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CMS-HIN-23-002 ; CERN-EP-2024-332
Evidence for similar collectivity of high transverse momentum particles in pPb and PbPb collisions
CMS Collaboration
10 February 2025
Submitted to Phys. Rev. Lett.
Abstract: Charged hadron elliptic anisotropies (v2) are presented over a wide transverse momentum (pT) range for proton-lead (pPb) and lead-lead (PbPb) collisions at nucleon-nucleon center-of-mass energies of 8.16 and 5.02 TeV, respectively. The data were recorded by the CMS experiment and correspond to integrated luminosities of 186 nb−1 and 0.607 nb−1 for the pPb and PbPb systems, respectively. A four-particle cumulant analysis is performed using subevents separated in pseudorapidity to effectively suppress non-collective effects. At high pT (pT> 8 GeV), significant positive v2 values are observed that are similar between pPb and PbPb collisions at comparable charged particle multiplicities. This observation suggests a common origin for the multi-particle collectivity for high-pT particles in the two systems.
Links: e-print arXiv:2502.07525 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;
Figures & Tables Summary References CMS Publications
Figures

png pdf 		Figure 1:
Different subevents in the CMS tracker acceptance used to correlate four particles. The denoted pseudorapidity ranges are used for the analysis. The letters denote the particles used in four-particle correlator. The asterisk (*) indicates particles with a negative azimuthal angle, and the prime symbol denotes the POI.

png pdf 		Figure 2:
Different subevents in the CMS tracker acceptance used to correlate four particles. The denoted pseudorapidity ranges are used for the analysis. The letters denote the particles used in four-particle correlator. The asterisk (*) indicates particles with a negative azimuthal angle, and the prime symbol denotes the POI.

png pdf 		Figure 3:
The v2{4} value vs. pT with 185 ≤Nofflinetrk< 250 for pPb (left) and PbPb (right) collisions. The red, blue, black and green markers are for the cases where tracks are not divided into subevents, and divided into two, three and four subevents, respectively. The solid lines and boxes represent the statistical and systematic uncertainties, respectively.

png pdf 		Figure 4:
The v2{4} value vs. pT with 185 ≤Nofflinetrk< 250 for pPb (left) and PbPb (right) collisions. The red, blue, black and green markers are for the cases where tracks are not divided into subevents, and divided into two, three and four subevents, respectively. The solid lines and boxes represent the statistical and systematic uncertainties, respectively.

png pdf 		Figure 5:
left: Comparison of pPb and PbPb four-subevent v2{4} values with 185 ≤Nofflinetrk< 250 vs. pT. right: Comparison of pPb and PbPb four-subevent v2{4} values with pPOIT> 6 GeV vs. ⟨Nofflinetrk⟩. The solid lines and boxes represent the statistical and systematic uncertainties, respectively.

png pdf 		Figure 5-a:
left: Comparison of pPb and PbPb four-subevent v2{4} values with 185 ≤Nofflinetrk< 250 vs. pT. right: Comparison of pPb and PbPb four-subevent v2{4} values with pPOIT> 6 GeV vs. ⟨Nofflinetrk⟩. The solid lines and boxes represent the statistical and systematic uncertainties, respectively.

png pdf 		Figure 5-b:
left: Comparison of pPb and PbPb four-subevent v2{4} values with 185 ≤Nofflinetrk< 250 vs. pT. right: Comparison of pPb and PbPb four-subevent v2{4} values with pPOIT> 6 GeV vs. ⟨Nofflinetrk⟩. The solid lines and boxes represent the statistical and systematic uncertainties, respectively.

png pdf 		Figure 6:
The four-subevent d2{4} values in generator level HIJING for 60 ≤Ngentrk< 120 shown by the closed green diamonds are compared to the experimental pPb results with 60 ≤Nofflinetrk< 120 (open blue squares) and 185 ≤Nofflinetrk< 250 (closed red circles).

png pdf 		Figure 7:
Comparison of v2{4} values as a function of ⟨Ncorrectedtrk⟩ between pPb and PbPb collisions. The solid lines and boxes indicate the statistical and systematic uncertainties, respectively.
Tables

png pdf
 Table 1:
⟨Nofflinetrk⟩ and ⟨Ncorrectedtrk⟩ values in various Nofflinetrk ranges.

png pdf
 Table 2:
⟨pT⟩ for pT bins of POI in pPb and PbPb for 185 ≤Nofflinetrk< 250. All pT values are given in units of GeV.
Summary
In summary, four-particle elliptic anisotropies (v2{4}) are measured over a large range of transverse momentum (pT) using multiparticle cumulant analyses based on the division of particle tracks into two, three, or four separate pseudorapidity ranges (``subevents''). Dividing the particles into separate subevents is shown to strongly suppress effects resulting from jet fragmentation. Significant and positive v2{4} values that extend to high pT are found for high-multiplicity pPb events. Results are shown for proton-lead (pPb) and lead-lead (PbPb) collisions at nucleon-nucleon center-of-mass energies of 8.16 TeV and 5.02 TeV, respectively. Similar magnitudes of v2{4} values at pT> 8 GeV are observed in pPb and PbPb collisions at comparable multiplicities. This observation suggests a common origin for the multi-particle collectivity for high-pT particles in the two systems.
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