CMS-PAS-HIN-21-004

CMS-PAS-HIN-21-004
Study of charm hadronization with prompt $\Lambda_c^+$ baryons in proton-proton and lead-lead collisions at $\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}=$ 5.02 TeV
CMS Collaboration
26 March 2023

Abstract: The production of prompt $\Lambda_c^+$ baryons is measured via the exclusive decay channel $\Lambda_c^+ \to \mathrm{p} \mathrm{K}^- \pi^+$ at a center-of-mass energy per nucleon pair of 5.02 TeV using 252 nb $^{-1}$ proton-proton (pp) and 0.607 nb $^{-1}$ lead-lead (PbPb) collisions collected at the CERN LHC in 2017 and 2018, respectively. The measurements are performed within the $\Lambda_c^+$ rapidity interval $\|y\| <$ 1.0 with transverse momentum ( $p_\mathrm{T}$ ) ranges of 3-30 and 6-40 $\mathrm{GeV}/c$ for pp and PbPb collisions, respectively. Compared to pp collisions scaled by the number of nucleon-nucleon interactions, the observed yields of $\Lambda_c^+$ with $p_\mathrm{T} >$ 10 GeV/ $c$ are strongly suppressed in PbPb collisions. The level of suppression depends significantly on both the collision centrality and the $p_\mathrm{T}$ of the $\Lambda_c^+$ baryon. The $\Lambda_c^+/\mathrm{D}^0$ production ratio in PbPb collisions is consistent with the result in pp collisions for $p_\mathrm{T} >$ 10 GeV/ $c$ , suggesting that the coalescence process of hadronization is not significant for PbPb collisions in the higher $p_\mathrm{T}$ region.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in pp collisions for $p_{\mathrm{T}} =$ 3-4 (left) and 20-30 GeV/ $c$ (right). The solid line represents the fit to the data and the dashed line represents the fit to the background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 1-a: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in pp collisions for $p_{\mathrm{T}} =$ 3-4 (left) and 20-30 GeV/ $c$ (right). The solid line represents the fit to the data and the dashed line represents the fit to the background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 1-b: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in pp collisions for $p_{\mathrm{T}} =$ 3-4 (left) and 20-30 GeV/ $c$ (right). The solid line represents the fit to the data and the dashed line represents the fit to the background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 2: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in PbPb collisions in $p_{\mathrm{T}} =$ 6.0-8.0 (left) and 30.0-40.0 GeV/ $c$ (middle) in the 0-90% centrality bin, and in $p_{\mathrm{T}} =$ 10.0-12.5 GeV/ $c$ in the 0-10% (right) centrality bin. The solid line represents the fit to the data and the dashed line represents the contribution from the combinatorial background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 2-a: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in PbPb collisions in $p_{\mathrm{T}} =$ 6.0-8.0 (left) and 30.0-40.0 GeV/ $c$ (middle) in the 0-90% centrality bin, and in $p_{\mathrm{T}} =$ 10.0-12.5 GeV/ $c$ in the 0-10% (right) centrality bin. The solid line represents the fit to the data and the dashed line represents the contribution from the combinatorial background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 2-b: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in PbPb collisions in $p_{\mathrm{T}} =$ 6.0-8.0 (left) and 30.0-40.0 GeV/ $c$ (middle) in the 0-90% centrality bin, and in $p_{\mathrm{T}} =$ 10.0-12.5 GeV/ $c$ in the 0-10% (right) centrality bin. The solid line represents the fit to the data and the dashed line represents the contribution from the combinatorial background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 2-c: The number of reconstructed $\Lambda_{c}^{+}$ candidates per 8 MeVc $^{2}$ invariant mass in PbPb collisions in $p_{\mathrm{T}} =$ 6.0-8.0 (left) and 30.0-40.0 GeV/ $c$ (middle) in the 0-90% centrality bin, and in $p_{\mathrm{T}} =$ 10.0-12.5 GeV/ $c$ in the 0-10% (right) centrality bin. The solid line represents the fit to the data and the dashed line represents the contribution from the combinatorial background. The lower panels show the pulls, obtained as the difference between the data points and the fit result, divided by the uncertainty in data.
png pdf	Figure 3: The product of acceptance and efficiency as a function of $p_{\mathrm{T}}$ for prompt $\Lambda_{c}^{+}$ in pp and PbPb collisions. The closed circles represent the value for pp. The $A\epsilon$ value for PbPb collisions in centrality bin 0-90, 0-10, 10-30, 30-50 and 50-90% are represented by symbols of star, square, triangle, inverted triangle and diamond, respectively. The horizontal error bars represent the bin widths.
png pdf	Figure 4: The $p_{\mathrm{T}}$ -differential cross sections for prompt $\Lambda_{c}^{+}$ production in pp collisions. Predictions for pp collisions are displayed for PYTHIA 8 with color reconnection (open crosses), GM-VFNS with fragmentation functions fit to the OPAL data only (open circles labeled GM-VFNS-1) and fit to the OPAL and Belle data (open triangles labeled GM-VFNS-2). The horizontal error bars represent the bin widths. The lower panel shows the data-to-prediction ratio for pp collisions with error bars and brackets corresponding to the statistical and total uncertainty in the data, respectively. The global fit uncertainty of 8.6% is not shown in the plot. The shaded boxes in the bottom panel represent the GM-VFNS uncertainty.
png pdf	Figure 5: The $p_{\mathrm{T}}$ -differential cross sections for prompt $\Lambda_{c}^{+}$ production in pp collisions (circles) and the $T_\text{AA}$ -scaled yields for within centrality regions of 0-90 (stars), 0-10 (squares), 10-30 (triangles), 30-50 (inverted triangles) and 50-90% (diamonds) in PbPb collisions. The boxes and error bars represent the systematic and statistical uncertainties, respectively. The horizontal error bars represent the bin widths.
png pdf	Figure 6: The nuclear modification factor $R_{\mathrm{AA}}$ versus $p_{\mathrm{T}}$ for prompt $\Lambda_{c}^{+}$ production in centrality region of 0-90 (stars), 0-10 (squares), 10-30 (triangles), 30-50 (inverted triangles) and 50-90% (diamonds) in PbPb collisions. The boxes and error bars represent the systematic and statistical uncertainties, respectively. The horizontal error bars represent the bin widths. The band at unity labeled global uncertainty includes the uncertainties for the luminosity of pp collisions, number of MB events in PbPb collisions, and tracking efficiency. The global uncertainty for $R_{\mathrm{AA}}$ is 16.5%.
png pdf	Figure 7: The ratio of the production cross sections of prompt $\Lambda_{c}^{+}$ to prompt D $^{0}$ versus $p_{\mathrm{T}}$ from pp collisions is represented by closed circles (left). The ratio for 0-90 (closed stars) and 0-10% (closed squares) centrality classes of PbPb collisions are compared to the pp result (right). The boxes and error bars represent the systematic and statistical uncertainties, respectively. The horizontal error bars represent the bin widths. The 6.6 and 7.3% normalization uncertainties in pp and PbPb collisions, respectively, are not included in the boxes representing the systematic uncertainties for each data point. Model calculations are displayed (see texts for details).
png pdf	Figure 7-a: The ratio of the production cross sections of prompt $\Lambda_{c}^{+}$ to prompt D $^{0}$ versus $p_{\mathrm{T}}$ from pp collisions is represented by closed circles (left). The ratio for 0-90 (closed stars) and 0-10% (closed squares) centrality classes of PbPb collisions are compared to the pp result (right). The boxes and error bars represent the systematic and statistical uncertainties, respectively. The horizontal error bars represent the bin widths. The 6.6 and 7.3% normalization uncertainties in pp and PbPb collisions, respectively, are not included in the boxes representing the systematic uncertainties for each data point. Model calculations are displayed (see texts for details).
png pdf	Figure 7-b: The ratio of the production cross sections of prompt $\Lambda_{c}^{+}$ to prompt D $^{0}$ versus $p_{\mathrm{T}}$ from pp collisions is represented by closed circles (left). The ratio for 0-90 (closed stars) and 0-10% (closed squares) centrality classes of PbPb collisions are compared to the pp result (right). The boxes and error bars represent the systematic and statistical uncertainties, respectively. The horizontal error bars represent the bin widths. The 6.6 and 7.3% normalization uncertainties in pp and PbPb collisions, respectively, are not included in the boxes representing the systematic uncertainties for each data point. Model calculations are displayed (see texts for details).

Tables
png pdf	Table 1: Systematic uncertainties from different sources

Summary

The differential cross section of prompt

$\Lambda_{c}^{+}$ baryons as a function of transverse momentum (

$p_{\mathrm{T}}$ ) is presented for both proton-proton (pp) and lead-lead (PbPb) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV in the central region

$|y| <$ 1.0. The measured

$p_{\mathrm{T}}$ ranges are 3

$< p_{\mathrm{T}} < 30 GeV/$ c

$and 6$ < p_{\mathrm{T}} < 40 GeV/

$c$ for the pp and PbPb collisions, respectively. The nuclear modification factors (

$R_{\mathrm{AA}}$ ) corresponding to the

$\Lambda_{c}^{+}$ yields divided by the yields expected by scaling up the pp results by the number of nucleon-nucleon collisions and the

$\Lambda_{c}^{+}/ \mathrm{D^0}$ production ratios have also been measured in various centrality classes for the PbPb collisions. The prompt

$\Lambda_{c}^{+}$ production for PbPb collisions is significantly suppressed compared to the pp collision results. The magnitude of the suppression is larger in more central collisions and varies with the

$p_{\mathrm{T}}$ value of

$\Lambda_{c}^{+}$ baryon, consistent with the effect originating from the parton energy loss of charm quarks traversing the quark-gluon plasma. A similar effect was previously observed for D

$^{0}$ mesons. The

$\Lambda_{c}^{+}$ baryon yields for pp collisions are much higher than predicted by GM-VFNS calculations that use fragmentation functions obtained by fitting data from the OPAL and Belle experiments, indicating a breakdown of the universality of charm quark fragmentation functions. Calculations based on PYTHIA 8 with the inclusion of color reconnection in the hadronization step can describe the pp data well for

$p_{\mathrm{T}} <$ 10 GeV/

$c$ , but is systematically lower than observed for the 10

$< p_{\mathrm{T}} <$ 30 GeV/

$c$ range. A model taking into account the contributions from the decays of excited charm baryons and a model involving both coalescence and fragmentation can also describe the

$\Lambda_{c}^{+}/ \mathrm{D^0}$ production ratios in pp collisions. For

$p_{\mathrm{T}} >$ 10 GeV/

$c$ , the

$\Lambda_{c}^{+}/ \mathrm{D^0}$ ratios for pp and PbPb collisions are consistent with each other. This suggests that the coalescence process does not play a significant role in

$\Lambda_{c}^{+}$ baryon production in this higher

$p_{\mathrm{T}}$ region. The current results extend the

$p_{\mathrm{T}}$ and centrality (for PbPb collisions) ranges in

$|y| <$ 1.0 over which the

$\Lambda_{c}^{+}$ baryon yields have been measured, thus further constraining model calculations that consider the role of quark coalescence in heavy ion collisions.

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