Elliptic flow of $D_s$ as evidence of the sequential hadronization mechanism in the hot QCD medium

Abstract

Heavy flavor production serves as an ideal probe of the hadronization mechanism in the hot and dense QCD medium created in relativistic heavy-ion collisions. Heavy quarks acquire elliptic flow through strong coupling with the medium, making $v_2$ a sensitive observable for probing the hadronization time. In this Letter, we study the production and elliptic flow of $D_s$ and $D^0$ mesons in $Pb+Pb$ collisions at $\sqrt{s_{\rm NN}}=5.02$~TeV within a Langevin dynamics framework for heavy quarks in the QCD medium, combined with the sequential or simultaneous coalescence plus fragmentation at the hadronization hypersurface. We find that, within the sequential hadronization framework, the earlier-produced $D_s$ exhibits a smaller $v_2$ than the $D^0$ in the intermediate $ p_{\mathrm{T}}$ region. This reversed ordering behavior aligns with preliminary ALICE measurements and provides strong evidence for the heavy flavor sequential hadronization mechanism.

Figures (4)

• Figure 1: Schematic diagram of sequential hadronization. The dash-dotted line represents the first hadronization hypersurface for the $D_{\rm s}$ meson at a temperature of $T_{D_s}=1.2 T_{\rm c}$. The dashed line represents another hadronization hypersurface at the temperature of $T_{\rm c}$ for other heavy-flavor hadrons.
• Figure 2: The coalescence probability for a charm quark to form $D^0+D^+$, $D^+_s$, $\Lambda_c$, $\Xi_c$, and $\Omega_c$ as a function of the charm quark momentum $p_{\rm c}$ in the simultaneous coalescence (left panel) and sequential coalescence (right panel) scenarios, respectively.
• Figure 3: Transverse momentum dependence of the elliptic flow $v_2$ of charm quark and light quarks on the $T_{D_s}$ and $T_c$ hypersurfaces. For comparison, the $v_2$ of initially produced charm quarks prior to medium evolution is also shown. All results are obtained for 30–50% central $Pb+Pb$ collisions at $\sqrt{s_{\rm{NN}}}$= 5.02 TeV.
• Figure 4: Transverse momentum dependence of the elliptic flow coefficient $v_2$ for $D^0$ and $D_{\rm s}$ mesons in 30-50% centrality $Pb+Pb$ collisions at $\sqrt{s_{\rm NN}}= 5.02$ TeV. Upper panel: Theoretical predictions considering the simultaneous coalescence, fragmentation, and hadronic rescattering mechanisms. Bottom panel: the same as above, but with the sequential coalescence mechanism. The corresponding contributions from coalescence and fragmentation alone are denoted by dotted and dash-dotted lines, respectively. The preliminary experimental data from the ALICE collaboration Torres:2025cicWu:2025byb.