Correlated Event-by-Event Fluctuations of Flow Harmonics in Pb-Pb Collisions at √sNN = 2.76 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Correlated Event-by-Event Fluctuations of Flow Harmonics in Pb-Pb Collisions at √sNN = 2.76 TeV ALICE Collaboration ALICE Collaboration. (2016). Correlated Event-by-Event Fluctuations of Flow Harmonics in Pb-Pb Collisions at √sNN = 2.76 TeV. Physical Review Letters, 117(18), Article 182301. https://doi.org/10.1103/PhysRevLett.117.182301 2016 Correlated Event-by-Event Fluctuations of Flow Harmonics in Pb-Pb Collisions at ffiffiffiffiffiffiffiffi sNN p=2.76 TeV J. Adam et al.* (ALICE Collaboration) (Received 4 May 2016; published 28 October 2016) We report the measurements of correlations between event-by-event fluctuations of amplitudes of anisotropic flow harmonics in nucleus-nucleus collisions, obtained for the first time using a new analysis method based on multiparticle cumulants in mixed harmonics. This novel method is robust against systematic biases originating from nonflow effects and by construction any dependence on symmetry planes is eliminated. We demonstrate that correlations of flow harmonics exhibit a better sensitivity to medium properties than the individual flow harmonics. The new measurements are performed in Pb-Pb collisions at the center-of-mass energy per nucleon pair of ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV by the ALICE experiment at the Large Hadron Collider. The centrality dependence of correlation between event-by-event fluctuations of the elliptic v2and quadrangular v4flow harmonics, as well as of anticorrelation between v2and triangular v3flow harmonics are presented. The results cover two different regimes of the initial state configurations: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions). Comparisons are made to predictions from Monte Carlo Glauber, viscous hydrodynamics, AMPT , and HIJING models. Together with the existing measurements of the individual flow harmonics the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions. DOI: 10.1103/PhysRevLett.117.182301 The properties of an extreme state of matter, the quark-gluon plasma (QGP), are studied by colliding heavy ions at BNL’s Relativistic Heavy Ion Collider (RHIC) and at CERN’s Large Hadron Collider (LHC). One of the most widely utilized physical phenomena in the exploration of QGP properties is collective anisotropic flow [1,2]. The large elliptic flow discovered at RHIC energies [3], which at the LHC energy of 2.76 TeV is 30% larger [4] and is recently reported in Ref. [5] to increase even further at 5.02 TeV, demonstrated that the QGP behaves like a strongly coupled liquid with a very small ratio of the shear viscosity to entropy density, η=s, which is close to a universal lower bound of 1=4π[6]. Anisotropic flow is traditionally quantified with harmonics vnand corresponding symmetry plane angles ψnin the Fourier series decomposition of the particle azimuthal distribution (parametrized with azimuthal angle φ) in the plane transverse to the beam direction [7]: dN dφ ∝1þ2X ∞ n¼1 vncos½nðφ−ψnÞ:ð1Þ The shape of the intersecting zone of two identical heavy ions in noncentral collisions is approximately ellipsoidal. This initial anisotropy is transferred via interactions among constituents and the pressure gradients developed in the QGP medium to an observable final-state anisotropic emission of particles with respect to the symmetry plane(s) of the intersecting zone. The resulting anisotropic flow for such an idealized ellipsoidal geometry is determined solely by even Fourier harmonics v2n, and only one symmetry plane (the reaction plane) exists. Recently, the importance of flow fluctuations and related additional observables has been identified. This has led to new concepts such as nonvanishing odd harmonics v2n−1at midrapidity [8], nonidentical symmetry plane angles ψnand their intercorrelations [9–14], the stochastic nature of the harmonic vnand its probability density function PðvnÞ[15–20], and, finally, the importance of higher order flow moments hvk ni(where the angular brackets denote an average over all events, and k≥2)[21]. Two distinct regimes for anisotropic flow development are nowadays scrutinized separately: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions) [11]. Anisotropic flow is generated by the initial anisotropic geometry and its fluctuations coupled with an expansion of the produced medium. The initial coordinate space anisotropy can be quantified in terms of the eccentricity coefficients εnand the corresponding symmetry plane angles Φn[8,15,22]. A great deal of effort is being invested *Full author list given at the end of the article. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 0031-9007=16=117(18)=182301(14) 182301-1 © 2016 CERN, for the ALICE Collaboration to understand the relations between the momentum space Fourier harmonics vnand the symmetry planes ψnon one side, and their spatial counterparts εnand Φnon the other side. These relations describe the response of the produced system to the initial coordinate space anisotropies, and therefore provide a rich repository of constraints for the system properties. In the early studies it was regularly assumed that, for small eccentricities, the harmonics vn respond linearly to the eccentricities εnof the same order, vn∝εn, and that ψn≃Φn[8,10,23,24]. However, for sizable eccentricities recent studies argue that the anisotropies in momentum and coordinate space are related instead with the matrix equation connecting a set of anisotropic flow harmonics fvngand a set of eccentricity coefficients fεng; it was demonstrated that the hydrodynamic response is both nondiagonal and nonlinear, and that in general ψn≠Φn[9,11,25,26]. The first realization led to the conclusion that a relationship between eventby-event fluctuations of the amplitudes of two different flow harmonics vmand vncan exist. This is hardly surprising for even flow harmonics in noncentral collisions because the ellipsoidal shape generates nonvanishing values for all even harmonics v2n[27], not only for elliptic flow. However, this simple geometrical argument cannot explain the possible relation between the even and odd flow harmonics in noncentral collisions, and the argument is not applicable in the central collisions, where all initial shapes are equally probable since they originate solely from fluctuations. Recently a linear correlation coefficient cða; bÞwas defined in this context, which becomes 1 (−1) if observables aand bare fully linearly (antilinearly) correlated and zero in the absence of correlation [25]. Model calculations of this new observable showed that neither v2and v3nor v2and v4are linearly correlated in noncentral collisions. Most importantly, it was demonstrated that cðv2;v 4Þdepends strongly both on η=s of the QGP and on the value of cðε2;ε4Þ, which quantifies the relationship between corresponding eccentricities in the initial state [25]. Therefore, it was concluded that new observables cðvn;v mÞ, depending on the choice of flow harmonics vnand vm, are sensitive both to the fluctuations of the initial conditions and to the transport properties of the QGP, with the potential to discriminate between the two respective contributions when combined with a measurement of individual flow harmonics [25]. In this Letter we study the relationship between eventby-event fluctuations of magnitudes of two different flow harmonics of order nand mby using a recently proposed four-particle observable [28]: ⟪cosðmφ1þnφ2−mφ3−nφ4Þ⟫c¼⟪cosðmφ1þnφ2−mφ3−nφ4Þ⟫ −⟪cos½mðφ1−φ2Þ⟫⟪cos½nðφ1−φ2Þ⟫ ¼hv2 mv2 ni−hv2 mihv2 nið2Þ with the condition m≠nfor two positive integers mand n. We refer to these new observables as the symmetric twoharmonic four-particle cumulant, and use the notation SCðm; nÞ, or just SC. The double angular brackets indicate that the averaging procedure has been performed in two steps—first, averaging over all distinct particle quadruplets in an event, and then in the second step weighting the singleevent averages with the “number of combinations.”The latter for single-event average four-particle correlations is mathematically equivalent to a unit weight for each individual quadruplet when the multiplicity differs event by event [29].In both two-particle correlators above all distinct particle pairs are considered in each case. The four-particle cumulant in Eq. (2) is less sensitive to nonflow correlations than any two- or four-particle correlator on the right-hand side taken individually [30,31]. The last equality is true only in the absence of nonflow effects [32]. The observable in Eq. (2) is zerointheabsenceofflow fluctuations,orifthemagnitudesof the harmonics vmand vnare uncorrelated [28].Itisalso unaffected by the relationship between the symmetry plane angles ψmand ψn. The four-particle cumulant in Eq. (2) is proportional to the linear correlation coefficient cða; bÞ introduced in Ref. [25] and discussed above, with a¼v2 m and b¼v2 n. Experimentally, it is more reliable to measure the higher order moments of the flow harmonics vk nðk≥2Þwith two- and multiparticle correlation techniques [31,33,34], than to measure the first moments vnwith the event plane method, due to the systematic uncertainties involved in the event-by-event estimation of the symmetry planes [35,36]. Therefore, we have used the new multiparticle observable in Eq. (2) as meant to be the least biased measure of the correlation between event-by-event fluctuations of magnitudes of the two different harmonics vmand vn[28]. The two- and four-particle correlations in Eq. (2) were evaluated in terms of Qvectors [33].TheQvector (or flow vector) in harmonic nfor a set of Mparticles, where throughout this Letter Mis the multiplicity of an event, is definedasQn≡PM k¼1einφk[7,37].Wehaveusedforasingleevent average two-particle correlation hcosðnðφ1−φ2ÞÞithe following definition and analytic result in terms of Qvectors: 1 ðM 2Þ2! X M i;j¼1 ði≠jÞ einðφi−φjÞ¼1 ðM 2Þ2! ½jQnj2−M:ð3Þ PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 182301-2 For four-particle correlation hcosðmφ1þnφ2−mφ3−nφ4Þi we used 1 ðM 4Þ4! X M i;j;k;l¼1 ði≠j≠k≠lÞ eiðmφiþnφj−mφk−nφlÞ¼1 ðM 4Þ4! fjQmj2jQnj2−2Re½QmþnQ mQ n−2Re½QmQ m−nQ n þjQmþnj2þjQm−nj2−ðM−4ÞðjQmj2þjQnj2ÞþMðM−6Þg:ð4Þ In order to obtain the all-event average correlations, denoted by ⟪⟫in Eq. (2), we have weighted singleevent expressions in Eqs. (3) and (4) with weights MðM−1Þand MðM−1ÞðM−2ÞðM−3Þ, respectively [29]. The data used in this analysis were obtained with the ALICE detector [38,39]. They consist of minimum-bias Pb-Pb collisions recorded during the 2010 LHC Pb-Pb run at ffiffiffiffiffiffiffiffi sNN p¼2.76 TeV. With the default event and track selection criteria described below, we have obtained in total about 1.8×105events per 1% centrality bin width. All individual systematic variations were combined in quadrature to obtain the final uncertainty. The centrality was determined with the V0 detector [40–42]. As a part of systematic checks the centrality was determined independently with the time projection chamber (TPC) [43] and the silicon pixel detector [44,45], which have slightly worse resolution [42]. A systematic difference of up to 3% was observed in the SCðm; nÞresults when using different centrality estimations. Charged particles were reconstructed with the TPC and the inner tracking system [44,45] immersed in a 0.5 T solenoidal field. The TPC is capable of detecting charged particles in the transverse momentum range 0.1<p T<100 GeV=c, with apTresolution of less than 6% for tracks below 20 GeV=c. Because of TPC dead zones between neighboring sectors, the track finding efficiency is about 75% for pT¼ 200 MeV=c and then it saturates at about 85% for pT> 1GeV=c in Pb-Pb collisions. The TPC covers the full azimuth and has a pseudorapidity coverage of jηj<0.9. Tracks reconstructed using the TPC and inner tracking system are referred to as global, while tracks reconstructed only with the TPC are referred to as TPC only. For online triggering, the V0 and silicon pixel detectors were used [39]. The reconstructed primary vertex is required to lie within 10 cm of the nominal interaction point in the longitudinal direction along the beam axis. The cut on the position of the primary vertex along the beam axis was varied from 12 to 6cm; the resulting SC measurements are consistent with those obtained with the default cut. The main analysis was performed with global tracks selected in the transverse momentum interval 0.2<p T< 5.0GeV=c and the pseudorapidity region jηj<0.8. With this choice of a low pTcutoff we are reducing event-by- event biases from a smaller reconstruction efficiency at lower pT, while the high pTcutoff was introduced to reduce the contribution to the anisotropies from the jets. Reconstructed tracks were required to have at least 70 TPC space points (out of a maximum of 159). Only tracks with a transverse distance of closest approach (DCA) to the primary vertex less than 3 mm are accepted to reduce the contamination from secondary tracks. Tracks with kinks (the tracks that appear to change direction due to multiple scattering, Kdecays) were rejected. An independent analysis was performed with TPC-only and hybrid tracks (see below). For TPC-only tracks, the DCA cut was relaxed to 3 cm, providing a different sensitivity to contamination from the secondary tracks. Both the azimuthal acceptance and the reconstruction efficiency as a function of transverse momentum differ between the TPC-only and global tracks. The resulting difference between independent analyses with global and TPC-only tracks was found to be 1%–5% in all the centrality ranges studied, both for SC(3,2) and SC(4,2). In another independent analysis with hybrid tracks, three different types of tracks were combined, in order to overcome the nonuniform azimuthal acceptance due to dead zones in the silicon pixel detector, and to achieve the best transverse momentum resolution [39]. In this analysis the DCA cut was set to 3.2 cm in the longitudinal and to 2.4 cm in the transverse direction. The results between the global and hybrid tracks differ by 3% to 5%, depending on the observable considered. One of the largest contributions to the systematic uncertainty originates from the nonuniform reconstruction efficiency as a function of transverse momentum. For the observables SC(3,2) and SC(4,2) the uncertainty is 7% and 8%, respectively. In order to correct the measurements of these azimuthal correlators for various detector inefficiencies, we have constructed the particle weights as a function of azimuthal angle φand transverse momentum pT, and used the prescription outlined in Ref. [28]. In particular, pT weights were constructed as a ratio of the transverse momentum distribution obtained from Monte Carlo generated tracks and from tracks reconstructed after they have passed through the detector simulated with GEANT 3[46]. We have used four Monte Carlo models in this Letter. The HIJING model [47,48] was utilized to obtain the pT weights [28]. Second, the HIJING model was used to estimate the strength of the nonflow correlations (typically few-particle correlations insensitive to the collision geometry). We have evaluated the observables of interest in coordinate space by modeling the initial conditions with a Monte Carlo Glauber model [49]. We have compared the PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 182301-3 centrality dependence of our observables with the theoretical model from Ref. [50], where the initial energy density profiles are calculated using a next-to-leading order perturbative-QCD+saturation model [51,52]. The subsequent spacetime evolution is described by relativistic dissipative fluid dynamics with different parametrizations for the temperature dependence of the shear viscosity to entropy density ratio η=sðTÞ. Each of the η=sðTÞparametrizations is adjusted to reproduce the measured vnfrom central to midperipheral collisions. Finally, we provide an independent estimate of the centrality dependence of our observables by utilizing the AMPT model [53]. The centrality dependence of SC(4,2) (red squares) and SC(3,2) (blue circles) is presented in Fig. 1. Positive values of SC(4,2) are observed for all centralities. This suggests a correlation between the event-by-event fluctuations of v2 and v4, which indicates that finding v2larger than hv2iin an event enhances the probability of finding v4larger than hv4iin that event. On the other hand, the negative results of SC(3,2) show the anticorrelation between the v2and v3 magnitudes, which further imply that finding v2larger than hv2ienhances the probability of finding v3smaller than hv3i. We have calculated the SC observables using HIJING , which does not include anisotropic collectivity but, e.g., azimuthal correlations due to jet production [47,48].Itis found that in HIJING both ⟪cosðmφ1þnφ2−mφ3−nφ4Þ⟫ and ⟪cos½mðφ1−φ2Þ⟫⟪cos½nðφ1−φ2Þ⟫are nonzero. However, the calculated SC observables from HIJING are compatible with zero for all centralities, which suggests that the SC measurements are nearly insensitive to nonflow correlations. We have also performed a study using the likesign technique, which is another powerful approach to estimate the nonflow effects [4]. It was found that the difference between the correlations for like-sign and all charged combinations is within 10%. This demonstrates that nonzero values of SC measurements cannot be explained by nonflow effects. A study based on the AMPT model showed that the observed (anti)correlations are also sensitive to the transport properties, e.g., the partonic and hadronic interactions [20,28]. Figure 2shows the comparison of SC(3,2) and Centrality percentile 0 10203040506070 SC(m,n) 2− 1− 0 1 2 3 6− 10× = 2.76 TeV NN sALICE Pb-Pb SC(4,2) SC(3,2) HIJING SC(4,2) SC(3,2) FIG. 1. Centrality dependence of the observables SC(4,2) (red filled squares) and SC(3,2) (blue filled circles) in Pb-Pb collisions at 2.76 TeV. Systematic errors are represented with boxes. The results for the HIJING model are shown with hollow markers. 10 20 30 40 50 60 70 SC(m,n) 3− 2− 1− 0 1 2 36− 10× = 2.76 TeV NN sALICE Pb-Pb SC(4,2) SC(3,2) Hydrodynamics /s=0.20ηSC(4,2), /s(T) param1ηSC(4,2), /s(T) param4ηSC(4,2), /s=0.20ηSC(3,2), /s(T) param1ηSC(3,2), /s(T) param4ηSC(3,2), AMPT: String Melting SC(3,2) SC(4,2) 10 20 30 40 50 60 70 〉 2 n v〈〉 2 m v〈SC(m,n)/ 0.6− 0.4− 0.2− 0 0.2 0.4 0.6 0.8 1〉 2 2 v〈〉 2 4 v〈SC(4,2)/ 〉 2 2 v〈〉 2 3 v〈SC(3,2)/ Hydrodynamics /s=0.20η,〉 2 2 v〈〉 2 4 v〈SC(4,2)/ /s(T) param1,2,3,4η,〉 2 2 v〈〉 2 4 v〈SC(4,2)/ /s=0.20η,〉 2 2 v〈〉 2 3 v〈SC(3,2)/ /s(T) param1,2,3,4η,〉 2 2 v〈〉 2 3 v〈SC(3,2)/ AMPT: String Melting 〉 2 2 v〈〉 2 3 v〈SC(3,2)/ 〉 2 2 v〈〉 2 4 v〈SC(4,2)/ Centrality percentile 0 10203040506070 〉 2 n ε〈〉 2 m ε〈/ ε SC(m,n) 0.2− 0 0.2 0.4 0.6 0.8 1 〉 2 2 v〈〉 2 4 v〈SC(4,2)/ 〉 2 2 v〈〉 2 3 v〈SC(3,2)/ MC-Glauber , WN〉 2 2 ε〈〉 2 4 ε〈/ ε SC(4,2) , BC〉 2 2 ε〈〉 2 4 ε〈/ ε SC(4,2) , WN〉 2 2 ε〈〉 2 3 ε〈/ ε SC(3,2) , BC〉 2 2 ε〈〉 2 3 ε〈/ ε SC(3,2) FIG. 2. AMPT model predictions are shown as hollow symbols in the (top) and (middle) panels. Top: comparison of the observables SC(4,2) (red filled squares) and SC(3,2) (blue filled circles) to the theoretical model from Ref. [50]. The solid lines indicate the predictions with constant η=s, while the dashed lines indicate predictions for different parametrizations of the η=s temperature dependence (labeled in the same way as in Fig. 1 in Ref. [50]). Middle: results divided by hv2 mihv2 ni. Bottom: comparison to the Monte Carlo Glauber model using wounded nucleon (WN) and binary collision (BC) weights. PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 182301-4 SC(4,2) to the AMPT calculations, which generally predict the correct sign but underestimate their magnitude. The comparison between experimental data and the theoretical calculations [50], which incorporate both the initial conditions and the system evolution, is shown in Fig. 2(top). The model captures qualitatively the centrality dependence, but not quantitatively. Most notably, there is no single centrality for which a given η=sðTÞparametrization describes simultaneously both SC(4,2) and SC(3,2). On the other hand, the same theoretical model captures quantitatively the centrality dependence of the individual v2,v3, and v4harmonics with a precision better than 10% in the central and midcentral collisions [50]. We therefore conclude that the individual flow harmonics vnand new SCðm; nÞobservables together provide a better handle on the initial conditions and η=sðTÞthan each of them alone. This is emphasized in Fig. 2(middle), where the SC(3,2) and SC(4,2) observables were divided with the products hv2 3ihv2 2iand hv2 4ihv2 2i, respectively, in order to obtain the normalized SC observables (the result for 60%–70% is omitted due to the large statistical uncertainty). These products were obtained with two-particle correlations and using a pseudorapidity gap of jΔηj>1.0to suppress biases from few-particle nonflow correlations. We have found that the normalized SC(4,2) observable exhibits much better sensitivity to different η=sðTÞparametrizations than the normalized SC(3,2) observable, see Fig. 2 (middle), and than the individual flow harmonics [50]. These findings indicate that the normalized SC(3,2) observable is sensitive mainly to the initial conditions, while the normalized SC(4,2) observable is sensitive to both the initial conditions and the system properties, which is consistent with the prediction from Ref. [25]. It can be seen in Fig. 1that SC(4,2) and SC(3,2) increase nonlinearly up to centrality 60%. Assuming only a linear response, vn∝εn, we expect that the normalized SCðm; nÞ evaluated in coordinate space can capture the measurement of the centrality dependence of the normalized SCðm; nÞin the momentum space. The correlations between the nth and mth order harmonics were estimated with calculations of ðhε2 nε2 mi−hε2 nihε2 miÞ=hε2 nihε2 mi, i.e., a normalized SC observable in the coordinate space, which we denote SCðm; nÞε=hε2 nihε2 mi. Here, εn(εm) is the nth (mth) order coordinate space anisotropy, following the definition in Ref. [8]. Different scenarios of the Monte Carlo Glauber model, named the wounded nucleon and binary collision weights, have been used. An increasing trend from central to peripheral collisions with different sign has been observed in Fig. 2(bottom) for SC(4,2) and SC(3,2). A dramatic deviation of SC(4,2) between data and the model calculation is observed for noncentral collisions. This deviation increases from midcentral to peripheral, which could be understood as the contribution of the nonlinear response (ε2contributes to v4) increasing as a function of centrality, which is consistent with that reported in Ref. [54]. Since the normalized SC(3,2) appears to be sensitive only to the initial conditions and not to η=sðTÞ, see Fig. 2(middle), the Monte Carlo Glauber model captures better its centrality dependence than it does for the normalized SC(4,2) observable, see Fig. 2(bottom). The relationship between the flow harmonics v2,v3,v4 has also been investigated by the ATLAS Collaboration using the event shape engineering technique [54–56].For events with a larger v2, the ATLAS Collaboration showed these have a smaller than average v3, and a larger than average v4. For events with a smaller v2, the opposite trend occurred. These observations are consistent with the patterns observed via the SC measurements presented in this Letter. The SC observables, however, provide a compact quantitative measure of these correlations, without fitting correlations between vnand vm. This simplifies the quantitative comparison of the SC observables with hydrodynamical calculations as shown in Fig. 2. In the most central collisions the anisotropies originate mainly from fluctuations; i.e., the initial ellipsoidal Centrality percentile 123456789 SC(m,n) 50− 0 9− 10× = 2.76 TeV NN sALICE Pb-Pb SC(4,2) SC(3,2) Centrality percentile 012345678910 〉 2 n v〈〉 2 m v〈SC(m,n)/ 0 0.1 0.2 = 2.76 TeV NN sALICE Pb-Pb 〉 2 2 v〈〉 2 4 v〈SC(4,2)/ 〉 2 2 v〈〉 2 3 v〈SC(3,2)/ MC-Glauber , WN〉 2 2 ε〈〉 2 4 ε〈/ ε SC(4,2) , BC〉 2 2 ε〈〉 2 4 ε〈/ ε SC(4,2) , WN〉 2 2 ε〈〉 2 3 ε〈/ ε SC(3,2) , BC〉 2 2 ε〈〉 2 3 ε〈/ ε SC(3,2) FIG. 3. Top: correlated and anticorrelated event-by-event fluctuations in coordinate (Monte Carlo Glauber model) and momentum space (data). Bottom: normalized SC observables, where the pseudorapidity gap jΔηj>1.0was applied in both twoparticle correlations in the denominator used to estimate the individual flow harmonics. PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 182301-5 geometry characteristic for midcentral collisions plays little role in this regime. Therefore, we have performed a separate analysis for the centrality range 0%–10% in centrality bins of 1%. The results are presented in Fig. 3. We observe that event-by-event fluctuations of v2 and v4remain correlated, and of v2and v3anticorrelated, also in this regime. However, the strength of the (anti) correlations exhibits a different centrality dependence than for the wider centrality range shown in Fig. 1. As seen in Fig. 3(top) the centrality dependence cannot be linearly extrapolated from the 0%–10% region to the full centrality range. Comparison with the two different parametrizations of the Monte Carlo Glauber initial conditions for the normalized SC observables presented in Fig. 3(bottom) suggests that the binary collision parametrization (binary collision weights) is favored by the data in most central collisions. This agreement may suggest the scaling with the number of quark participants [57–61] in central collisions at the LHC energies. In summary, we have measured for the first time the new multiparticle observables, the symmetric two-harmonic four-particle cumulants, which quantify the relationship between the event-by-event fluctuations of two different flow harmonics. We have found that the fluctuations of v2 and v3are anticorrelated in all centralities; however, the details of the centrality dependence differ in the fluctutation-dominated (most central) and the geometry-dominated (midcentral) regimes. The fluctuations of v2and v4are correlated for all centralities. The SC observables were used to discriminate between the state-of-the-art hydro model calculations with different parametrizations of the temperature dependence of η=s, for all of which the centrality dependence of elliptic, triangular, and quadrangular flow has a weaker sensitivity at the LHC. In particular, the centrality dependence of SC(4,2) cannot be captured with the constant η=s. We have also used our results to discriminate between two different parametrizations of the initial conditions and have demonstrated that in the fluctuation-dominated regime (in central collisions) the Monte Carlo Glauber initial conditions with binary collision weights are favored over wounded nucleon weights. The ALICE Collaboration would like to thank Harri Niemi for providing the latest predictions from the state-of- the-art hydrodynamic model. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centers and the Worldwide LHC Computing Grid (WLCG) Collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); Ministry of Science and Technology of China (MSTC), National Natural Science Foundation of China (NSFC) and Ministry of Education of China (MOEC); Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia; Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; the Helsinki Institute of Physics and the Academy of Finland; the French Centre national de la recherche scientifique-Institut national de physique nucléaire et de physique des particules (CNRS-IN2P3), the “Region Pays de Loire,”“Region Alsace,”“Region Auvergne”and CEA, France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; National Research, Development and Innovation Office (NKFIH), Hungary; Council of Scientific and Industrial Research (CSIR), New Delhi; Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi—Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi,”Italy; Japan Society for the Promotion of Science (JSPS) KAKENHI and MEXT, Japan; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico (DGAPA), México, Amerique Latine Formation academique—European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin American Network); Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); Pontificia Universidad Católica del Perú; National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and National Council of Scientific Research in Higher Education (CNCSI-UEFISCDI), Romania; Joint Institute for Nuclear Research, Dubna; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio PRL 117, 182301 (2016) PHYSICAL REVIEW LETTERS week ending 28 OCTOBER 2016 182301-6 de Economía y Competitividad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); National Science and Technology Development Agency (NSDTA), Suranaree University of Technology (SUT) and Office of the Higher Education Commission under NRU project of Thailand; Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); the United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio. 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