Update of a multiphase transport model with modern parton distribution functions and nuclear shadowing

A multiphase transport (AMPT) model has been successful in explaining a wide range of observables in relativistic heavy ion collisions. In this work, we implement a newer set of free proton parton distribution functions and an impact parameter-dependent nuclear shadowing in the AMPT model. After refitting the parameters of the two-component initial condition model to the experimental data on $pp$ and $p\overline{p}$ total and inelastic cross sections from $\sqrt{s}\sim$ 4 GeV to 13 TeV, we study particle productions in $pp$ and $AA$ collisions. We show that the updated AMPT model with string melting can reasonably describe the overall particle yields and transverse momentum spectra for both $pp$ and $AA$ collisions at RHIC and LHC energies after we introduce a nuclear scaling of the minijet transverse momentum cutoff for $AA$ collisions at LHC energies that is motivated by the color glass condensate. Since heavy flavor and $\mathrm{high}-p_{\mathrm{T}}$ particles are produced by perturbative-QCD processes and thus directly depend on parton distribution functions of nuclei, the updated AMPT model is expected to provide a more reliable description of these observables.

Figure 1

Parton density distributions of the free proton (at $Q^2=10 {\mathrm{GeV}}^2$) from the CTEQ6.1M set (solid curves) in comparison with the old Duke-Owens set (dot-dashed) and the more recent CJ15 set (dashed).

Figure 2

Comparison of the nuclear shadowing functions of gluons at the center of a lead nucleus from the EPS09s NLO set at two different $Q^2$ values and from the HIJING 2 parametrization with two different values for the $s_g$ parameter.

Figure 3

Total and elastic cross sections versus the colliding energy of $pp$ collisions from the experimental data (symbols) in comparison with the AMPT results (solid and dot-dashed curves); jet cross section per $pp$ collision, ${\sigma }_{\mathrm{jet}}$, is also shown for $pp$ (dotted) and $AA$ (dashed) collisions.

Figure 4

Fitted ${\sigma }_{\mathrm{soft}}$ function (solid curve), $p_0^{pp}$ function for $pp$ collisions (dotted), and the $q$ function (dot-dashed, scaled up by a factor of 100) versus the colliding energy. Dashed curve represents the $p_0^{AA}$ function for central $AA$ collisions after the nuclear scaling of $p_0$ of Eq. (10). Note that $p_0$ is only relevant at $\sqrt{s_{\text{NN}}}>10$ GeV.

Figure 5

Pseudorapidity distributions of charged particles in NSD $pp$ collisions at $\sqrt{s}=7$ TeV [29], NSD $p\overline{p}$ collisions at $200,546,900$ [30], and $1800\mathrm{GeV}$ [31], and inelastic $pp$ collisions at 23.6 GeV, 53 GeV [32], and 13 TeV [33] from AMPT (curves) in comparison with the experimental data.

Figure 6

Invariant cross sections of charged particles versus $p_{\mathrm{T}}$ in $pp$ collisions at $\sqrt{s}=23.6$ and 53 GeV [32], $p\overline{p}$ collisions at 200, 546, 900 [34], and 1800 GeV [35], and $pp$ collisions at 7 [36] and 13 TeV [37] from the AMPT model in comparison with the experimental data.

Figure 7

Identified particle $dN/dy$ and particle ratios at midrapidity in $pp$ collisions versus the colliding energy; curves represent the AMPT results with $a=0.8$ and $b=0.4{\mathrm{GeV}}^{-2}$, while symbols represent the experimental data from NA61SHINE [38], PHENIX [39], STAR [40], and ALICE [41, 42, 43].

Figure 8

Identified particle $dN/dy$ distributions (upper panels) and $p_{\mathrm{T}}$ spectra (lower panels) for 0–5% central $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV (left panels) and 0–5% central $\mathrm{Pb}+\mathrm{Pb}$ collisions at 2.76 TeV (right panels). Curves represent the AMPT results using the same Lund fragmentation parameters and $p_0$ as for $pp$ collisions, while symbols represent experimental data [44, 46].

Figure 9

Same as Fig. 8 but using Lund $b=0.15{\mathrm{GeV}}^{-2}$ and $A$-scaled minijet cutoff $p_0^{AA}\left(s\right)$ for central $AA$ collisions; note that $p_0^{AA}=p_0^{pp}$ at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$.

Figure 10

Identified particle $dN/dy$ and particle ratios at midrapidity in 0–5% central $\mathrm{Au}+\mathrm{Au}$ or $\mathrm{Pb}+\mathrm{Pb}$ collisions versus the colliding energy; curves represent AMPT results, while symbols represent experimental data at 62.4 GeV [45], 130 GeV [48], 200 GeV [44], and 2.76 TeV [46].

Figure 11

Centrality dependence of midrapidity $d{N}_{ch}/d\eta$ divided by $N_{\mathrm{part}}/2$ from the AMPT model for $\mathrm{Au}+\mathrm{Au}$ collisions at 200 GeV and $\mathrm{Pb}+\mathrm{Pb}$ collisions at 2.76 TeV and 5.02 TeV with (solid) and without (dotted) nuclear shadowing in comparison with data [49, 50, 51]. Also shown are AMPT results at LHC energies without using the $A$-scaling of $p_0$ for central $AA$ collisions with (dot-dashed) or without (dashed) nuclear shadowing, which are applicable to very peripheral $AA$ collisions.