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CMS-HIN-19-004 ; CERN-EP-2022-036
Strange hadron collectivity in pPb and PbPb collisions
CMS Collaboration
29 April 2022
JHEP 05 (2023) 007
Abstract: The collective behavior of K0S and Λ/¯Λ strange hadrons is studied by measuring the elliptic azimuthal anisotropy (v2) using the scalar-product and multiparticle correlation methods. Proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy √sNN= 8.16 TeV and lead-lead (PbPb) collisions at √sNN= 5.02 TeV collected by the CMS experiment at the LHC are investigated. Nonflow effects in the pPb collisions are studied by using a subevent cumulant analysis and by excluding events where a jet with transverse momentum greater than 20 GeV is present. The strange hadron v2 values extracted in pPb collisions via the four- and six-particle correlation method are found to be nearly identical, suggesting the collective behavior. Comparisons of the pPb and PbPb results for both strange hadrons and charged particles illustrate how event-by-event flow fluctuations depend on the system size.
Links: e-print arXiv:2205.00080 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;
Figures Summary References CMS Publications
Figures

png pdf 		Figure 1:
Invariant mass distributions of K0S mesons (left) and Λ baryons (right) candidates within |y|< 1 and 2.8 <pT< 3.6 GeV in 10-30% centrality PbPb collisions at 5.02 TeV. The blue lines show the fitted signal peak. The red lines indicate the fitted background component.

png pdf 		Figure 1-a:
Invariant mass distribution of K0S meson candidates within |y|< 1 and 2.8 <pT< 3.6 GeV in 10-30% centrality PbPb collisions at 5.02 TeV. The blue lines show the fitted signal peak. The red lines indicate the fitted background component.

png pdf 		Figure 1-b:
Invariant mass distribution of Λ baryon candidates within |y|< 1 and 2.8 <pT< 3.6 GeV in 10-30% centrality PbPb collisions at 5.02 TeV. The blue lines show the fitted signal peak. The red lines indicate the fitted background component.

png pdf 		Figure 2:
The scalar-product and multiparticle cumulant v2 results of PbPb collisions at √sNN= 5.02 TeV for charged hadrons (upper), K0S mesons (middle), and Λ baryons (lower) in different centrality intervals. The shaded bands are hydrodynamic calculations of v2{2} and v2{4} values with AMPT initial conditions. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 2-a:
The scalar-product and multiparticle cumulant v2 results of PbPb collisions at √sNN= 5.02 TeV for charged hadrons in different centrality intervals. The shaded bands are hydrodynamic calculations of v2{2} and v2{4} values with AMPT initial conditions. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 2-b:
The scalar-product and multiparticle cumulant v2 results of PbPb collisions at √sNN= 5.02 TeV for K0S mesons in different centrality intervals. The shaded bands are hydrodynamic calculations of v2{2} and v2{4} values with AMPT initial conditions. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 2-c:
The scalar-product and multiparticle cumulant v2 results of PbPb collisions at √sNN= 5.02 TeV for Λ baryons in different centrality intervals. The shaded bands are hydrodynamic calculations of v2{2} and v2{4} values with AMPT initial conditions. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 3:
The scalar-product and multiparticle cumulant v2 results of pPb collisions at √sNN= 8.16 TeV for charged hadrons (upper), K0S mesons (middle), and Λ baryons (lower) in different Nofflinetrk ranges. The scalar-product results are based on Q-vectors that are determined in either the p-going (p-SP) or Pb-going (Pb-SP) side HF calorimeter. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 3-a:
The scalar-product and multiparticle cumulant v2 results of pPb collisions at √sNN= 8.16 TeV for charged hadrons in different Nofflinetrk ranges. The scalar-product results are based on Q-vectors that are determined in either the p-going (p-SP) or Pb-going (Pb-SP) side HF calorimeter. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 3-b:
The scalar-product and multiparticle cumulant v2 results of pPb collisions at √sNN= 8.16 TeV for K0S mesons in different Nofflinetrk ranges. The scalar-product results are based on Q-vectors that are determined in either the p-going (p-SP) or Pb-going (Pb-SP) side HF calorimeter. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 3-c:
The scalar-product and multiparticle cumulant v2 results of pPb collisions at √sNN= 8.16 TeV for Λ baryons in different Nofflinetrk ranges. The scalar-product results are based on Q-vectors that are determined in either the p-going (p-SP) or Pb-going (Pb-SP) side HF calorimeter. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 4:
The charged-particle v2{4} and v2{6} distributions for pPb collisions at √sNN= 8.16 TeV with and without rejecting jet events (upper) and their ratios (lower) in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 4-a:
The charged-particle v2{4} and v2{6} distributions for pPb collisions at √sNN= 8.16 TeV with and without rejecting jet events in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 4-b:
Ratio of the charged-particle v2{4} and v2{6} distributions for pPb collisions at √sNN= 8.16 TeV with and without rejecting jet events in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 5:
The standard 4-particle v2{4} and subevent 4-particle v2{4,Sub} values from PbPb collisions at √sNN= 5.02 TeV and pPb collisions at √sNN= 8.16 TeV for charged hadrons (upper), K0S mesons (middle), and Λ baryons (lower) in different centrality intervals. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 5-a:
The standard 4-particle v2{4} and subevent 4-particle v2{4,Sub} values from PbPb collisions at √sNN= 5.02 TeV and pPb collisions at √sNN= 8.16 TeV for charged hadrons in different centrality intervals. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 5-b:
The standard 4-particle v2{4} and subevent 4-particle v2{4,Sub} values from PbPb collisions at √sNN= 5.02 TeV and pPb collisions at √sNN= 8.16 TeV for K0S mesons in different centrality intervals. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 5-c:
The standard 4-particle v2{4} and subevent 4-particle v2{4,Sub} values from PbPb collisions at √sNN= 5.02 TeV and pPb collisions at √sNN= 8.16 TeV for Λ baryons in different centrality intervals. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 6:
The v2{4} and v2{4,Sub} values from pPb collisions at √sNN= 8.16 TeV for charged hadrons in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 7:
The v2{4,Sub}/v2{4} ratios for charged hadrons, K0S mesons and Λ baryons, for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals (upper) and pPb collisions at √sNN= 8.16 TeV in different Nofflinetrk ranges (lower). The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 7-a:
The v2{4,Sub}/v2{4} ratios for charged hadrons, K0S mesons and Λ baryons, for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 7-b:
The v2{4,Sub}/v2{4} ratios for charged hadrons, K0S mesons and Λ baryons, for pPb collisions at √sNN= 8.16 TeV in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 8:
The v2 fluctuation results for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals and pPb collisions at √sNN= 8.16 TeV with 185 ≤Nofflinetrk< 250 using v2{Pb-SP} for charged hadrons (upper), K0S mesons (middle), and Λ baryons (lower). The shaded bands are hydrodynamic calculations of v2 fluctuations with AMPT and TRENTo initial conditions in PbPb collisions [64]. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 8-a:
The v2 fluctuation results for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals and pPb collisions at √sNN= 8.16 TeV with 185 ≤Nofflinetrk< 250 using v2{Pb-SP} for charged hadrons. The shaded bands are hydrodynamic calculations of v2 fluctuations with AMPT and TRENTo initial conditions in PbPb collisions [64]. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 8-b:
The v2 fluctuation results for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals and pPb collisions at √sNN= 8.16 TeV with 185 ≤Nofflinetrk< 250 using v2{Pb-SP} for K0S mesons. The shaded bands are hydrodynamic calculations of v2 fluctuations with AMPT and TRENTo initial conditions in PbPb collisions [64]. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 8-c:
The v2 fluctuation results for PbPb collisions at √sNN= 5.02 TeV in different centrality intervals and pPb collisions at √sNN= 8.16 TeV with 185 ≤Nofflinetrk< 250 using v2{Pb-SP} for Λ baryons. The shaded bands are hydrodynamic calculations of v2 fluctuations with AMPT and TRENTo initial conditions in PbPb collisions [64]. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 9:
The v2 fluctuation results of pPb collisions at √sNN= 8.16 TeV for charged hadrons derived from different multiparticle correlations in different Nofflinetrk ranges. The vertical bars and shaded boxes show the statistical and systematic uncertainties.

png pdf 		Figure 10:
The v2{6}/v2{4} (upper) and v2{8}/v2{4} (lower) ratios in PbPb collisions at √sNN= 5.02 TeV for charged hadrons, K0S mesons, and Λ baryons in different centrality intervals. The vertical bars show the statistical uncertainties. The uncertainties are treated as uncorrelated for the ratios.

png pdf 		Figure 10-a:
The v2{6}/v2{4} ratio in PbPb collisions at √sNN= 5.02 TeV for charged hadrons, K0S mesons, and Λ baryons in different centrality intervals. The vertical bars show the statistical uncertainties. The uncertainties are treated as uncorrelated for the ratios.

png pdf 		Figure 10-b:
The v2{8}/v2{4} ratio in PbPb collisions at √sNN= 5.02 TeV for charged hadrons, K0S mesons, and Λ baryons in different centrality intervals. The vertical bars show the statistical uncertainties. The uncertainties are treated as uncorrelated for the ratios.
Summary
The elliptic azimuthal anisotropy v2 values have been measured using the scalar-product and the multiparticle Q-cumulant methods for K0S mesons, Λ baryons, and charged hadrons in lead-lead (PbPb) collisions at √sNN= 5.02 TeV and high-multiplicity proton-lead (pPb) collisions at √sNN= 8.16 TeV. For the first time, multiparticle correlations of identified strange particles have been studied for pPb collisions. The hydrodynamic model calculations of scalar-product v2{SP} and 4-particle v2{4} values for K0S mesons, Λ baryons and charged hadrons using different initial conditions are found to be qualitatively consistent with the observations in PbPb collisions. No obvious particle species dependence of the fluctuations in the v2 values is observed for either the PbPb or pPb systems, indicating an origin of observed v2 fluctuations from the initial-state geometry. The flow fluctuations are observed to be larger in pPb collisions at low transverse momentum (pT). However, once jet correlations are removed, the pPb fluctuations are similar to those observed in peripheral PbPb collisions at high pT.

Nonflow effects are studied for the multiparticle cumulant results. A large difference between the 4- and 6-particle v2 values in pPb collisions that is not present for PbPb collisions can be explained by jet-related correlations. These nonflow correlations are suppressed by rejecting events with at least one jet with pT> 20 GeV. A subevent cumulant method is also performed to reduce short-range correlation effects in pPb and PbPb collisions. Whereas the jet rejection study results in higher values for v2{4} as compared to the standard method, the subevent method leads to smaller values. This difference may be attributed to the effect of flow decorrelations on the subevent cumulant method.
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