Three-particle Azimuthal Correlations Research Articles

Harmonic decomposition of three-particle azimuthal correlations at energies available at the BNL Relativistic Heavy Ion Collider

We present measurements of three-particle correlations for various harmonics in Au+Au collisions at energies ranging from $\sqrt{s_{{\rm NN}}}=7.7$ to 200 GeV using the STAR detector. The quantity $\langle\cos(m\phi_1+n\phi_2-(m+n)\phi_3)\rangle$ is evaluated as a function of $\sqrt{s_{{\rm NN}}}$, collision centrality, transverse momentum, $p_T$, pseudo-rapidity difference, $\Delta\eta$, and harmonics ($m$ and $n$). These data provide detailed information on global event properties like the three-dimensional structure of the initial overlap region, the expansion dynamics of the matter produced in the collisions, and the transport properties of the medium. A strong dependence on $\Delta\eta$ is observed for most harmonic combinations consistent with breaking of longitudinal boost invariance. Data reveal changes with energy in the two-particle correlation functions relative to the second-harmonic event-plane and provide ways to constrain models of heavy-ion collisions over a wide range of collision energies.

Constraints on the chiral magnetic effect using charge-dependent azimuthal correlations in pPb and PbPb collisions at the CERN Large Hadron Collider

Charge-dependent azimuthal correlations of same- and opposite-sign pairs with respect to the second- and third-order event planes have been measured in pPb collisions at sNN=8.16TeV and PbPb collisions at 5.02 TeV with the CMS experiment at the LHC. The measurement is motivated by the search for the charge separation phenomenon predicted by the chiral magnetic effect (CME) in heavy ion collisions. Three- and two-particle azimuthal correlators are extracted as functions of the pseudorapidity difference, the transverse momentum (pT) difference, and the pT average of same- and opposite-charge pairs in various event multiplicity ranges. The data suggest that the charge-dependent three-particle correlators with respect to the second- and third-order event planes share a common origin, predominantly arising from charge-dependent two-particle azimuthal correlations coupled with an anisotropic flow. The CME is expected to lead to a v2-independent three-particle correlation when the magnetic field is fixed. Using an event shape engineering technique, upper limits on the v2-independent fraction of the three-particle correlator are estimated to be 13% for pPb and 7% for PbPb collisions at 95% confidence level. The results of this analysis, both the dominance of two-particle correlations as a source of the three-particle results and the similarities seen between PbPb and pPb, provide stringent constraints on the origin of charge-dependent three-particle azimuthal correlations and challenge their interpretation as arising from a chiral magnetic effect in heavy ion collisions.18 MoreReceived 4 August 2017DOI:https://doi.org/10.1103/PhysRevC.97.044912Published 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.©2018 CERN, for the CMS CollaborationPhysics Subject Headings (PhySH)Research AreasRelativistic heavy-ion collisionsNuclear Physics

Predictions on three-particle azimuthal correlations in proton--proton collisions

The ridge signal, which is long-ranged in rapidity, in the di-hadron correlations in high-multiplicity p-p and p-A collisions opened up a whole new research area in high-energy QCD. Although the ridge had been observed in A-A collisions and interpreted as a result of the radial flow of quark-gluon plasma, it had not appeared until recently in the data of small collision systems such as p-p and p-A, nor had it been predicted theoretically or seen in the event generators. There are two competing approaches that attempt to explain the systematics of the di-hadron ridge signal; hydrodynamics and gluon saturation physics (color glass condensate/glasma). In this work, we present predictions for the transverse momentum and rapidity dependence of the three-particle correlation function within the gluon saturation physics. Tri-hadron correlations can be measured, and the data can possibly rule out one of the two alternative approaches.

Study of charge-dependent azimuthal correlations using reaction-plane-dependent balance functions

STAR has recently reported charge-dependent azimuthal correlations that are sensitive to the charge separation effect in Au+Au collisions at = 200 GeV. Qualitatively, these results agree with some of the theoretical predictions for local parity violation in heavy-ion collisions. However, a study using reaction-plane-dependent balance functions shows an alternative origin of this signal. The balance function, which measures the correlation between oppositely charged pairs, is sensitive to the mechanisms of charge formation and the subsequent relative diffusion of the balancing charges. The reaction-plane-dependent balance function measurements can be related to STAR's charge-dependent azimuthal correlations. We report reaction-plane-dependent balance functions for Au+Au collisions at = 200, 62.4, 39, 11.5 and 7.7 GeV using the STAR detector. The model of Schlichting and Pratt incorporating local charge conservation and elliptic flow reproduces most of the three-particle azimuthal correlation results at 200 GeV. The experimental charge-dependent azimuthal charge correlations observed at 200 GeV can be explained in terms of local charge conservation and elliptic flow.

Indications of Conical Emission of Charged Hadrons at the BNL Relativistic Heavy Ion Collider

Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at sqrt[s_{NN}]=200 GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and peripheral Au+Au collisions. An additional structure is observed in central Au+Au data, signaling conical emission of correlated charged hadrons. The conical emission angle is found to be theta=1.37+/-0.02(stat)-0.07+0.06(syst), independent of p_{ perpendicular}.

Analysis method for jet-like three-particle azimuthal correlations

Jet-like three-particle azimuthal correlations can discriminate various physical scenarios that have been proposed to explain the observed strong modification to two-particle azimuthal correlations. The three-particle correlation analysis is notoriously difficult in heavy-ion collisions due to the large combinatorial backgrounds. We describe the general idea behind the jet-like three-particle azimuthal correlation analysis, with emphasis on the subtraction of the combinatorial backgrounds. We discuss in detail various sources of systematic uncertainties in such an analysis.

METHODS FOR JET STUDIES WITH THREE-PARTICLE CORRELATIONS

We present a method based on three-particle azimuthal correlation cumulants for the study of the interaction of jets with the medium produced in heavy ion collisions at RHIC and LHC where jets cannot be reconstructed on an event-by-event basis with conventional jet finding algorithms. The method is specifically designed to distinguish a range of jet interaction mechanisms such as Mach cone emission, gluon Cerenkov emission, jet scattering, and jet broadening. We discuss how anisotropic flow background conti-butions of second order (e.g. [Formula: see text]) are suppressed in three-particle azimuthal correlation cumulants, and discussed specific model representations of di-jets, away-side scattering, and Mach cone emission.

Three-particle azimuthal correlations and Mach shocks

Measurements of angular correlations of hadrons with a (semi)hard trigger hadron in heavy-ion collisions at RHIC show large angular structures opposite to the trigger which were a priori unexpected. These away side large angle correlations were first observed in two-particle correlations [S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. Lett. 97, 052301 (2006) and J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 95, 152301 (2005)] and have recently also been investigated in three-particle correlation measurements [J. G. Ulery (STAR Collaboration), Nucl. Phys. A774, 581 (2006) and F. Wang, arXiv:nucl-ex/0610027]. We show that the correlation signal can be understood in terms of sonic shockwaves (`Mach cones') excited by hard partons supersonically traversing the medium. The propagation of such shocks through the medium evolution is treated in a Monte Carlo (MC) framework [T. Renk and J. Ruppert, Phys. Rev. C 73, 011901(R) (2006) and Phys. Lett. B646, 19 (2007)]. We demonstrate that two- and especially three-particle correlations offer nontrivial insight into the medium-averaged speed of sound and the evolution of flow. Our findings imply that the assumption of ``deflected jets'' is not necessary to account for the observed correlations.

Two- and three-particle correlations in a partonic transport model

Two- and three-particle azimuthal correlations have been investigated in central Au + Au collisions at = 200 GeV with a partonic transport model. A hump structure on the away side, so-called ‘Mach-like structure’, which was observed in two- (three-) particle correlations in RHIC experiments, has been reproduced by the partonic transport model. It indicates that a strong parton cascade can couple partons together to show an apparent collective behaviour, i.e. a partonic Mach-like shock wave, due to a big partonic interaction cross section.

Search for conical emission with three-particle correlations

We present preliminary STAR results on three-particle azimuthal angle correlation studies in Au+Au collisions at GeV. The studies are carried out at mid-rapidity between a trigger particle with 3 ⩽p⊥⩽ 4 GeV/c and two associated particles in 1 ⩽p⊥⩽ 2 GeV/c. A cumulant analysis reveals finite three-particle azimuthal correlations, dominated by near- and away-side particle correlations consistent with jet production, and jet-flow correlations. We use a two-component model to remove underlying background correlations. This analysis indicates the presence of the conical emission signals in central Au+Au collisions within the model assumptions about background composition and normalization.

Two- and three-particle azimuthal correlations of high-pt charged hadrons in Pb–Au collisions at 158 A GeV/c

Azimuthal correlations of hadrons with high transverse momenta serve as a measure to study the energy loss and the fragmentation pattern of jets emerging from hard parton–parton interactions in heavy-ion collisions. Preliminary results from the CERES experiment on two- and three-particle correlations in central Pb–Au collisions are presented. A strongly non-Gaussian shape on the away-side of the two-particle correlation function is observed, indicating significant interactions of the emerging partons with the medium. Mechanisms like deflection of the initial partons or the evolution of a mach cone in the medium can lead to similar modifications of the jet structure on the away-side. An analysis based on three-particle correlations is presented which helps us to shed light on the origin of the observed away-side pattern.

Comparison of 3-Particle Correlation Methods

Three-particle azimuthal correlations can be used to explore the away-side structure of jets in heavy ion collisions. The combinatorial background in such measurements has angular structure similar to the signal. This paper discusses two different techniques for subtracting the backgrounds. A simple simulation model is used for illustration. A brief review of the current experimental results is also given.

Three-Particle Azimuthal Correlations

Two-particle azimuthal correlations reveal broadened and softened away-side correlations. Several different physics mechanisms are possible: large angle gluon radiation, deflected jets, and conical flow or Cerenkov radiation. Three-particle correlations are investigated to try to discriminate these mechanisms. We present results on 3-particle azimuthal correlations between a trigger particle of 3 < p T < 4 GeV / c and two softer particles of 1 < p T < 2 GeV / c for pp, d+Au and Au+Au collisions at s N N = 200 GeV . Implications of the results are discussed.

Hadron Azimuthal Correlations and Mach-like Structures in a Partonic/Hadronic Transport Model

With a multi-phase transport model (AMPT) with both partonic and hadronic interactions, two- and three-particle azimuthal correlations in Au + Au collisions at s N N = 200 GeV have been studied by the mixing-event technique. A Mach-like structure has been observed in two- and three-particle correlations in central collisions. It has been found that both partonic and hadronic dynamical mechanisms contribute to the Mach-like structure. However, only hadronic rescattering is unable to reproduce experimental amplitude of Mach-like structure, and parton cascade process is indispensable. The results of three-particle correlation indicate a partonic Mach-like shock wave can be produced by strong parton cascade in central Au+Au collisions.

Methods for jet studies with three-particle correlations

We present a method based on three-particle azimuthal correlation cumulants for studying jet interactions with the medium produced in heavy ion collisions (at RHIC) where jets cannot be reconstructed on an event-by-event basis with conventional jet-finding algorithms. The method is specifically designed to distinguish a range of jet interaction mechanisms such as Mach cone emission, gluon Cerenkov emission, jet scattering, and jet broadening. We describe how anisotropic flow contributions of second order (e.g., ${v}_{2}^{2})$ are suppressed in three-particle azimuthal correlation cumulants, and discuss specific model representations of dijets, away-side scattering, and Mach cone emission.

Analysis of directed flow from three-particle correlations

We present a new method for analysing directed flow, based on a three-particle azimuthal correlation. It is less biased by nonflow correlations than two-particle methods, and requires less statistics than four-particle methods. It is illustrated on NA49 data.