Evidence of different $Λ_{\rm c}$-baryon and D-meson elliptic flow in Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}}}$ = 5.36 TeV with ALICE at the LHC

Abstract

The ALICE collaboration reports the azimuthal-anisotropy coefficient $v_2$ of prompt D$^0$, D$^+$, D$^+_{\rm s}$ mesons and the first measurement of $v_2$ of prompt $Λ_{\rm c}$ baryons in semicentral Pb$-$Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.36$ TeV. The D mesons and $Λ_{\rm c}$ baryons are reconstructed in their hadronic decays at midrapidity ($ |y|<0.8$) in the transverse-momentum interval $0.5< p_{\rm T} < 24$ GeV/$c$. Similar $v_2$ values are measured for D$^0$ and D$^+$, while a hint of a difference ($2.6σ$) emerges between D$^0$ and D$^+_{\rm s}$ mesons in the $1 < p_{\rm T} < 5 $ GeV/$c$ interval. A larger $v_2$ for $Λ_{\rm c}$ baryons with respect to D$^0$ mesons is observed with $3.7σ$ significance for $4 < p_{\rm T} < 12$ GeV/$c$, providing evidence for the partonic origin of charm-hadron $v_2$ and hadron formation via quark coalescence. This interpretation is further supported by comparisons with theoretical calculations of charm-quark transport in a hydrodynamically expanding medium.

Figures (3)

• Figure 1: Elliptic flow of prompt $\mathrm{D^0}$, $\mathrm{D^+}$, $\mathrm{D_s^+}$ mesons and $\Lambda_\mathrm{c}^+$ baryons at midrapidity ($\left | y \right |~<~0$.8) in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace = 5.36$ TeV in the 30$-$50% centrality class as a function of the charm-hadron $p_{\rm T}$. The $v_{\rm 2}$ of charged pions measured at $\sqrt{s_{\mathrm{NN}}}\xspace = 5.02$ TeV in the 30$-$40% centrality class ALICE:2018yph is also shown for comparison.
• Figure 1: Left: simultaneous fit of the candidate yield (top) and total $v_{\rm 2}\xspace$ as a function of the candidate invariant mass (bottom) of $\Lambda_\mathrm{c}^+$ baryons in $6 < p_{\rm T}\xspace < 8~\mathrm{GeV}/c\xspace$ in Pb--Pb collisions for the 30--50% centrality class. The combinatorial background and the total fit functions are depicted as dotted red curves and blue lines, respectively. The BDT-based selection and the corresponding signal $v_{\rm 2}$ are reported in the bottom panel. Top right: raw-yield distribution as a function of the minimum threshold on the BDT prompt score of $\Lambda_\mathrm{c}^+$ baryons in $6 < p_{\rm T}\xspace < 8~\mathrm{GeV}/c\xspace$ for the 30--50% centrality class. The prompt and non-prompt populations, along with their sum, are reported in red, blue, and green, respectively. Bottom right: linear fit to the signal $v_{\rm 2}\xspace$ of $\Lambda_\mathrm{c}^+$ baryons in $6 < p_{\rm T}\xspace < 8~\mathrm{GeV}/c\xspace$ for the 30--50% centrality class as a function of the non-prompt fraction. The blue band represents the $1\sigma$ confidence interval obtained from the fit.
• Figure 2: Elliptic flow of prompt $\mathrm{D^0}$ (top), $\mathrm{D_s^+}$ (middle), and $\Lambda_\mathrm{c}^+$ (bottom) hadrons at midrapidity ($\left | y \right |~<~0$.8) in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}\xspace = 5.36$ TeV in the 30–-50% centrality class as a function of $p_{\rm T}$. The data are compared with predictions from TAMU He:2019vgs, Catania Scardina:2017ipoSambataro:2024mkr, POWLANG Beraudo:2022dpzBeraudo:2023nlq, LBT-PNP Ke:2018tshXing:2021xwc, EPOS4HQ Zhao:2024ecc, and Langevin Li:2021xbdLi:2020kax transport models at $\sqrt{s_{\mathrm{NN}}}\xspace = 5.02$ TeV.