CMS-PAS-HIN-19-004

CMS-PAS-HIN-19-004
Strange particle collectivity in pPb and PbPb
CMS Collaboration
May 2021

Abstract: The collective behavior of $\mathrm{K^0_S}$ and $\Lambda/\bar{\Lambda}$ strange hadrons is studied using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at 8.16 TeV and lead-lead collisions at 5.02 TeV are studied. The data samples were collected by the CMS experiment at the CERN LHC. Nonflow effects in the pPb collisions are investigated by a subevent cumulant analysis and by including a veto on events where a jet with $p_{\mathrm{T}} >$ 20 GeV is observed. For the first time, the collectivity of strange particles is observed in proton-lead collisions. A comparison of the proton-lead and lead-lead results for both strange particles and charged-hadrons is used to show how flow fluctuations are affected by system size.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to JHEP.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $v_2$ results of PbPb collisions at 5.02 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of 2- and 4-particle $v_2$ with AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 1-a: The $v_2$ results of PbPb collisions at 5.02 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of 2- and 4-particle $v_2$ with AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 1-b: The $v_2$ results of PbPb collisions at 5.02 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of 2- and 4-particle $v_2$ with AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 1-c: The $v_2$ results of PbPb collisions at 5.02 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of 2- and 4-particle $v_2$ with AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 2: The $v_2$ results of pPb collisions at 8.15 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 2-a: The $v_2$ results of pPb collisions at 8.15 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 2-b: The $v_2$ results of pPb collisions at 8.15 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 2-c: The $v_2$ results of pPb collisions at 8.15 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 3: The charge particle $v_2\{4\}$ and $v_2\{6\}$ for pPb collisions at 8.16 TeV with and without rejecting jet events (top), and their ratios (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 3-a: The charge particle $v_2\{4\}$ and $v_2\{6\}$ for pPb collisions at 8.16 TeV with and without rejecting jet events (top), and their ratios (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 3-b: The charge particle $v_2\{4\}$ and $v_2\{6\}$ for pPb collisions at 8.16 TeV with and without rejecting jet events (top), and their ratios (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 4: The $v_2\{4\}$ and $v_2\{4,\text {Sub}\}$ for PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 4-a: The $v_2\{4\}$ and $v_2\{4,\text {Sub}\}$ for PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 4-b: The $v_2\{4\}$ and $v_2\{4,\text {Sub}\}$ for PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 4-c: The $v_2\{4\}$ and $v_2\{4,\text {Sub}\}$ for PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 5: The $v_2\{4\}$ and $v_2\{4,\text {Sub}\}$ for pPb collisions at 8.16 TeV for charge hadron. The shaded boxes are systematic uncertainties.
png pdf	Figure 6: The $v_2\{4,\text {Sub}\}/v_2\{4\}$ ratios of charge hadron, ${\mathrm{K^0_S}}$ and $\Lambda$ for PbPb collisions at 5.02 TeV (top) and pPb collisions at 8.16 TeV (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 6-a: The $v_2\{4,\text {Sub}\}/v_2\{4\}$ ratios of charge hadron, ${\mathrm{K^0_S}}$ and $\Lambda$ for PbPb collisions at 5.02 TeV (top) and pPb collisions at 8.16 TeV (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 6-b: The $v_2\{4,\text {Sub}\}/v_2\{4\}$ ratios of charge hadron, ${\mathrm{K^0_S}}$ and $\Lambda$ for PbPb collisions at 5.02 TeV (top) and pPb collisions at 8.16 TeV (bottom). The shaded boxes are systematic uncertainties.
png pdf	Figure 7: The $v_2$ fluctuation results of PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of $v_2$ fluctuations with Trento and AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 7-a: The $v_2$ fluctuation results of PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of $v_2$ fluctuations with Trento and AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 7-b: The $v_2$ fluctuation results of PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of $v_2$ fluctuations with Trento and AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 7-c: The $v_2$ fluctuation results of PbPb collisions at 5.02 TeV and pPb collisions at 8.16 TeV for charge hadron (top), ${\mathrm{K^0_S}}$ (middle) and $\Lambda$ (bottom). The shaded bands are hydrodynamic calculations of $v_2$ fluctuations with Trento and AMPT initial conditions. The shaded boxes are systematic uncertainties.
png pdf	Figure 8: The $v_2$ fluctuation results of pPb collisions at 8.16 TeV for charge hadron derived from different multiparticle correlations. The shaded boxes are systematic uncertainties.
png pdf	Figure 9: The $v_2\{6\}/v_2\{4\}$ (top) and $v_2\{8\}/v_2\{4\}$ (bottom) in PbPb collisions at 5.02 TeV for charge hadron, ${\mathrm{K^0_S}}$ , and $\Lambda$ . The uncertainties are treated as uncorrelated for the ratios.
png pdf	Figure 9-a: The $v_2\{6\}/v_2\{4\}$ (top) and $v_2\{8\}/v_2\{4\}$ (bottom) in PbPb collisions at 5.02 TeV for charge hadron, ${\mathrm{K^0_S}}$ , and $\Lambda$ . The uncertainties are treated as uncorrelated for the ratios.
png pdf	Figure 9-b: The $v_2\{6\}/v_2\{4\}$ (top) and $v_2\{8\}/v_2\{4\}$ (bottom) in PbPb collisions at 5.02 TeV for charge hadron, ${\mathrm{K^0_S}}$ , and $\Lambda$ . The uncertainties are treated as uncorrelated for the ratios.

Summary

In summary, the elliptic azimuthal anisotropies

$v_2$ values have been measured using the scalar-product and the multiparticle Q-cumulant methods for identified

${\mathrm{K^0_S}}$ ,

$\Lambda$ , and charged hadrons in PpPb collisions at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 5.02 TeV and high-multiplicity pPb collisions at

${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} =$ 8.16 TeV. Nonflow effects are studied for the 4-, 6- particle cumulant results. The large difference between the 4- and 6-particle correlation results can be explained by jet-related nonflow correlations, which can be suppressed by rejecting events with a jet

${p_{\mathrm{T}}}$ greater than 20 GeV. Another method for reducing short-range nonflow correlation, the subevent cumulant method, is also studied. The difference for the amount of nonflow suppression observed with the two methods is attributed to the effect of event plane decorrelation on the subevent method. For the first time, the collective multiparticle correlations of identified strange particles have been observed in pPb collisions. No obvious particle species dependence of the fluctuations in the

$v_2$ values arising from the initial-state geometry is observed for either the PpPb or pPb systems. These flow fluctuation are observed to be larger in pPb collisions compared to PpPb collisions.

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