Abstract
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
Highlights
It has been suggested that in high-energy nucleus-nucleus (AA) collisions, metastable domains of gluon fields with nontrivial topological configurations may form [1,2,3,4]. These domains can carry an imbalance between left- and righthanded quarks arising from interactions of chiral quarks with topological gluon fields, leading to a local parity (P ) violation [3,4]. This chirality imbalance, in the presence of the extremely strong magnetic field, which can be produced in a noncentral AA collision, is expected to lead to an electric current perpendicular to the reaction plane, resulting in a final-state charge separation phenomenon known as the chiral magnetic effect (CME) [5,6,7]
Measurements of the charge-dependent three-particle (γ112, γ123) and two-particle (δ) correlators are shown in Fig. 3 as functions of the pseudorapidity difference (| η| ≡ |ηα − ηβ|)
The same sign (SS) and opposite sign (OS) of δ correlators are shown with different markers to differentiate the twoparticle correlation from the three-particle correlation with a particle c in the forward rapidity
Summary
It has been suggested that in high-energy nucleus-nucleus (AA) collisions, metastable domains of gluon fields with nontrivial topological configurations may form [1,2,3,4] These domains can carry an imbalance between left- and righthanded quarks arising from interactions of chiral quarks with topological gluon fields, leading to a local parity (P ) violation [3,4]. This chirality imbalance, in the presence of the extremely strong magnetic field, which can be produced in a noncentral AA collision, is expected to lead to an electric current perpendicular to the reaction plane, resulting in a final-state charge separation phenomenon known as the chiral magnetic effect (CME) [5,6,7].
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