To study heavy ion collisions at energies available from the Relativistic Heavy Ion Collider (RHIC), we have developed a multiphase transport model that includes both initial partonic and final hadronic interactions. Specifically, the Zhang’s parton cascade (ZPC) model, which uses as input the parton distribution from the heavy ion jet interaction generator (HIJING) model, is extended to include the quark-gluon–to–hadronic-matter transition and also final-state hadronic interactions based on a relativistic transport (ART) model. Predictions of the model for central Au on Au collisions at RHIC are reported.

Using a chiral kinetic approach based on initial conditions from a multiphase transport model, we study the spin polarizations of quarks and antiquarks in noncentral heavy ion collisions at the BNL Relativistic Heavy Ion Collider. Because of the nonvanishing vorticity field in these collisions, quarks and antiquarks are found to acquire appreciable spin polarizations in the direction perpendicular to the reaction plane. Converting quarks and antiquarks to hadrons via the coalescence model, we further calculate the spin polarizations of $\mathrm{\Lambda }$ and anti-$\mathrm{\Lambda }$ hyperons and find their values comparable to those measured in experiments by the STAR Collaboration.

The multiplicity and elliptic flow of charged particles produced in Pb-Pb collisions at center of mass energy $\sqrt{s_{\mathit{NN}}}=2.76$ TeV from the Large Hadron Collider are studied in a multiphase transport (AMPT) model. With the standard parameters in the HIJING model, which is used as initial conditions for subsequent partonic and hadronic scatterings in the AMPT model, the resulting multiplicity of final charged particles at mid-pseudorapidity is consistent with the experimental data measured by the ALICE Collaboration. This value is, however, increased by about $25\%$ if the final-state partonic and hadronic scatterings are turned off. Because of final-state scatterings, particularly those among partons, the final elliptic flow of charged hadrons is also consistent with the ALICE data if a smaller but more isotropic parton scattering cross section than previously used in the AMPT model for describing the charged hadron elliptic flow in heavy ion collisions at the Relativistic Heavy Ion Collider is used. The resulting transverse momentum spectra of charged particles as well as the centrality dependence of their multiplicity density and the elliptic flow are also in reasonable agreement with the ALICE data. Furthermore, the multiplicities, transverse momentum spectra, and elliptic flows of identified hadrons such as protons, kaons, and pions are predicted.

Heavy baryon/meson ratios ${\Lambda }_c/D^0$ and ${\Lambda }_b/{\overline{B}}^0$ in relativistic heavy ion collisions are studied in the quark coalescence model. For heavy baryons, we include production from coalescence of heavy quarks with free light quarks as well as with bounded light diquarks that might exist in the strongly coupled quark-gluon plasma produced in these collisions. Including the contribution from decays of heavy hadron resonances and also that due to fragmentation of heavy quarks that are left in the system after coalescence, the resulting ${\Lambda }_c/D^0$ and ${\Lambda }_b/{\overline{B}}^0$ ratios in midrapidity ($\left|y\right|⩽0.5$) from central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV are about a factor of five and ten, respectively, larger than those given by the thermal model, and about a factor of ten and twelve, respectively, larger than corresponding ratios in the PYTHIA model for $\mathit{pp}$ collisions. These ratios are reduced by a factor of about 1.6 if there are no diquarks in the quark-gluon plasma. The transverse momentum dependence of the heavy baryon/meson ratios is found to be sensitive to the heavy quark mass, with the ${\Lambda }_b/{\overline{B}}^0$ ratio being much flatter than the ${\Lambda }_c/D^0$ ratio. The latter peaks at the transverse momentum $p_T\simeq 0.8$ GeV but the peak shifts to $p_T\simeq 2$ GeV in the absence of diquarks.

Using a covariant and angular-momentum-conserved chiral transport model, which takes into account the spin-orbit interactions of chiral fermions in their scatterings via the side jumps, we study the quark spin polarization in quark matter. For a system of rotating and unpolarized massless quarks in an expanding box, we find that side jumps can dynamically polarize the quark spin and result in a final quark spin polarization consistent with that of thermally equilibrated massless quarks in a self-consistent vorticity field. For the quark matter produced in noncentral relativistic heavy ion collisions, we find that in the medium rest frame both the quark local spin polarizations in the direction perpendicular to the reaction plane and along the longitudinal beam direction show an azimuthal angle dependence in the transverse plane similar to those observed in experiments for the Lambda hyperon.