Transverse-momentum and event-shape dependence of D-meson flow harmonics in Pb-Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV

Abstract: The elliptic and triangular flow coefficients $v_2$ and $v_3$ of prompt D$^{0}$, D$^{+}$, and D$^{*+}$ mesons were measured at midrapidity ($|y|<0.8$) in Pb-Pb collisions at the centre-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 5.02$ TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval $1< p_{\rm T}<36$ GeV/$c$ in central (0-10%) and semi-central (30-50%) collisions. Compared to pions, protons, and J/$\psi$ mesons, the average D-meson $v_{n}$ harmonics are compatible within uncertainties with a mass hierarchy for $p_{\rm T} \lesssim 3$ GeV/$c$, and are similar to those of charged pions for higher $p_{\rm T}$. The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson $v_2$ and $p_{\rm T}$-differential yields. The D-meson $v_2$ is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event D-meson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium.

Analysis of D-Meson Flow Harmonics in Pb--Pb Collisions at 5.02 TeV

The paper, authored by the ALICE Collaboration, presents an in-depth investigation into the flow harmonics of D mesons in lead-lead (Pb--Pb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV, using data collected by the ALICE detector at the Large Hadron Collider (LHC). The focus of this study is on the elliptic ($v_2$) and triangular ($v_3$) flow coefficients of the prompt D mesons, which include the $D^0$, $D^+$, and $D^{*+}$ particles, particularly concerning their transverse momentum ($p_T$) and event-shape dependence.

Key Findings

1. Flow Coefficient Measurements:

   • The $v_2$ and $v_3$ coefficients of D mesons were measured at midrapidity ($|y|<0.8$) across a $p_T$ range of 1 to 36 GeV/c for different centrality classes (0–10% and 30–50%).
   • For $p_T \lesssim 3$ GeV/c, a mass hierarchy in the flow coefficients was observed, aligning with expectations: $v_n(\text{D}) < v_n(\text{p}) < v_n(\pi)$ for lower transverse momentum.
   • At higher $p_T$, D-meson $v_n$ harmonics converge to values similar to those of charged pions, suggesting similar underlying mechanisms, possibly related to quark coalescence or charm quark hydrodynamic interactions.

2. Theoretical Comparisons:

   • The experimental results were compared to predictions from theoretical models incorporating charm quark transport and recombination in a quark-gluon plasma (QGP). Notably, various models like TAMU, POWLANG, and Catania have provided consistent descriptions of the D-meson flow coefficients, although discrepancies remain, especially in the intermediate $p_T$ region for some models.
   • Theoretical models generally fit within the bounds of experimental uncertainty, adding credibility to the models' descriptions of charm quark dynamics and QGP properties.

3. Event-Shape Engineering (ESE):

   • The study applied ESE techniques to understand the correlation between D-meson $v_2$ and event geometry by categorizing events based on their charged-particle flow.
   • D-meson elliptic flow was found to vary with the event shape, signifying a connection with the collective flow of the bulk medium.
   • The per-event yield ratios in ESE-selected samples were consistent with unity, suggesting minimal ESE influence on D-meson production, consistent across differing event topologies.

Implications and Future Research

The reported findings highlight the sensitivity of D mesons to the collective flow within the QGP, providing insight into the charm quark's participation in the medium's dynamical evolution. These results not only advance our comprehension of heavy flavor flow in high-energy nuclear collisions but also furnish constraints for theoretical models of QGP transport properties, particularly concerning charm quarks.

Future research could enhance the understanding of heavy quark dynamics by leveraging larger datasets and improved experimental techniques to reduce statistical uncertainties, providing more decisive comparisons between various theoretical frameworks. Additionally, exploration into finer centrality binning and higher precision in $v_n$ measurements would further elucidate the charm quark's role in QGP properties and its hadronization processes, potentially leading to refined models with higher predictive power.

The study by ALICE not only expands the landscape of heavy-ion collision physics but also benchmarks sophisticated methods to probe the QGP's microscopic characteristics, paving the way for future inquiries into the fundamental aspects of QCD matter at extreme conditions.