Further developments of a multi-phase transport model for relativistic nuclear collisions

NUCL SCI TECH (2021)32:113 https://doi.org/10.1007/s41365-021-00944-5 (0123456789().,-volV) (0123456789().,-volV) Further developments of a multi-phase transport model for relativistic nuclear collisions Zi-Wei Lin1 • Liang Zheng2 Received: 15 June 2021 / Revised: 2 August 2021 / Accepted: 26 August 2021 Ó The Author(s) 2021 Abstract 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. Keywords QGP  Transport model  Heavy-ion collisions 1 Introduction In high energy heavy ion collisions [1], a hot and dense matter made of parton degrees of freedom, the quark-gluon plasma (QGP), has been expected to be created [2]. Experimental data from the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) [3–8] Z.-W.L. is supported in part by the National Science Foundation under Grant No. PHY-2012947. L.Z. is supported in part by the National Natural Science Foundation of China under Grant No. 11905188. & Zi-Wei Lin 1 Department of Physics, East Carolina University, Greenville, NC 27858, USA 2 School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan 430074, China strongly indicate that the QGP is indeed created in heavy ion collisions at high energies [9]. Comprehensive comparisons beween the experimental data and theoretical models are essential for the extraction of key properties of the high density matter, including the structure of the QCD phase diagram at high temperature and/or high net-baryon density. Many theoretical models including transport models [10–14], hydrodynamic models [15–18], and hybrid models [19–21] have been constructed to simulate and study the phase space evolution of the QGP. A multi-phase transport (AMPT) model [13] is one such model. The AMPT model aims to apply the kinetic theory approach to describe the evolution of heavy-ion collisions as it contains four main components: the fluctuating initial condition, partonic interactions, hadronization, and hadronic interactions. The default version of the AMPT model [11, 22] was first constructed. Its initial condition is based on the Heavy Ion Jet INteraction Generator (HIJING) twocomponent model [23, 24], then minijet partons enter the parton cascade and eventually recombine with their parent strings to hadronize via the Lund string fragmentation [25]. The default AMPT model can well describe the rapidity distributions and transverse momentum (pT ) spectra of identified particles observed in heavy ion collisions at SPS and RHIC. However, it significantly underestimates the elliptic flow (v2 ) at RHIC. Since the matter created in the early stage of high energy heavy ion collisions is expected to have a very high energy density and thus should be in parton degrees of freedom, the string melting version of the AMPT (AMPT-SM) model [26] was then constructed, where all the excited strings from a heavy ion collision are converted into partons and a spatial quark coalescence model is invented to describe the hadronization process. String melting increases the parton density and produces an over-populated 123 113 Page 2 of 33 partonic matter [27], while quark coalescence further enhances the elliptic flow of hadrons [26, 28]. As a result, the string melting AMPT model is able to describe the large elliptic flow in Au?Au collisions at RHIC energies with a rather small parton cross section [26, 29]. The source code of the AMPT model was first publicly released online around April 2004, and a subsequent publication [13] provided detailed descriptions of the model such as the included physics processes and modeling assumptions. The AMPT model has since been widely used to simulate the evolution of the dense matter created in high energy nuclear collisions. In particular, the string melting version of the AMPT model [13, 26] can well describe the anisotropic flows and particle correlations in collisions of small or large systems at both RHIC and LHC energies [13, 26, 30–33]. The AMPT model is also a useful test bed of different ideas. For example, the connection between the triangular flow and initial geometrical fluctuations was discovered with the help of AMPT simulations [34], and the model has also been applied to studies of vorticity and polarization in heavy ion collisions [35–37]. Experimental data from heavy ion collisions fit with hydrodynamics-inspired models suggest that particles are locally thermalized and possess a common radial flow velocity [38]. Large momentum anisotropies such as the elliptic flow [39] have been measured in large collision systems, as large as the hydrodynamics predictions [7, 40]. This suggests that the collision system is strongly interacting and close to local thermal equilibrium [9]. Transport models can also generate large anisotropic flows. The string melting AMPT model [13, 26] can describe the large anisotropic flows with a rather small parton cross section of  3 mb [26] and the flow enhancement from quark coalescence [26, 28, 29, 41, 42]. Without the quark coalescence, a pure parton transport for minijet gluons requires an unusually large parton cross section of  40  50 mb [29, 43] for the freezeout gluons to have a similar magnitude of elliptic flow as charged hadrons in the experiments. This minijet gluon system, despite a factor of  2:5 lower parton multiplicity at mid-rapidity, has a factor of  6 smaller mean free path than the string melting AMPT model for 200A GeV Au?Au collisions at impact parameter b ¼ 8 fm [29]. In general, for large systems at high energies, transport models tend to approach hydrodynamics since the average number of collisions per particle is large and thus the bulk matter is close to local equilibrium. Hydrodynamics models and transport models are also complementary to each other. For example, hydrodynamics models provide a direct access to the equation of state and transport coefficients, while transport models can address non-equilibrium dynamics and provide a microscopic picture of the interactions. 123 Z.-W. Lin, L. Zheng Recent data from small systems, however, hint at significant anisotropic flows in high multiplicity pp and pPb collisions at the LHC [44] and p=d=3 He?Au collisions at RHIC [45, 46]. Hydrodynamic calculations seem to describe the experimental data well [47, 48]. The AMPTSM model also seems to describe the measured correlations [30]. This suggests that the coll (...truncated)