Constraints on the chiral magnetic effect using charge-dependent azimuthal correlations in The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Sirunyan, A. M. et al. “Constraints on the Chiral Magnetic Effect Using Charge-Dependent Azimuthal Correlations in pPb and PbPb Collisions at the CERN Large Hadron Collider.” Physical Review C 97, 4 (April 2018): 044912 © 2018 CERN, for the CMS Collaboration As Published http://dx.doi.org/10.1103/PhysRevC.97.044912 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/115083 Terms of Use Creative Commons Attribution 3.0 Unported license Detailed Terms http://creativecommons.org/licenses/by/3.0/ PHYSICAL REVIEW C 97, 044912 (2018) Constraints on the chiral magnetic effect using charge-dependent azimuthal correlations in pPb and PbPb collisions at the CERN Large Hadron Collider A. M. Sirunyan et al.∗ (CMS Collaboration) (Received 4 August 2017; published 23 April 2018) Charge-dependent azimuthal correlations of same- and opposite-sign√ pairs with respect to the second- and third- p s = . order event planes have been measured in Pb collisions at NN 8 16 TeV 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. DOI: 10.1103/PhysRevC.97.044912 I. INTRODUCTION observed, which is qualitatively consistent with the expectation of charge separation from the CME. No strong collision energy It has been suggested that in high-energy nucleus-nucleus dependence of the signal is observed going from RHIC to LHC (AA) collisions, metastable domains of gluon fields with energies, although some theoretical predictions suggested that nontrivial topological configurations may form [1–4]. These the possible CME signal could be much smaller at the LHC domains can carry an imbalance between left- and right- than at RHIC because of a shorter lifetime of the magnetic field handed quarks arising from interactions of chiral quarks [17]. Nevertheless, theoretical estimates of the time evolution with topological gluon fields, leading to a local parity (P ) of the magnetic field have large uncertainties [17]. violation [3,4]. This chirality imbalance, in the presence of The experimental evidence for the CME in heavy ion the extremely strong magnetic field, which can be produced in collisions remains inconclusive because of several identified a noncentral AA collision, is expected to lead to an electric sources of background correlations that can account for part or current perpendicular to the reaction plane, resulting in a all of the observed charge-dependent azimuthal correlations final-state charge separation phenomenon known as the chiral [18–20]. Moreover, the charge-dependent azimuthal correla- magnetic effect (CME) [5–7]. Such macroscopic phenomena tion in high-multiplicity pPb collisions has been recently found arising from quantum anomalies are a subject of interest for to have a nearly identical value to that observed in PbPb a wide range of physics communities. The chiral-anomaly- collisions [21]. This is a strong indication that the observed induced phenomena have been observed in magnetized rela- effect in heavy ion collisions might predominantly result tivistic matter in three-dimensional Dirac and Weyl materials from background contributions. The CME-induced charge [8–10]. The search for the charge separation from the CME separation effect is predicted to be negligible in pPb collisions, in AA collisions was first carried out at RHIC at BNL as the angle between the magnetic field direction and the event [11–15] and later at the CERN LHC [16] at various center- plane is expected to be randomly distributed [21,22]. of-mass energies. In these measurements, a charge-dependent The charge separation can be characterized by the first P - azimuthal correlation with respect to the reaction plane was odd sine term (a1) in a Fourier decomposition of the charged- particle azimuthal distribution [23]: dN ∗ ∝ + {v n φ− + a n φ− }, Full author list given at the end of the article. dφ 1 2 n cos[ ( RP)] n sin[ ( RP)] n Published by the American Physical Society under the terms of the (1) Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) where φ − RP represents the particle azimuthal angle with and the published article’s title, journal citation, and DOI. respect to the reaction plane angle RP in heavy ion collisions 2469-9985/2018/97(4)/044912(34) 044912-1 ©2018 CERN, for the CMS Collaboration A. M. SIRUNYAN et al. PHYSICAL REVIEW C 97, 044912 (2018) (determined by the impact parameter and beam axis), and vn paper is to conduct a systematic investigation of how much of and an denote the coefficients of P -even and P -odd Fourier the observed charge-dependent correlations in the data can be terms, respectively. Although the reaction plane is not an accounted for by this mechanism. experimental observable, it can be approximated in heavy ion Although the background contribution from local charge collisions by the second-order event plane 2, determined by conservation is well defined in Eq. (3) and has been long the direction of the beam and the maximal particle density in recognized [17,20,24], it is still not known to what extent the elliptic azimuthal anisotropy. The P -odd terms will vanish background contributions account for the observed γ112 cor- after averaging over events, because the sign of the chirality relator. The main difficulty lies in determining the unknown imbalance changes event by event. Therefore, the observation value of κ2 in a model-independent way. The other difficulty of such an effect is only possible through the measurement v γ bkg is to demonstrate directly the linear dependence on 2 of 112 , of particle azimuthal correlations. An azimuthal three-particle which is nontrivial as one has to ensure that the magnetic field, correlator γ112 proposed to explore the first coefficient a1 of and thus the CME, does not change when selecting events with P the -odd Fourier terms characterizing the charge separation different v2 values. Therefore, selecting events with a quantity [23]is that directly relates to the magnitude of v2 is essential. This paper aims to overcome the difficulties mentioned γ ≡cos(φα + φβ − 2 ) 112 2 above and achieve a better understanding as to the contribution = φ − φ − cos( α 2) cos( β 2) of the local charge conservation background to the charge- dependent azimuthal correlation data. The results should serve −sin(φα − 2)sin(φβ − 2). (2) as a new baseline for the search for the CME in heavy ion α β Here, and denote particles with the same or opposite collisions. Two approaches are employed as outlined below. electric charge sign and the angle brackets reflect an averaging (1) Higher-order harmonic three-particle correlator: in α β over particles and events. Assuming particles and are heavy ion collisions, the charge separation effect from the uncorrelated, except for their individual correlations with CME is only expected along the direction of the induced respect to the event plane, the first term on the right-hand magnetic field normal to the reaction plane, approximated side of Eq. (2) becomes v ,αv ,β , which is generally small 1 1 by the second-order event plane 2. As the symmetry plane and independent of the charge [12], while the second term of the third-order Fourier term (“triangular flow” [25]) 3 is is sensitive to the charge separation and can be expressed as expected to have a weak correlation with [26], the charge a a 2 1,α 1,β . separation effect with respect to is expected to be negligible. p 3 While the similarity of the Pb and PbPb data at 5.02 TeV By constructing a charge-dependent correlator with respect to analyzed by the CMS experiment pose a considerable chal- the third-order event plane, lenge to the CME interpretation of the charge-dependent γ ≡ φ + φ − , azimuthal correlations observed in AA collisions [21], impor- 123 cos( α 2 β 3 3) (4) tant questions still remain to be addressed: is the correlation p charge-dependent background effects unrelated to the CME signal observed in Pb collisions entirely a consequence of can be explored. In particular, in the context of the local charge background correlations? What is the underlying mechanism conservation mechanism, the γ123 correlator is also expected for those background correlations that are almost identical in to have a background contribution, with pPb and PbPb collisions? Can the background contribution be quantitatively constrained with data and, if so, is there still γ bkg = κ φ − φ φ − 123 3 cos( α β ) cos 3( β 3) evidence for a statistically significant CME signal? = κ δv , (5) In particular, among the proposed mechanisms for back- 3 3 ground correlations, one source is related to the charge- similar to that for the γ112 correlator as given in Eq.