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Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at$$ \\sqrt{s_{\\textrm{NN}}} $$s NN = 5 . 02 TeV are extended to a pseudorapidity gap of ∆ η ~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆ η ~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v 2 ( η ), is extracted from the long-range correlations. The v 2 ( η ) results are presented for a wide pseudorapidity range of –3 . 1 < η < 4 . 8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the v 2 ( η ) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.

The second-order ( v 2 ) and third-order ( v 3 ) Fourier coefficients describing the azimuthal anisotropy of prompt and nonprompt (from b-hadron decays) J / ψ, as well as prompt ψ(2S) mesons are measured in lead-lead collisions at a center-of-mass energy per nucleon pair of$$ \\sqrt{s_{\\textrm{NN}}} $$s NN = 5 . 02 TeV. The analysis uses a data set corresponding to an integrated luminosity of 1.61 nb − 1 recorded with the CMS detector. The J / ψ and ψ(2S) mesons are reconstructed using their dimuon decay channel. The v 2 and v 3 coefficients are extracted using the scalar product method and studied as functions of meson transverse momentum and collision centrality. The measured v 2 values for prompt J / ψ mesons are found to be larger than those for nonprompt J / ψ mesons. The prompt J / ψ v 2 values at high p T are found to be underpredicted by a model incorporating only parton energy loss effects in a quark-gluon plasma medium. Prompt and nonprompt J / ψ meson v 3 and prompt ψ(2S) v 2 and v 3 values are also reported for the first time, providing new information about heavy quark interactions in the hot and dense medium created in heavy ion collisions.

A bstract Multi-particle azimuthal cumulants are measured as a function of centrality and transverse momentum using 470 μ b − 1 of Pb+Pb collisions at s NN = 5 . 02 TeV with the ATLAS detector at the LHC. These cumulants provide information on the event-by-event fluctuations of harmonic flow coefficients v n and correlated fluctuations between two harmonics v n and v m . For the first time, a non-zero four-particle cumulant is observed for dipolar flow, v 1 . The four-particle cumulants for elliptic flow, v 2 , and triangular flow, v 3 , exhibit a strong centrality dependence and change sign in ultra-central collisions. This sign change is consistent with significant non-Gaussian fluctuations in v 2 and v 3 . The four-particle cumulant for quadrangular flow, v 4 , is found to change sign in mid-central collisions. Correlations between two harmonics are studied with three- and four-particle mixed-harmonic cumulants, which indicate an anti-correlation between v 2 and v 3, and a positive correlation between v 2 and v 4 . These correlations decrease in strength towards central collisions and either approach zero or change sign in ultra-central collisions. To investigate the possible flow fluctuations arising from intrinsic centrality or volume fluctuations, the results are compared between two different event classes used for centrality definitions. In peripheral and mid-central collisions where the cumulant signals are large, only small differences are observed. In ultra-central collisions, the differences are much larger and transverse momentum dependent. These results provide new information to disentangle flow fluctuations from the initial and final states, as well as new insights on the influence of centrality fluctuations.

A bstract The first measurements of elliptic flow of π ± , K ± , p + p ¯ , K S 0 , Λ + Λ ¯ , ϕ , Ξ − + Ξ ¯ + , and Ω − + Ω ¯ + using multiparticle cumulants in Pb–Pb collisions at s NN = 5 . 02 TeV are resented. Results obtained with two- ( v 2 {2}) and four-particle cumulants ( v 2 {4}) are shown as a function of transverse momentum, p T , for various collision centrality intervals. Combining the data for both v 2 {2} and v 2 {4} also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous p T -differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low p T and the approximate scaling with the number of constituent quarks at intermediate p T , are similarly present in the four-particle correlations and the combinations of v 2 {2} and v 2 {4}. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide p T range.

Abstract Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s$$ \\sqrt{s} $$= 13 TeV and p–Pb collisions at s NN$$ \\sqrt{s_{\\textrm{NN}}} $$= 5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆φ and pseudorapidity separation ∆η for pairs of primary charged particles within the pseudorapidity interval |η| < 0.9 and the transverse-momentum interval 1 < p T < 4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6 < |∆η| < 1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.

A bstract Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s = 13 TeV and p–Pb collisions at s NN = 5 . 02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆ φ and pseudorapidity separation ∆ η for pairs of primary charged particles within the pseudorapidity interval | η | < 0 . 9 and the transverse-momentum interval 1 < p T < 4 GeV/ c . Flow coefficients are extracted for the long-range correlations (1 . 6 < |∆ η | < 1 . 8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.

A bstract Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at s NN = 5 . 02 TeV are extended to a pseudorapidity gap of ∆ η ~ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆ η ~ 8 for the first time in p–Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v 2 ( η ), is extracted from the long-range correlations. The v 2 ( η ) results are presented for a wide pseudorapidity range of –3 . 1 < η < 4 . 8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small collision systems, the v 2 ( η ) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small collision systems.

A bstract The second-order ( v 2 ) and third-order ( v 3 ) Fourier coefficients describing the azimuthal anisotropy of prompt and nonprompt (from b-hadron decays) J / ψ, as well as prompt ψ(2S) mesons are measured in lead-lead collisions at a center-of-mass energy per nucleon pair of s NN = 5 . 02 TeV. The analysis uses a data set corresponding to an integrated luminosity of 1.61 nb − 1 recorded with the CMS detector. The J / ψ and ψ(2S) mesons are reconstructed using their dimuon decay channel. The v 2 and v 3 coefficients are extracted using the scalar product method and studied as functions of meson transverse momentum and collision centrality. The measured v 2 values for prompt J / ψ mesons are found to be larger than those for nonprompt J / ψ mesons. The prompt J / ψ v 2 values at high p T are found to be underpredicted by a model incorporating only parton energy loss effects in a quark-gluon plasma medium. Prompt and nonprompt J / ψ meson v 3 and prompt ψ(2S) v 2 and v 3 values are also reported for the first time, providing new information about heavy quark interactions in the hot and dense medium created in heavy ion collisions.

The first measurements of elliptic flow of π ± , K ± ,$$ \\textrm{p}+\\overline{\\textrm{p}} $$p + p ¯ ,$$ {\\textrm{K}}_{\\textrm{S}}^0 $$K S 0 ,$$ \\Lambda +\\overline{\\Lambda} $$Λ + Λ ¯ , ϕ ,$$ {\\Xi}^{-}+{\\overline{\\Xi}}^{+} $$Ξ − + Ξ ¯ + , and$$ {\\varOmega}^{-}+{\\overline{\\varOmega}}^{+} $$Ω − + Ω ¯ + using multiparticle cumulants in Pb–Pb collisions at$$ \\sqrt{s_{\\textrm{NN}}} $$s NN = 5 . 02 TeV are resented. Results obtained with two- ( v 2 2) and four-particle cumulants ( v 2 4) are shown as a function of transverse momentum, p T , for various collision centrality intervals. Combining the data for both v 2 2 and v 2 4 also allows us to report the first measurements of the mean elliptic flow, elliptic flow fluctuations, and relative elliptic flow fluctuations for various hadron species. These observables probe the event-by-event eccentricity fluctuations in the initial state and the contributions from the dynamic evolution of the expanding quark–gluon plasma. The characteristic features observed in previous p T -differential anisotropic flow measurements for identified hadrons with two-particle correlations, namely the mass ordering at low p T and the approximate scaling with the number of constituent quarks at intermediate p T , are similarly present in the four-particle correlations and the combinations of v 2 2 and v 2 4. In addition, a particle species dependence of flow fluctuations is observed that could indicate a significant contribution from final state hadronic interactions. The comparison between experimental measurements and CoLBT model calculations, which combine the various physics processes of hydrodynamics, quark coalescence, and jet fragmentation, illustrates their importance over a wide p T range.