Benchmarking a Multi-Phase Transport for Relativistic Heavy Ion Collisions

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The soup of elementary particles, quarks and gluons, that characterized the universe for a few millionths of a second after the Big Bang, known as the quark-gluon plasma (QGP), has been created in high energy heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) in the USA and the Large Hadron Collider (LHC) in Europe. Such experiments allow physicists to study the QGP properties and understand the strong interaction at extreme temperatures and densities. A rich set of data has been measured from these experiments, including the recent evidence that the QGP is even formed in small collision systems. Hydrodynamics-based models as well as transport models have been quite successful in describing many observables. However, because transport models include non-equilibrium dynamics, they are expected to be different from hydrodynamic models for small systems. Open questions include the onset and origin of the observed collective motion of the produced particles from small to large systems, and how an interacting system transitions from the dilute limit to the hydrodynamic limit. This project focuses on using the improved transport method to address these questions, with the PI mentoring students in this research at both graduate and undergraduate levels. The improved multi-phase transport model as a public code also serves as a convenient event generator and testbed for the nuclear physics community.The main goal of this project is to address open questions regarding the QGP properties by improving the theoretical foundation and accuracy of a multi-phase transport model. This model contains a parton cascade phase and a bulk hadronization model in addition to fluctuating initial conditions and a later hadron cascade phase. The objectives include the following: i) benchmark and improve the accuracy of the parton transport method in describing the evolution of the dense matter; ii) improve the hadronization process, including starting the quark coalescence around a critical energy density instead of the parton kinetic freeze-out and applying fragmentation to heavy-flavor partons at high transverse momentum; iii) improve a multi-phase transport model with the verified parton cascade and improved hadronization and then make direct comparisons with experimental observables such as anisotropic flows. With the improved transport model constrained by bulk observables, properties of the dense matter such as the shear viscosity can also be quantitatively studied and compared with results from other approaches such as viscous-hydrodynamic models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.