Open charm production and $Λ_{c}^{+}/D^{0}$ ratio in pp and Au+Au collisions at the RHIC

TL;DR

The study addresses how charm quarks evolve and hadronize in the quark-gluon plasma produced in $pp$ and Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. It adopts an updated string-melting AMPT framework in which charm–anticharm pairs are generated from the HIJING initial state, propagate through a partonic medium with Cronin broadening, and hadronize via a hybrid coalescence–fragmentation mechanism, with a transverse-mass dependent transition to regulate hadronization. The results show that $D^{0}$ spectra and $R_{\mathrm{AA}}$ are well described, and the $\Lambda_{c}^{+}/D^{0}$ ratio is enhanced in Au+Au at low–mid $p_T$ due to coalescence, while fragmentation dominates at high $p_T$; a decomposition into hadronization channels reveals a clear transition from coalescence-dominated to fragmentation-dominated production as $p_T$ increases. This work supports a unified, microscopic description of charm dynamics in heavy-ion collisions and provides quantitative constraints on the relative importance of coalescence and fragmentation for heavy-quark hadronization, guiding future measurements of charm baryons at RHIC.

Abstract

We study open charm hadrons production in pp and Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$~GeV using an improved a multi-phase transport (AMPT) model. Specifically, we show the transverse-momentum spectra and nuclear modification factors $R_{\mathrm{AA}}$ of $D^{0}$ mesons and $Λ_{c}^{+}$ baryons, as well as the $Λ_{c}^{+}/D^{0}$ ratio in pp and Au+Au collisions. The results obtained from the AMPT model simulations are compared with the STAR experimental data and found to be consistent. We further investigate the $Λ_{c}^{+}/D^{0}$ ratio by evaluating contributions from coalescence, fragmentation, and the combined coalescence+fragmentation mechanisms, and we find that fragmentation alone underestimates the pronounced enhancement in Au+Au relative to pp at low and intermediate $p_{\mathrm{T}}$, whereas the coalescence+fragmentation mechanism reproduces the observed trend significantly better. These results indicate that coalescence plays a key role in charm baryon productions and helps constrain the relative importance of different hadronization mechanisms in the ultra-relativistic nuclear collisions.

Figures (5)

• Figure 1: The $p_{\mathrm{T}}$ spectra of charm quarks at mid-rapidity in central Au+Au collisions extracted from the HIJING initial condition and from the string-melting process of the AMPT model at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV.
• Figure 2: Transverse momentum spectra of (a) $D^{0}$ mesons and (b) $\Lambda_{c}^{\pm}$ baryons at mid-rapidity ($|y|<1.0$) in $pp$ and Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 200 GeV. Model calculations from the updated string-melting AMPT setup are shown for $pp$ and for several Au+Au centrality classes, and are compared with the published STAR data where available. For $\Lambda_{c}^{\pm}$, experimental spectra are currently only available for 10--80% Au+Au collisions. Model results are obtained with the same kinematic cuts and centrality selections as the measurements.
• Figure 3: Nuclear modification factors $R_{\mathrm{AA}}$ of (a) $D^{0}$ mesons and (b) $\Lambda_{c}^{+}$ baryons at mid-rapidity in Au+Au collisions at $\sqrt{s_{\text{NN}}}$ = 200 GeV for three centrality intervals. The bands denote the results from the updated string-melting AMPT model, while the markers in panel (a) show the STAR data for $D^{0}$ mesons. For $\Lambda_{c}^{+}$, no experimental $R_{\mathrm{AA}}$ measurements are currently available at this energy. The average number of binary nucleon–nucleon collisions $\langle N_{\mathrm{coll}}\rangle$ for each centrality bin are taken from the corresponding experimental Glauber analysis referenced in the txet.
• Figure 4: Transverse momentum dependence of the $\Lambda_{c}^{+}/D^{0}$ yield ratio at mid-rapidity in $pp$ and 10-80% Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV. Curves show results from the updated string-melting AMPT model, while the data points denote the STAR measurement STAR:2018zdy.
• Figure 5: Transverse momentum dependence of the $\Lambda_{c}^{+}/D^{0}$ yield ratio at mid-rapidity in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV from the updated AMPT model, separated into contributions from charm-quark coalescence, independent fragmentation, and their mix.