Correlations In Pb-Pb Collisions Research Articles

Resolving medium properties using high-pT jets with jet and in-jet correlations in PbPb collisions at 5.02 TeV with the CMS detector

Even though the quark-gluon plasma produced in high-energy heavy ion collisions has been studied in detail for years, there are still properties that are not known to a good precision. One example of such property is the medium resolution length, which is related to the smallest angle of emission where the medium can still resolve the daughter particles as individual particles. This can be probed for example through the energy-energy correlator. The medium resolution length has implications for parton energy loss, since two particles will emit energy more vigorously compared to one. Also other details of the parton energy loss, like the path-length dependency, could use precision measurements to provide better discriminating power between available theoretical models. Jets are a good observable to experimentally tackle these questions. In this talk, we present recent, high-precision CMS jet measurements in lead-lead collisions at √SNN = 5.02 TeV aimed to explore the properties of the quark-gluon plasma.

ΛK femtoscopy in Pb-Pb collisions at sNN=2.76 TeV

The first measurements of the scattering parameters of ΛK pairs in all three charge combinations (ΛK+, ΛK−, and ΛKS0) are presented. The results are achieved through a femtoscopic analysis of ΛK correlations in Pb-Pb collisions at sNN=2.76 TeV recorded by ALICE at the Large Hadron Collider. The femtoscopic correlations result from strong final-state interactions and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the nonfemtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the ΛK+ interaction and attractive in the ΛK− interaction. The data hint that the ΛKS0 interaction is attractive; however, the uncertainty of the result does not permit such a decisive conclusion. The results suggest an effect arising either from different quark-antiquark interactions between the pairs (ss¯ in ΛK+ and uu¯ in ΛK−) or from different net strangeness for each system (S=0 for ΛK+, and S=−2 for ΛK−). Finally, the ΛK systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space-time of the single-particle Λ and K source distributions.3 MoreReceived 5 June 2020Accepted 30 March 2021DOI:https://doi.org/10.1103/PhysRevC.103.055201Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.©2021 CERN, for the ALICE CollaborationPhysics Subject Headings (PhySH)Research AreasParticle & resonance productionParticle correlations & fluctuationsParticle interactionsRelativistic heavy-ion collisionsNuclear Physics

Non-identical charged kaon femtoscopy in Pb—Pb collisions at by ALICE

We present the result of the femtoscopic analysis of non-identical charged kaon correlations in Pb-Pb collisions at obtained in ALICE at the LHC. One-dimensional K+K− correlation functions were analyzed in three centrality classes and eight transverse momentum ranges. The femtoscopic correlations of K+K− pairs are the result of Coulomb final-state interactions and formation of a 0(980) and f 0(980) resonances. The K+K− correlation function was fit with the R. Lednicky and V. Luboshitz model [1]. For the first time, f 0(980) mass and couplings were extracted from the K+K− correlation functions fit with the constraint on the radii to be close to the corresponding radii from identical charged kaon correlations.

Constraining the Chiral Magnetic Effect with charge-dependent azimuthal correlations in Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 and 5.02 TeV

Systematic studies of charge-dependent two- and three-particle correlations in Pb-Pb collisions at sqrt{s_{mathrm{NN}}} = 2.76 and 5.02 TeV used to probe the Chiral Magnetic Effect (CME) are presented. These measurements are performed for charged particles in the pseudorapidity (η) and transverse momentum (pT) ranges |η| < 0.8 and 0.2 < pT< 5 GeV/c. A significant charge-dependent signal that becomes more pronounced for peripheral collisions is reported for the CME-sensitive correlators γ1, 1 = 〈cos(φα + φβ − 2Ψ2)〉 and γ1, − 3 = 〈cos(φα − 3φβ + 2Ψ2)〉. The results are used to estimate the contribution of background effects, associated with local charge conservation coupled to anisotropic flow modulations, to measurements of the CME. A blast-wave parametrisation that incorporates local charge conservation tuned to reproduce the centrality dependent background effects is not able to fully describe the measured γ1,1. Finally, the charge and centrality dependence of mixed-harmonics three-particle correlations, of the form γ1, 2 = 〈cos(φα + 2φβ − 3Ψ3)〉, which are insensitive to the CME signal, verify again that background contributions dominate the measurement of γ1,1.

Flow Dominance and Factorization of Transverse Momentum Correlations in Pb-Pb Collisions at the LHC.

We present the first measurement of the two-particle transverse momentum differential correlation function, P_{2}≡⟨Δp_{T}Δp_{T}⟩/⟨p_{T}⟩^{2}, in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV. Results for P_{2} are reported as a function of the relative pseudorapidity (Δη) and azimuthal angle (Δφ) between two particles for different collision centralities. The Δϕ dependence is found to be largely independent of Δη for |Δη|≥0.9. In the 5% most central Pb-Pb collisions, the two-particle transverse momentum correlation function exhibits a clear double-hump structure around Δφ=π (i.e., on the away side), which is not observed in number correlations in the same centrality range, and thus provides an indication of the dominance of triangular flow in this collision centrality. Fourier decompositions of P_{2}, studied as a function of the collision centrality, show that correlations at |Δη|≥0.9 can be well reproduced by a flow ansatz based on the notion that measured transverse momentum correlations are strictly determined by the collective motion of the system.

Study of h-h and V0-h angular correlations in Pb-Pb collisions at = 2.76 TeV

Two-particle angular correlations provide a powerful tool to study jets and their modification in ultra-relativistic heavy-ion collisions. The study of the particle species dependence of correlation structures as a function of transverse momentum provides additional information on particle production mechanisms. In this contribution we present studies of a near-side jet-like correlation peak in h-h correlations in Pb-Pb collisions at = 2.76 TeV. We observe significant broadening of the jet-like peak that increases with decreasing ptrigT and passocT and with increasing centrality and additional broadening of the jet-like peak in Δη with respect to broadening in Δϕ in most central collisions.The prospects of expanding these studies to include strange particles (Λ, and K0S) is explored.

Very High-pT Triggered Dihadron Correlations in PbPb Collisions at 2.76 TeV with CMS

Measurements of dihadron correlations triggered by very high-pT particles in 2.76 TeV PbPb collisions are shown. The data set, which corresponds to an integrated luminosity of 150 μb−1, collected by the CMS detector is utilized. Long-range correlations are measured up to pT ~ 50 GeV/c. These correlations are driven by single-particle azimuthal anisotropies and characterized by the Fourier harmonics, vn. Once the V2 − v4 harmonic components are subtracted the associated particle yields on the near and away side of the jet-like correlations are studied. These measurements are done over a wide kinematic range in the associated and trigger particle pT and as a function of centrality. The data are compared to pp collisions at the same energy. A suppression of about 50% from the pp reference in the away-side associated yield is observed for passocT > 3 GeV/c. For lower momentums, passocT ~ 0.5 GeV/c, the yield is found to be enhanced significantly by up to a factor of 3–4 on the away-side. On the near side, there is some evidence of a moderate enhancement.

Overview of results on flow and correlations from the CMS collaboration

Abstract Highlights of the results obtained by the CMS experiment at the LHC on collective flow and dihadron correlations in PbPb collisions are presented. At lower transverse momenta of the outgoing particles and for a wide range of collision centralities, the 2 nd - through 6 th -order Fourier components of the azimuthal distribution have been found using different analysis techniques with different sensitivities to non-flow and flow fluctuation effects. An ultra-central collision trigger allows the Fourier components to be extended to a region of centrality where initial-state effects are expected to be small. A high- p T single-track trigger has allowed the elliptic anisotropy ( ν 2 ) and higher-orderharmonics to be explored up to ≈ 60 GeV/c , as a function of pseudorapidity and collision centrality. The CMS results provide new information on both the hydrodynamic properties of the medium at low p T and the path-length dependence of in-medium parton energy loss at high p T . By subtracting the long-range in pseudorapidity anisotropy components arising from hydrodynamic flow from dihadron correlation results, it is possible to gain further insight into the medium effect on dijets. This is done with a study of the “near-side” and “away-side” yields of dijets measured in PbPb collisions compared to those found in pp collisions.

Hadron spectra, flow and correlations in PbPb collisions at the LHC: interplay between soft and hard physics

The started LHC heavy ion program makes it possible to probe new frontiers of the high temperature Quantum Chromodynamics. It is expected that the role of hard and semi-hard particle production processes may be significant at ultra-high energies even for bulk properties of the created matter. In this paper, the LHC data on multiplicity, hadron spectra, elliptic flow and femtoscopic correlations from PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed in the framework of the HYDJET++ model. The influence of the jet production mechanism on these observables is discussed.