Improved quark coalescence for a multi-phase transport model

The string melting version of a multi-phase transport model is often applied to high-energy heavy-ion collisions since the dense matter thus formed is expected to be in parton degrees of freedom. In this work we improve its quark coalescence component, which describes the hadronization of the partonic matter to a hadronic matter. We removed the previous constraint that forced the numbers of mesons, baryons, and antibaryons in an event to be separately conserved through the quark coalescence process. A quark now could form either a meson or a baryon depending on the distance to its coalescence partner(s). We then compare results from the improved model with the experimental data on hadron $dN/dy,p_{{}_{\mathrm{T}}}$ spectra, and $v_2$ in heavy-ion collisions from $\sqrt{s_{{}_{\mathrm{NN}}}}=62.4$ GeV to 5.02 TeV. We show that, besides being able to describe these observables for low-$p_{{}_{\mathrm{T}}}$ pions and kaons, the improved model also better describes the low-$p_{{}_{\mathrm{T}}}$ baryon observables in general, especially the baryon $p_{{}_{\mathrm{T}}}$ spectra and antibaryon-to-baryon ratios for multistrange baryons.

Figure 1

The average relative distance among coalescing partons in a meson or a (anti)baryon as a function of hadron rapidity from the new (thick curves) and old (thin curves) quark coalescence model for central $\text{Au}+\text{Au}$ collisions at 200 GeV.

Figure 2

The average coalescence time of partons in mesons and (anti)baryons as a function of the hadron rapidity from the new (thick curves) and old (thin curves) quark coalescence for central $\text{Au}+\text{Au}$ collisions at 200 GeV. The circles represent the $cosh{y}_{{}_{\text{H}}}$ dependence for comparison.

Figure 3

Normalized distributions of (a) $\mathrm{\Delta }E$ (the hadron energy minus the total energy of its coalescing partons) and (b) the relative distance among coalescing partons for the coalescence to mesons and (anti)baryons at midrapidity in central $\text{Au}+\text{Au}$ collisions at 200 GeV from the new (thick curves) and old (thin curves) quark coalescence.

Figure 4

$dN/dy$ of (a) charged pions and (b) charged kaons for 0%–5% central $\text{Au}+\text{Au}$ collisions at 200 GeV and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV from the improved string-melting AMPT model (curves) in comparison with the experimental data.

Figure 5

The $p_{\mathrm{T}}$ spectra of (a) charged pions and (b) charged kaons at midrapidity for 0%–5% central $\text{Au}+\text{Au}$ collisions at 200 GeV and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV from the improved string-melting AMPT model (curves) in comparison with the experimental data.

Figure 6

Results on $v_2\left\{\mathrm{EP}\right\}$ of charged pions (upper panels) and charged kaons (lower panels) at midrapidity from the new (solid curves) and old (dashed curves) quark coalescence for 20%–30% central $\text{Au}+\text{Au}$ collisions at 200 GeV (left panels) and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV (right panels) in comparison with the experimental data.

Figure 7

$dN/dy$ (upper panels), $p_{\mathrm{T}}$ spectra at midrapidity (middle panels), and $v_2\left\{\mathrm{EP}\right\}$ at midrapidity (lower panels) of (anti)protons from the new (thick curves) and old (thin curves) quark coalescence for $\text{Au}+\text{Au}$ collisions at 200 GeV (left panels) and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV (right panels) in comparison with the experimental data.

Figure 8

$dN/dy$ of $\mathrm{\Lambda }$ (upper panels), $\mathrm{\Xi }$ (middle panels), and $\mathrm{\Omega }$ (lower panels) including the antiparticles from the new (thick curves) and old (thin curves) quark coalescence for central $\text{Au}+\text{Au}$ collisions at 200 GeV and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV in comparison with the STAR and ALICE data.

Figure 9

The $p_T$ spectra of $\mathrm{\Lambda }$ (upper panels), $\mathrm{\Xi }$ (middle panels), and $\mathrm{\Omega }$ (lower panels) including the antiparticles from the new (thick curves) and old (thin curves) quark coalescence for central $\text{Au}+\text{Au}$ collisions at 200 GeV and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV in comparison with the STAR and ALICE data.

Figure 10

Antiparticle-to-particle ratios around midrapidity for central $\text{Au}+\text{Au}$ collisions at 200 GeV (left panel) and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV (right panel) from the new (solid curves) and old (dashed curves) quark coalescence in comparison with the experimental data.

Figure 11

Centrality dependencies of antibaryon-to-baryon ratios around midrapidity for (a) protons, (b) $\mathrm{\Lambda }$, (c) $\mathrm{\Xi }$, and (d) $\mathrm{\Omega }$ in $\text{Au}+\text{Au}$ collisions at 200 GeV and $\text{Pb}+\text{Pb}$ collisions at 2.76 TeV from the new (thick curves) and old (thin curves) quark coalescence in comparison with the experimental data.

Figure 12

Energy dependencies of antibaryon-to-baryon ratios around midrapidity in central $\text{Au}+\text{Au}$ at RHIC energies and $\text{Pb}+\text{Pb}$ collisions at LHC energies from the new (left panel) and old (right panel) quark coalescence in comparison with the experimental data.