Zi-Wei Lin

It has been challenging to quantitatively understand the stopping of incoming nucleons in nuclear collisions, and recently it has been proposed that comparing the baryon stopping with electric charge stopping can help address the question. Here we focus on the $$B/Q\times Z/A$$ B / Q × Z / A ratio, which can strongly depend on rapidity although its value is one for the full phase space. We find that this ratio is very sensitive to the difference between strange and anti-strange rapidity distributions (the $$s-\bar{s}$$ s - s ¯ asymmetry), and slightly more anti-strange quarks at mid-rapidity would lead to a ratio well below one. This is the case for Zr + Zr and Ru + Ru isobar collisions at 200 A GeV from a multi-phase transport (AMPT) model. Without the $$s-\bar{s}$$ s - s ¯ asymmetry, the AMPT model would give a mid-rapidity $$B/Q\times Z/A$$ B / Q × Z / A ratio at or above one. In addition, the AMPT model gives $$B/\Delta Q\times \Delta Z/A<1$$ B / Δ Q × Δ Z / A < 1 at mid-rapidity for isobar collisions at all centralities, which strongly contradicts the recent data from the STAR Collaboration. We further find that the $$B/\Delta Q\times \Delta Z/A$$ B / Δ Q × Δ Z / A ratio is very sensitive to the net-light quark ( u , d ) stoppings, but it is less sensitive to the $$s-\bar{s}$$ s - s ¯ asymmetry than the $$B/Q\times Z/A$$ B / Q × Z / A ratio by a factor of 3.

The analytical expressions of the shear viscosity of both one and two particle species with Boltzmann statistics and $2 \rightarrow 2$ elastic scatterings are known from the Chapman-Enskog method and have been shown to be quite accurate. The expression for a multi-species hadronic gas under $2 \rightarrow 2$ elastic scatterings is also known. Here we use the Chapman-Enskog method to derive the shear viscosity of a massless quark-gluon gas of $N_f$ quark flavors in chemical equilibrium subjected to all $2 \rightarrow 2$ parton scatterings including for the first time inelastic scatterings. We then verify the relation in a general single-species limit, where the shear viscosity of the quark-gluon gas should reduce to the result for a single particle species. In addition, we show the explicit analytical result in terms of the seven independent cross sections for the special case of isotropic and energy-independent cross sections. The analytical relations derived here can be useful for determining the shear viscosity of parton transport models with any $2 \rightarrow 2$ scattering cross sections. They can also be coupled with finite temperature QCD cross sections to help study the shear viscosity of the quark gluon plasma.

It has been challenging to quantitatively understand the stopping of incoming nucleons in nuclear collisions, and recently it has been proposed that comparing the baryon stopping with electric charge stopping can help address the question. Here we focus on the $B/Q\times Z/A$ ratio, which can strongly depend on rapidity although its value is one for the full phase space. We find that this ratio is very sensitive to the difference between strange and anti-strange rapidity distributions (the $s-\bar s$ asymmetry), and slightly more anti-strange quarks at mid-rapidity would lead to a ratio well below one. This is the case for Zr+Zr and Ru+Ru isobar collisions at $200A$ GeV from a multi-phase transport (AMPT) model. Without the $s-\bar s$ asymmetry, the AMPT model would give a mid-rapidity $B/Q\times Z/A$ ratio at or above one. In addition, the AMPT model gives $B/ΔQ\times ΔZ/A<1$ at mid-rapidity for isobar collisions at all centralities, which strongly contradicts the recent data from the STAR Collaboration. We further find that the $B/ΔQ\times ΔZ/A$ ratio is very sensitive to the net-light quark ($u,d$) stoppings, but it is less sensitive to the $s-\bar s$ asymmetry than the $B/Q\times Z/A$ ratio by a factor of 3.

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 $$\sqrt{s}=13$$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 evolution, 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 and can thus be regarded as unique evidence related to the creation of deconfined parton matter in high energy pp collisions.

We study the production of $$D^0$$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 is 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 a good description of $$D^0$$D 0 meson spectra in p-Pb collisions, the $$Q_{\textrm{pPb}}$$Q pPb data at different centralities and $$R_{\textrm{pPb}}$$R 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 $$D^{0}$$D 0 meson production and $$R_{\textrm{pPb}}$$R pPb . Our results indicate that using the same strength of the Cronin effect (i.e $$\delta $$δ value) as that obtained from the mid-rapidity data leads to a considerable overestimation of the $$D^0$$D 0 meson spectra and $$R_{\textrm{pPb}}$$R pPb data at high $$p_{\textrm{T}}$$p T in the backward rapidity. As a result, the $$\delta $$δ is determined via a $$\chi ^2$$χ 2 fitting of the $$R_{\textrm{pPb}}$$R 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.