Investigating charm-quark dynamics in the QGP via the charm-hadron elliptic flow in Pb-Pb collisions with ALICE

TL;DR

ALICE investigates charm-quark dynamics in the QGP by measuring the elliptic flow $v_{2}$ of prompt and non-prompt charm hadrons in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.36$ TeV. The analysis uses the Scalar Product method and machine-learning-based separation of prompt versus non-prompt contributions to reconstruct $D^{0}$, $D^{+}$, $D_{s}^{+}$, and $\Lambda_{c}^{+}$ via hadronic decays at midrapidity, enabling precise $v_{2}$ measurements down to low $p_{T}$ and across centralities. The results reveal a strong charm baryon–meson $v_{2}$ splitting at intermediate $p_{T}$ with $v_{2}(\Lambda_{c}^{+})$ exceeding $v_{2}(D^{0})$, and show that prompt D mesons have $v_{2}$ comparable to pions at low to intermediate $p_{T}$, indicating partial charm thermalization. Non-prompt $v_{2}$ remains positive but smaller than prompt, while comparisons with transport models begin to constrain the heavy-quark diffusion coefficient $D_S$ and hadronization dynamics such as coalescence.

Abstract

In these proceedings, the elliptic flow ($v_{2}$) measurement of prompt and non-prompt charm hadrons - originating respectively from the fragmentation of a charm quark and from the decay of hadrons with beauty-quark content - in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36~\text{TeV}$, using the latest data collected during LHC Run 3 by the ALICE detector, is presented. The analysis is performed at midrapidity ($|y| < 0.8$), and hadronic decay channels are used to reconstruct the signal candidates. The $v_{2}$ coefficient is measured via the Scalar Product (SP) technique. The prompt $v_{2}$ in semicentral collisions is reported for the $D^{0}$, $D^{+}$, $D_{s}^{+}$ mesons and, for the first time, for the $Λ_{c}^{+}$ baryon. These results achieve unprecedented precision and low-$p_{\mathrm{T}}$ reach for the D mesons and highlight the first observation of the splitting of baryon and meson elliptic flow at intermediate $p_{\mathrm{T}}$ in the charm sector. The $v_{2}$ is also measured for prompt $D^{0}$, $D^{+}$ and $D_{s}^{+}$ in peripheral collisions, offering new insights into the features of the fireball generated in ultrarelativistic heavy-ion collisions. Lower $v_{2}$ values are observed for non-prompt $D^{0}$ and $Λ_{c}^{+}$ hadrons in semicentral collisions, as expected from the larger beauty-quark mass.

Figures (5)

• Figure 1: Left: simultaneous fit of the invariant mass distribution and $v_{\rm 2}\xspace$ as a function of the mass. Top right: $\Lambda_\mathrm{c}^+\xspace$ raw yields as a function of the BDT prompt score selection with templates from prompt and non-prompt contributions obtained from Monte Carlo simulations. Bottom right: linear fit to the observed inclusive $v_{\rm 2}\xspace$ values for different values of non-prompt fraction.
• Figure 2: Prompt $\mathrm{D^0}\xspace$, $\mathrm{D^+}\xspace$, $\mathrm{D_s^+}\xspace$$v_{\rm 2}\xspace$ in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36$ TeV for the centrality percentiles $30$–$40\%$, $40$–$50\%$ and $60$–$80\%$. The grey markers indicate the pion $v_{\rm 2}\xspace$ from Ref. pionv2.
• Figure 3: Left: elliptic flow of prompt $\mathrm{D^0}\xspace$, $\mathrm{D_s^+}\xspace$ and $\Lambda_\mathrm{c}^+\xspace$, and inclusive $\mathrm{J}/\uppsi\xspace$ (from Ref. jpsi_v2) in semicentral Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36$ TeV superimposed with TAMU model He:2019vgs predictions. Right: superposition of prompt $\mathrm{D^0}\xspace$ and $\Lambda_\mathrm{c}^+\xspace$$v_{\rm 2}\xspace$ measurements with $\Lambda$ and $\text{K}^0_\text{S}$$v_{\rm 2}\xspace$ from Ref. ALICE:2022zks.
• Figure 4: Prompt $\mathrm{D^0}\xspace$ and $\Lambda_\mathrm{c}^+\xspace$ elliptic flow measurements in the $30$–$50\%$ centrality class superimposed with phenomenological models.
• Figure 5: Prompt and non-prompt $\mathrm{D^0}\xspace$ and $\Lambda_\mathrm{c}^+\xspace$ elliptic flow in semicentral Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.36$ TeV.