important for describing both the magnitude of the two-particle cumulant and the sign of the four-particle cumulants. We emphasize that the strong mass ordering of the elliptic flow results from the coalescence process in the transport model can thus be regarded as unique evidence related to the creation of deconfined parton matter in high energy pp collisions.

The impact of nuclear structure has garnered considerable attention in the high-energy nuclear physics community in recent years. This work focuses on studying the potential nuclear cluster structure in $^{16}\text{O}$ nuclei using anisotropic flow observables in $\rm O+O$ collisions at 200 GeV. Employing an improved AMPT model with various cluster structure configurations, we find that an extended effective parton formation time is necessary to align with the recent STAR experimental data. In addition, we reveal that the presented flow observables serve as sensitive probes for differentiating configurations of $\alpha$-clustering of $^{16}\text{O}$ nuclei. The systematic AMPT calculations presented in this paper, along with comprehensive comparisons to forthcoming experimental measurements at RHIC and the LHC, pave the way for a novel approach to investigate the $\alpha$-clustering structure of $^{16}\text{O}$ nuclei using $\rm O+O$ collisions at the ultra-relativistic energies., Comment: 10 pages, 11 figures

We study the production of $D^0$ meson in $p$+$p$ and $p-$Pb collisions using the improved AMPT model considering both coalescence and independent fragmentation of charm quarks after the Cronin broadening are included. After a detailed discussion of the improvements implemented in the AMPT model for heavy quark production, we show that the modified AMPT model can provide good description of $D^0$ meson spectra in $p-$Pb collisions, the $Q_{\rm pPb}$ data at different centrality and $R_{\rm pPb}$ data in both mid- and forward (backward) rapidities. We also studied the effects of nuclear shadowing and parton cascade on the rapidity dependence of $D^{0}$ meson production and $R_{\rm pPb}$. Our results indicate that having the same strength of the Cronin (i.e $\delta$ value) obtained from the mid-rapidity data leads to a considerable overestimation of the $D^0$ meson spectra and $R_{\rm pPb}$ data at high $p_{T}$ in the backward rapidity. As a result, the $\delta$ is determined via a $\chi^2$ fitting of the $R_{\rm pPb}$ data across various rapidities. This work lays the foundation for a better understanding of cold-nuclear-matter (CNM) effects in relativistic heavy-ion collisions.

Fluctuations of conserved charges, such as the net-baryon number fluctuations, are influenced by different dynamical evolution processes. In this paper, we investigate the influence of hadronic rescatterings on different orders of cumulants of the net-baryon number distribution. At the start of hadronic rescatterings, we introduce net-baryon number distributions reconstructed based on net-baryon cumulants of different orders obtained from computation in functional renormalization group (FRG), where the distributions were constructed using the maximum entropy method. This way we introduce the critical fluctuations of Quantum Chromodynamics (QCD) into the AMPT model. Firstly, we find that hadronic rescatterings have distinct effects on cumulant ratios of different orders for the net-baryon number. Secondly, we observe that the effect of hadronic rescatterings is more significant for critical fluctuations than dynamical fluctuations, because the two-, three- and four-particle correlation functions due to critical fluctuations are weakened more significantly by hadronic rescatterings., Comment: 10 pages, 8 figures

We present the first study of collectivity inside jets with high charged multiplicity $N^j_{\rm ch}$ in proton-proton collisions at the Large Hadron Collider. By incorporating final-state partonic and hadronic interactions through cascade models among jet shower partons and final hadrons, we investigate and compare to the CMS experimental data on multiplicity distribution, pseudorapidity distribution, and elliptic anisotropy coefficient $v^{j}_2$ of two-particle correlations within the jet. We show that final-state partonic interactions are essential for producing the flow-like long-range correlation, which leads to the enhanced tail in the $N^j_{\rm ch}$ dependence of $v^{j}_2$ above the non-flow correlation from jet parton showering at high multiplicities ($N^j_{\rm ch}\gtrsim70$) as observed in the CMS experimental data. In addition, we provide predictions for the pseudorapidity-gap dependence of $v^{j}_2$ that can be tested in future experimental measurements., Comment: 6 pages, 4 fiures

The rapidity-odd directed flows ($v_{\rm 1}$) of identified hadrons are expected to follow the coalescence sum rule when the created matter is initially in parton degrees of freedom and then hadronizes through quark coalescence. A recent study has considered the $v_{\rm 1}$ of produced hadrons that do not contain $u$ or $d$ constituent quarks. It has constructed multiple hadron sets with a small mass difference but given difference in electric charge $\Delta q$ and strangeness $\Delta S$ between the two sides, where a nonzero and increasing $\Delta v_{\rm 1}$ with $\Delta q$ has been proposed to be a consequence of electromagnetic fields. In this study, we examine the consequence of coalescence sum rule on the $\Delta v_{\rm 1}$ of the hadron sets in the absence of electromagnetic fields. We find that in general $\Delta v_{\rm 1} \neq 0$ for a hadron set with nonzero $\Delta q$ and/or $\Delta S$ due to potential $v_{\rm 1}$ differences between $\bar u$ and $\bar d$ and between $s$ and $\bar s$ quarks. We further propose methods to extract the coefficients for the $\Delta q$- and $\Delta S$-dependences of the direct flow difference, where a nonzero constant term would indicate the breaking of the coalescence sum rule. The extraction methods are then demonstrated with transport model results., Comment: 7 pages, 4 figures

It has been difficult to reconcile the experimental data on the $D^0$ meson nuclear modification factor and elliptic flow in $p-$Pb collisions at LHC energies. Here we study these observables with the string melting version of a multi-phase transport model, which has been improved with the implementation of the Cronin effect (or transverse momentum broadening) and independent fragmentation for charm quarks. Using a strong Cronin effect allows us to provide the first simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$ data at $p_{\rm T} \leq$ 8 GeV$/c$. The model also provides a reasonable description of the $D^0$ meson $p_{\rm T}$ spectra and the low-$p_{\rm T}$ (below $\sim$ 2 GeV$/c$) charged hadron spectra in $p+p$ and $p-$Pb collisions as well as $R_{\rm pA}$ and $v_2$ in $p-$Pb collisions. We find that both parton scatterings and the Cronin effect are important for the $D^0$ meson $R_{\rm pA}$, while parton scatterings are mostly responsible for the $D^0$ meson $v_2$. Our results indicate that it is crucial to include the Cronin effect for the simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$. Since the Cronin effect is expected to grow with the system size, this work implies that the Cronin effect could also be important for heavy hadrons in large systems., Comment: 6 pages, 3 figures, proceedings for the 11th International Conference on Hard and Electromagnetic Probes of High-Energy Nuclear Collisions

In nuclear collisions at RHIC energies, an excess of $\Omega$ hyperons over $\bar{\Omega}$ is observed, indicating that $\Omega$ carries a net baryon number despite $s$ and $\bar{s}$ quarks being produced in pairs. The baryon number in $\Omega$ could have been transported from the incident nuclei and/or produced in baryon-pair production of $\Omega$ with other types of anti-hyperons, such as $\bar{\Xi}$. To investigate these two scenarios, we propose to measure correlations between $\Omega$ and $K$, as well as between $\Omega$ and anti-hyperons. We will use two versions, the default and string-melting, of a multiphase transport (AMPT) model to illustrate the method to measure the correlation and to demonstrate the general shape of the correlation. We will present the $\Omega$-hadron correlations from simulated $\mathrm{Au}$+$\mathrm{Au}$ collisions at $\sqrt{s_{NN}} = 7.7$ and $14.6 \ \mathrm{GeV}$, and discuss the dependence on collision energy and on the hadronization scheme in these two AMPT versions. These correlations can be used to explore the mechanism of baryon number transport and the effects of baryon number and strangeness conservation in nuclear collisions.

Although strange quarks are produced in $s\bar{s}$ pairs, the ratio of $\Omega^{-}$ to ${\bar{\Omega}}^{+}$ is greater than one in heavy-ion collisions at lower RHIC energies. Thus the produced $\Omega$ hyperons must carry net baryon quantum numbers from the colliding nuclei. We present results of $K$-$\Omega$ correlations from AMPT model simulations of Au+Au collisions at $\sqrt{s_{NN}}$ = 14.6 GeV, to probe dynamics for baryon number transport to mid-rapidities at this beam energy. We use both the default and string-melting versions to illustrate how hadronization schemes of quark coalescence and string fragmentations could leave imprints on such correlations. Implications on the measurements of these correlations with the STAR experiment at RHIC will also be discussed., Comment: submitted to SQM 2022 conference proceedings

At low to moderate collision energies where the parton formation time $\tau_F$ is not small compared to the nuclear crossing time, the finite nuclear thickness significantly affects the energy density $\epsilon(t)$ and net conserved-charge densities such as the net-baryon density $n_B(t)$ produced in heavy ion collisions. As a result, at low to moderate energies the trajectory in the QCD phase diagram is also affected by the finite nuclear thickness. Here, we first discuss our semi-analytical model and its results on $\epsilon(t)$, $n_B(t)$, $n_Q(t)$, and $n_S(t)$ in central Au+Au collisions. We then compare the $T(t)$, $\mu_B(t)$, $\mu_Q(t)$, and $\mu_S(t)$ extracted with the ideal gas equation of state (EoS) with quantum statistics to those extracted with a lattice QCD-based EoS. We also compare the $T-\mu_B$ trajectories with the RHIC chemical freezeout data. Finally, we discuss the effect of transverse flow on the trajectories., Comment: 4 pages, proceedings for the 20th International Conference on Strangeness in Quark Matter (SQM2022)

It has been a challenge to understand the experimental data on both the nuclear modification factor and elliptic flow of $D^0$ mesons in $p-$Pb collisions at LHC energies. In this work, we study these collisions with an improved multi-phase transport model. By including the Cronin effect (i.e., transverse momentum broadening) and independent fragmentation for charm quarks, we provide the first simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$ data at $p_{\rm T} \leq 8$ GeV$/c$. The model also reasonably describes the $D^0$ meson $p_{\rm T}$ spectra and the low-$p_{\rm T}$ charged hadron spectra, $R_{\rm pA}$ and $v_2$. Our results show that both parton interactions and the Cronin effect are important for the $D^0$ meson $R_{\rm pA}$, while parton interactions are mostly responsible for the $D^0$ meson $v_2$. It is thus essential to include the Cronin effect for the simultaneous description of the $D^0$ meson $R_{\rm pA}$ and $v_2$. This work implies that the Cronin effect could also be important for heavy hadrons in large systems., Comment: 6 pages, 3 figures, the Cronin strength is reduced, comparisons with the HVQMNR code are corrected

The finite nuclear thickness affects the energy density $\epsilon(t)$ and conserved-charge densities such as the net-baryon density $n_B(t)$ produced in heavy ion collisions. While the effect is small at high collision energies where the Bjorken energy density formula for the initial state is valid, the effect is large at low collision energies, where the nuclear crossing time is not small compared to the parton formation time. The temperature $T(t)$ and chemical potentials $\mu(t)$ of the dense matter can be extracted from the densities for a given equation of state (EOS). Therefore, including the nuclear thickness is essential for the determination of the $T$-$\mu_B$ trajectory in the QCD phase diagram for relativistic nuclear collisions at low to moderate energies such as the RHIC-BES energies. In this proceeding, we will first discuss our semi-analytical method that includes the nuclear thickness effect and its results on the densities $\epsilon(t), n_B(t), n_Q(t)$, and $n_S(t)$. Then, we will show the extracted $T(t), \mu_B(t), \mu_Q(t)$, and $\mu_S(t)$ for a quark-gluon plasma using the ideal gas EOS with quantum or Boltzmann statistics. Finally, we will show the results on the $T$-$\mu_B$ trajectories in relation to the possible location of the QCD critical end point. This semi-analytical model provides a convenient tool for exploring the trajectories of nuclear collisions in the QCD phase diagram., Comment: 7 pages, 5 figures, proceedings for the 37th Winter Workshop on Nuclear Dynamics

The shear viscosity $\eta$ of a quark-gluon plasma in equilibrium can be calculated analytically using multiple methods or numerically using the Green-Kubo relation. It has been realized, which we confirm here, that the Chapman-Enskog method agrees well with the Green-Kubo result for both isotropic and anisotropic two-body scatterings. We then apply the Chapman-Enskog method to study the shear viscosity of the parton matter from a multi-phase transport model. In particular, we study the parton matter in the center cell of central and midcentral Au+Au collisions at $200A$ GeV and Pb+Pb collisions at $2760A$ GeV, which is assumed to be a plasma in thermal equilibrium but partial chemical equilibrium. As a result of using a constant Debye mass or cross section $\sigma$ for parton scatterings, the $\eta/s$ ratio increases with time (as the effective temperature decreases), contrary to the trend preferred by Bayesian analysis of the experimental data or pQCD results that use temperature-dependent Debye masses. At $\sigma=3$ mb that enables the transport model to approximately reproduce the elliptic flow data of the bulk matter, the average $\eta/s$ of the parton matter in partial equilibrium is found to be very small, between one to two times $1/(4\pi)$., Comment: Added subsection on the Green-Kubo relation to better match published version

Recent beam energy scan (BES) experiments at RHIC by the STAR Collaboration (PLB {\bf 827},137003 (2022) and PRL {\bf 128}, 202303 (2022)) found that hadronic interactions dominate the collective flow and the proton cumulant ratios are driven by baryon number conservation in a region of high baryon density in $\sqrt{s_{NN}}$ = 3 GeV Au+Au reactions, indicating the dense medium formed in such collisions is likely hadronic matter. Within an updated ART (A Relativistic Transport) model with momentum dependent isoscalar and isovector single-nucleon mean-field potentials corresponding to different symmetry energies at suprasaturation densities, the $n/p$, $\pi^{-}/\pi^{+}$, $K_{s}^{0}/K^{+}$, $\Sigma^{-}/\Sigma^{+}$ and $\Xi^{-}/\Xi^{0}$ ratios are studied for central Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV where the maximum central density reaches about $(3.6\sim 4.0)\rho_0$. The doubly strange $\Xi^{-}/\Xi^{0}$ ratio is found to have the strongest sensitivity to the variation of high-density nuclear symmetry energy. Thus, the $\Xi^{-}/\Xi^{0}$ ratio in relativistic heavy-ion reactions at $\sqrt{s_{NN}} \sim 3$ GeV may help probe sensitively the poorly known symmetry energy of dense neutron-rich matter critically important for understanding various properties of neutron stars., Comment: With minor revisions. Phys. Rev. C in press

During the early development of Quantum Chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the midrapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near midrapidity indeed follows an exponential distribution with a slope of $-0.61\pm0.03$ as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes ($\gamma+p$/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and $e+p$/A collisions at the EIC., Comment: 12 pages, 6 figures, Submitted to EPJC

The near-side depression in two-proton or two-antiproton azimuthal correlations in $pp$ collisions at $\sqrt{s}=$7 TeV has been observed experimentally and then qualitatively reproduced in our earlier studies with a multi-phase transport model. In this study, we further investigate the origin of the depression feature in two-baryon correlations in small collision systems. We find that the initial parton-level spatial correlation, a finite expansion in the parton stage, and quark coalescence are important ingredients leading to the near-side depression. In particular, we find that a finite expansion of the parton system leads to a finite space-momentum correlation at hadronization, which then converts the near-side depression in the coordinate space to that in the momentum space. These results suggest that a partonic matter with a finite lifetime is formed in the pp collisions. Further studies are needed to determine whether the partonic matter is near local equilibrium and can thus be called a QGP or far away from local equilibrium., Comment: 7 pages, 4 figures

Flow-like signals including the ridge structure observed in small collision systems that are similar to those in large collision systems have led to questions about the onset of collectivity in nuclear collisions. In this study, we use the string melting version of a multi-phase transport (AMPT) model with or without the sub-nucleon geometry for the proton to study multiparticle cumulants in $p+p$ collisions at 13 TeV. Both versions of the model produce negative $c_{2}\{4 \}$ values at high multiplicities. In addition, the dependence of $c_{2}\{4 \}$ on the parton cross section is non-monotonous, where only a range of parton cross section values leads to negative $c_{2} \{4 \}$. Furthermore, the AMPT model with sub-nucleon geometry better describes the multiplicity dependence of $c_{2} \{4 \}$, demonstrating the importance of incorporating the sub-nucleon geometry in studies of small collision systems., Comment: 8 pages, 8 figures (added two figures)

We extend a semi-analytical model that includes the finite nuclear thickness to calculate the energy density $\epsilon(t)$ and conserved-charge densities including the net-baryon density $n_{\rm _B}(t)$ produced at mid-spacetime-rapidity in central Au+Au collisions. Assuming the formation of a quark-gluon plasma with an ideal gas equation of state of either quantum or Boltzmann statistics or with a lattice QCD-based equation of state, we extract the temperature $T$ and chemical potentials $\mu_{\rm _B}$, $\mu_{\rm _Q}$ and $\mu_{\rm _S}$ as functions of time. This then allows us to semi-analytically calculate the $T-\mu_{\rm _B}$ trajectory of relativistic nuclear collisions in the QCD phase diagram, which should benefit the studies of high density physics including the search for the critical end point. This model is also useful for exploring the trajectories in the more general $T-\mu_{\rm _B}-\mu_{\rm _Q}-\mu_{\rm _S}$ QCD phase space., Comment: 19 pages, 12 figures, corrected typo in Eq.(41) and citations to Figs.11-12

A multi-phase transport (AMPT) model was constructed as a self-contained kinetic theory-based description of relativistic nuclear collisions as it contains four main components: the fluctuating initial condition, a parton cascade, hadronization, and a hadron cascade. Here, we review the main developments after the first public release of the AMPT source code in 2004 and the corresponding publication that described the physics details of the model at that time. We also discuss possible directions for future developments of the AMPT model to better study the properties of the dense matter created in relativistic collisions of small or large systems., Comment: 31 pages, 20 figures

Recently the splitting of elliptic flow $v_2$ at finite rapidities has been proposed as a result of the global vorticity in non-central relativistic heavy ion collisions. In this study, we find that this left-right (i.e., on opposite sides of the impact parameter axis) splitting of the elliptic flow at finite rapidities is a result of the non-zero directed flow $v_1$, with the splitting magnitude $\approx 8v_1(1-3v_2)/(3\pi)$. We also use a multi-phase transport model, which automatically includes the vorticity field and flow fluctuations, to confirm the $v_2$ splitting. In addition, we find that the analytical expectations for the $v_2$ splitting work for the raw $v_2$ and $v_1$ (i.e., before event plane resolutions are applied) measured relative to either the first- or second-order event plane. Since the $v_2$ splitting is mostly driven by $v_1$, it vanishes at zero transverse momentum ($p_{\rm T}$), and its magnitude and sign may have non-trivial dependencies on $p_{\rm T}$, centrality, collision energy, and hadron species., Comment: 8 pages, 10 figures; v2 splitting results are now shown to 2nd order, added a new section with six figures to show results from the event plane method