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| CMS-PAS-HIN-25-022 | ||
| Azimuthal anisotropy of prompt $ \Lambda_{\mathrm{c}}^{\pm} $ baryons in PbPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 2026-08-03 | ||
| Abstract: The azimuthal anisotropy of $ \Lambda_{\mathrm{c}}^{\pm} $ baryons in lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.36 TeV is presented. The data were collected by the CMS experiment at the CERN LHC in 2023 and correspond to an integrated luminosity of 1.48 $ \mathrm{nb}^{-1} $. The anisotropy is characterized by the second-order Fourier coefficient ($ v_{2} $) of the azimuthal distribution. Using the exclusive hadronic decay channel $ \Lambda_{\mathrm{c}}^{\pm} \to \mathrm{p}\mathrm{K}^{\mp}\pi^{\pm} $, the $ \Lambda_{\mathrm{c}}^{\pm} $ candidates with transverse momenta 4 $ < p_{\mathrm{T}} < $ 40 GeV are measured in the mid-rapidity range $ |y| < $ 1.0. Results for the 30 $ -50% $ centrality interval, where 0% centrality corresponds to full overlap of the colliding nuclei, are compared with a prompt $ \mathrm{D}^{0} $ meson measurement and with theoretical transport models. Models that include radiative energy loss of high-$ p_{\mathrm{T}} $ quarks in the created medium can describe the experimental observations. Crucially, at high $ p_{\mathrm{T}} $ ($ > $ 10 GeV), the $ \Lambda_{\mathrm{c}}^{\pm} $ baryon and $ \mathrm{D}^{0} $ meson $ v_{2} $ coefficients are comparable, as determined with improved precision compared to previous measurements. This convergence indicates that hadronization-specific collective effects subside at high momentum, where the path-length dependence of the partonic energy loss becomes the dominant mechanism shaping the charm-flavor anisotropy. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The left panel shows an example of the invariant mass ($ m_{\text{inv}} $) spectrum fit for $ \Lambda_{\mathrm{c}}^{\pm} $ candidates in the $ \Lambda_{\mathrm{c}}^{\pm} $ $ p_{\mathrm{T}} $ range 10--15 GeV for the centrality interval 30--50%. The right panel shows an example of the simultaneous fit where the $ v_{2} $ value of the combined signal and background ($ {v_2}^\text{Sig+Bkg} $) is modeled as a function of the invariant mass. For both panels, the horizontal lines represent the $ m_{\text{inv}} $ mass binning and the vertical lines represent the statistical uncertainties. |
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Figure 2:
The $ v_{2} $ coefficients for prompt $ \Lambda_{\mathrm{c}}^{\pm} $ baryons (filled red circles) as a function of $ p_{\mathrm{T}} $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV in the 30--50% centrality interval. The results are compared with prompt $ \mathrm{D^0} $ meson measurements (open blue circles) from CMS [16] and $ \Lambda_{\mathrm{c}}^{\pm} $ measurements (filled purple boxes) from the ALICE Collaboration [44]. The vertical bars and shaded boxes represent statistical and systematic uncertainties, respectively. Horizontal bars indicate the $ p_{\mathrm{T}} $ bin widths. |
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Figure 3:
The elliptic flow coefficient $ v_{2} $ of prompt $ \Lambda_{\mathrm{c}}^{\pm} $ baryons as a function of $ p_{\mathrm{T}} $ in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV, for the 30--50% centrality interval and $ |y| < $ 1.0. The CMS results (red markers) are compared with theoretical model predictions calculated at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV, including EPOS4HQ (solid blue curve), POWLANG HTL (dashed orange curve) and lQCD (dotted purple curve), LBT (dot-dashed pink curve), and Langevin-based calculations (dashed green curve) [45,46,47,48,49,50,51], as well as the TAMU prediction (green shaded band) [52]. Vertical bars and open boxes represent statistical and systematic uncertainties, respectively. |
| Tables | |
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Table 1:
Relative systematic and statistical uncertainties (% of $ v_{2}^{\text{nom}} $) for $ \Lambda_{\mathrm{c}}^{\pm} $ $ v_{2} $ in the 30--50% centrality interval. Each entry is $ \delta_{\text{sys},i}/v_{2}^{\text{nom}}\times 100% $ and the total systematic row is the quadrature sum. |
| Summary |
| The elliptic flow Fourier coefficient ($ v_{2} $) of prompt $ \Lambda_{\mathrm{c}}^{\pm} $ baryons is measured by the CMS Collaboration in lead-lead collisions at a center-of-mass energy per nucleon pair of 5.36 TeV. Collisions are selected in the 30--50% centrality range, where centrality is based on the fraction of the total inelastic cross section, with 0% centrality corresponding to complete overlap of the colliding nuclei. The $ v_{2} $ values are found to be positive across the measured transverse momentum range (4 $ < p_{\mathrm{T}} < $ 40 GeV). In the lower $ p_{\mathrm{T}} $ range (4 $ < p_{\mathrm{T}} < $ 8 GeV), the magnitude of the $ v_{2} $ values indicates that charm quarks participate significantly in the collective expansion of the hot, dense medium created in the collision, known as the quark-gluon plasma (QGP). When compared with prompt $ \mathrm{D^0} $ mesons, larger prompt $ \Lambda_{\mathrm{c}}^{\pm} $ values in the 6 $ < p_{\mathrm{T}} < $ 8 GeV range are qualitatively consistent with quark coalescence, a hadronization mechanism in which a baryon forms by combining three nearby quarks from the medium, so that its flow reflects the combined motion of its three constituents rather than that of a single quark, as for a meson. However, the current statistical uncertainties are too large to claim increased baryon $ v_{2} $ values with confidence. At higher transverse momenta ($ p_{\mathrm{T}} > $ 10 GeV), significantly improved precision over previous measurements is achieved. Here, the $ v_{2} $ values of $ \Lambda_{\mathrm{c}}^{\pm} $ baryons and $ \mathrm{D^0} $ mesons are found to be comparable. In this range, the positive $ v_{2} $ values are predominantly attributed to the path-length dependence of partonic energy loss as quarks traverse the QGP, rather than to collective expansion of the medium. The experimental results are well described by models that combine quark coalescence at lower $ p_{\mathrm{T}} $ with parton energy loss in the QGP at higher $ p_{\mathrm{T}} $, reinforcing this two-mechanism picture of charm-flavor anisotropy. |
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