News on collectivity in Pb-Pb collisions from the ALICE experiment

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1 EPJ Web of Conferences, 000 (07) DOI: 0.05/ epjconf/07000 ISMD 06 News on collectiity in Pb-Pb collisions from the ALICE experiment D.J Kim,a for the ALICE Collaboration Uniersity of Jyäskylä and Helsinki Institute of Physics (HIP), Finland Abstract. The collectie expansion of the color-deconfined fireball created in ultrarelatiistic heay-ion collisions maps the initial state of the quark-gluon plasma (QGP) to the final-state particle spectrum. The ALICE experiment has been leading important roles for completing the indiidual flow harmonic measurements at the highest energies to date as well as improing flow harmonic correlation techniques to understand the properties of the QGP and the full eolution of the heay-ion collisions. In this article, a brief summary of the indiidual flow harmonic measurements, the details of the new obserables deeloped in recent years from ALICE collaboration and their implications to future studies are discussed. Introduction The main emphasis of the ultra-relatiistic heay-ion collisions at the Relatiistic Heay Ion Collider (RHIC) and the Large Hadron Collider (LHC) is to study deconfined phase of the strongly interacting nuclear matter, the Quark-Gluon Plasma (QGP). This matter exhibits strong collectie and anisotropic flow in the plane transerse to the beam direction, which is drien by the anisotropic pressure gradients, resulting in more particles emitted in the direction of the largest gradients. The large elliptic flow discoered at RHIC energies [] continues to increase also at LHC energies [, 3]. This has been predicted by calculations utilizing iscous hydrodynamics [ 9]. These calculations also demonstrated that the shear iscosity to the entropy density ratio (η/s) of QGP is close to a uniersal lower bound /π [0] in heay-ion collisions at RHIC and LHC energies. The temperature dependence of the η/s has some generic features that most of the known fluids obey. For instance, one such general behaior is that the ratio typically reaches its minimum alue close to the phase transition region []. It was shown, using kinetic theory and quantum mechanical considerations [], that η/s 0. would be the correct order of magnitude for the lowest possible shear iscosity to entropy ratio alue found in nature. Later it was demonstrated that an exact lower bound (η/s) min = /π 0.08 can be calculated using the AdS/CFT correspondence [0]. Hydrodynamical simulations support as well the iew that the QGP matter is close to that limit [8]. This in turn may hae important implications for other fundamental physics goals. It is argued that such a low alue might imply that thermodynamic trajectories for the expanding matter would lie close to the quantum chromodynamics (QCD) critical end point, which is another subject of intensie experimental quest []. a djkim@jyu.fi The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creatie Commons Attribution License.0 (

2 EPJ Web of Conferences, 000 (07) DOI: 0.05/ epjconf/07000 ISMD 06 Anisotropic flow [3] is traditionally quantified with harmonics n and corresponding symmetry plane angles Ψ n in the Fourier series decomposition of the azimuthal particle distribution in the plane transerse to the beam direction []: E d3 N dp = d N { + 3 n (p T,η) cos[n(ϕ Ψ n )] }, () π p T dp T dη where E, N, p, p T, ϕ and η are the energy, particle yield, total momentum, transerse momentum, azimuthal angle and pseudorapidity of particles, respectiely, and Ψ n is the azimuthal angle of the symmetry plane of the n th -order harmonic. The n th -order flow coefficients are denoted as n and can be calculated as n = cos[n(ϕ Ψ n )], where the brackets denote an aerage oer all particles in all eents. The anisotropic flow in heay-ion collisions is understood as the hydrodynamic response of produced matter to spatial deformations of the initial energy density profile [5]. This profile fluctuates eent-by-eent due to fluctuations of the positions of the constituents inside the colliding nuclei, which in turn implies that the flow also fluctuates [6, 7]. The recognition of the importance of flow fluctuations has led to measurements of triangular flow and higher flow harmonics [8, 9] as well as the correlations between different Fourier harmonics [0, ]. The higher order harmonics are expected to be particularly sensitie to fluctuations in the initial conditions and to the η/s [, 3], while correlations hae the potential to discriminate the two respectie contributions to anisotropic flow deelopment [0]. Howeer, difficulties on extracting η/s in heay-ion collisions can be attributed mostly to the fact that it strongly depends on the specific choice of the initial conditions [, 3, ]. The iscous effects reduce the magnitude of the elliptic flow. Furthermore, the magnitude of η/s used in hydrodynamic model calculations should be considered as an aerage oer the temperature history of the expanding fireball as it is known that η/s of other fluids depends on temperature. In addition, part of the elliptic flow can also originate from the hadronic phase [5 7]. Therefore, knowledge of both the temperature dependence and the relatie contributions from the partonic and hadronic phases should be understood better to quantify η/s of the partonic fluid. The ALICE experiment has been leading important roles for completing the indiidual flow harmonic measurements at the highest energies to date as well as improing flow harmonic correlation techniques to understand the properties of the QGP and the full eolution of the heay-ion collisions. In this article, few selected results from recently published papers from the ALICE experiment are discussed in the following sections on the emphasis of extracting η/s. n= Results Firstly, we show the multiparticle obserables, the Symmetric -harmonic -particle Cumulants (SC), which quantify the relationship between eent-by-eent fluctuations of two different flow harmonics [8]. These obserables are particularly robust against few-particle non-flow correlations and they proide orthogonal information to recently analyzed symmetry plane correlators. It was demonstrated that they are sensitie to the temperature dependence of η/s of the expanding medium and therefore simultaneous descriptions of different order harmonic correlations would constrain both the initial conditions and the medium properties [8, 9]. These results are discussed in Sec... Second, pseudorapidity dependent charge particle, 3 and are measured in wide range of pseudorapidity and shown in Sec... These results can proide access to a range of arying medium properties, een at a fixed collision energy. Thirdly, the first results of the charged hadron n with the highest LHC beam energy are presented in Sec..3.

3 EPJ Web of Conferences, 000 (07) DOI: 0.05/ epjconf/07000 ISMD 06. Eent-by-eent fluctuations of two different flow harmonics SC(m,n) ALICE Pb-Pb s NN SC(,) =.76 TeV SC(3,) 0 3 SC(,), η/s=0.0 SC(,), η/s(t) param SC(,), η/s(t) param SC(3,), η/s=0.0 SC(3,), η/s(t) param SC(3,), η/s(t) param AMPT: String Melting SC(3,) SC(,) SC(m,n)/ n m SC(,)/ SC(3,)/ 3 SC(,)/, η/s=0.0 AMPT: String Melting SC(,)/, η/s(t) param,,3, SC(3,)/, η/s=0.0 SC(3,)/ 3 3 SC(3,)/, η/s(t) param,,3, SC(,)/ Centrality percentile Figure. (Top Left) Comparison of obserables SC(,) (red filled squares) and SC(3,) (blue filled circles) to theoretical model from [30]. (Left Bottom) The results from top left panel hae been rescaled with m n. (Right) The η/s(t) parameterizations used in theoretical model [30]. The SC obserables are defined as: cos(mϕ +nϕ mϕ 3 nϕ ) c = cos(mϕ +nϕ mϕ 3 nϕ ) cos[m(ϕ ϕ )] cos[n(ϕ ϕ )] = mn m n, () with the condition m n for two positie integers m and n. The complete discussion can be found in Section IV C of Ref. [3]. SC(m,n) can be normalized with the product m n to obtain normalized symmetric cumulants [8, 3]. Normalized symmetric cumulants reflect only the degree of the correlation which is expected to be insensitie to the magnitudes of m and n, while SC(m, n) contains both the degree of the correlations between two different flow harmonics and indiidual n harmonics. That products in the denominator are obtained with two-particle correlations and using a pseudorapidity gap of Δη >.0 to suppress biases from few-particle nonflow correlations. On the other hand, in the two two-particle correlations which appear in the definition of SC(m, n) in Eq. the pseudorapidity gap is not needed, since nonflow is suppressed by construction in SC obserable, as the study based on HIJING model has clearly demonstrated in Ref. [8]. The measurements of SC obserables hae reealed that fluctuations of and 3 are anticorrelated, while fluctuations of and are correlated in all centralities as shown in Figure. (Top Left). The comparison between experimental data and the theoretical calculations [30], which incorporate both the initial conditions and system eolution, is shown in Figure. (Top Left). The model captures qualitatiely the centrality dependence, but not quantitatiely. Most notably, there is 3

4 EPJ Web of Conferences, 000 (07) DOI: 0.05/ epjconf/07000 ISMD 06 n 0.08 Pb-Pb s NN =.76 TeV 0-30% ALICE {} 3 {} {} {} 3 {} {} n 0. ALICE Pb-Pb TeV {, Δη >} 3 {, Δη >} {, Δη >} {} {6} {8}.76 TeV {, Δη >} 3 {, Δη >} {, Δη >} {} 5.0 TeV, Ref.[7] {, Δη >} 3 {, Δη >} n η 0-50% Ratio Ratio... 3, Ref.[5] η /s(t), param η/s = 0.0 (a). (c) Centrality percentile Figure. (Left) Comparisons to hydrodynamics predictions [33], where input parameters (temperature dependence of η/s) hae been tuned to RHIC data for the Pb-Pb 0-30% (top) and 0-50% (Bottom) centralities. (Right) The first anisotropic flow measurements of charged particles in Pb Pb collisions at s NN = 5.0 TeV. (b) no single centrality for which a gien η/s(t) parameterization describes simultaneously both SC(, ) and SC(3, ). On the other hand, the same theoretical model captures quantitatiely the centrality dependence of indiidual, 3 and harmonics with a precision better than 0% in central and midcentral collisions [30]. We therefore conclude that indiidual flow harmonics n and new SC(m, n) obserables together proide a better handle on the initial conditions and η/s(t) than each of them alone. This is further emphasized in Figure (Bottom), where SC(3, ) and SC(, ) obserables were diided with the products 3 and, respectiely. We hae found that the normalized SC(, ) obserable exhibits stronger sensitiity to different η/s(t) parameterizations than the normalized SC(3, ) obserable, see Figure (Bottom), and than the indiidual flow harmonics [30]. These findings indicate that the normalized SC(3, ) obserable is sensitie mainly to the initial conditions, while the normalized SC(, ) obserable is sensitie to both the initial conditions and the system properties like η/s, which is consistent with the prediction from [0].. Moing forward to constrain the shear iscosity of QCD matter ALICE recently published peudorapidity dependence of flow harmonics up to th order harmonics [3]. At forward rapidities, the system will spend less time in the QGP phase. This implies that the iscosity from the hadronic phase would play a greater role in affecting the flow harmonics [33, 35]. Therefore, the relatie decrease of the flow harmonics in different η ranges may help to disentangle the iscous effects from the hadronic phase with those from the QGP phase. The results are shown on the left panel in Fig., the shape of n (η) is largely independent of centrality for all order harmonic coefficients measured (, 3 and ). The results are compared to hydrodynamic calculations tuned to RHIC data [33]. The tuning inoles finding a parameterization of the temperature dependence of η/s, so that the hydrodynamical calculations describe PHOBOS measurements of (η) [36, 37]. It is

5 EPJ Web of Conferences, 000 (07) DOI: 0.05/ epjconf/07000 ISMD 06 clear that the same parameterization does not describe the LHC data. For both centralities shown on the left in Figure, the elliptic flow coefficient is generally underestimated, while the higher order coefficients 3 and are oerestimated. These results could proide better independent constraints for the initial state fluctuations in three-dimensional space and η/s(t) een at a fixed collision energy..3 Anisotropic flow of charged particles at s NN =5.0 TeV We hae presented the first anisotropic flow measurements of charged particles in Pb Pb collisions at s NN = 5.0 from the data taken in Noember 05 in Run at the LHC in Figure [3]. Only one low luminosity run (with trigger rate of 7 Hz) was used, being least affected by pile-up and distortions from space charge in the main tracking detector, the Time Projection Chamber (TPC). In the right panel of Figure (a), the centrality dependence of, 3 and is presented from two- and multi-particle cumulants, integrated oer the p T range 0. < p T < 5.0 GeV/c, for.76 and 5.0 TeV Pb Pb collisions. To elucidate the energy eolution of, 3 and, the ratios of anisotropic flow measured at 5.0 TeV to.76 TeV are presented in Figure right panel (b) and (c). The predictions of anisotropic flow coefficients n from the hydrodynamic model [38] are compared to the measurements in Figure. (a). The predictions are compatible with the measured anisotropic flow n coefficients. At the same time, a different hydrodynamic calculation [39], which employs both constant η/s = 0.0 and temperature dependent η/s, can also describe the increase in anisotropic flow measurements of. The increase of and 3 from the two energies is rather moderate, while for it is more pronounced. An increase of (3.0±0.6)%, (.3±.)% and (0.±3.8)%, is obtained for elliptic, triangular and quadrangular flow, respectiely, oer the centrality range 0 50% in Pb Pb collisions when going from.76 TeV to 5.0 TeV. 3 Summary The ALICE experiment continues to play a critical role for completing the indiidual flow harmonic measurements at the highest energies to date. We are improing flow harmonic correlation techniques to understand the properties of the QGP and the full eolution of the heay-ion collisions i.e different order flow harmonic correlations and peudorapidity dependence of flow harmonics. The different order flow harmonic correlation measurements proide strong constraints on the temperature dependence of the shear iscosity to the entropy density ratio in hydrodynamics in combination with the indiidual flow harmonics. Pseudorapidity dependence of,3, results can proide access to a range of arying medium properties, een at a fixed collision energy. The indiidual flow harmonics with the highest LHC energy were measured and show a quantitatie agreement with arious models. Many analyses with Run Pb-Pb data with the full statistics are underway and the statistical and systematic errors will be reduced in the future to gie aluable inputs to model calculations with discriminating powers on initial conditions and the transport properties of nuclear matter in heay-ion collisions. References [] K.H. Ackermann et al. (STAR), Phys. Re. Lett. 86, 0 (00), nucl-ex/00090 [] K. Aamodt et al. (ALICE), Phys. Re. Lett. 05, 530 (00), 0.39 [3] J. Adam et al. (ALICE), Phys. Re. Lett. 6, 330 (06), [] P. Romatschke, U. Romatschke, Phys. Re. Lett. 99, 730 (007), [5] C. Shen, U. Heinz, P. Huoinen, H. Song, Phys. Re. C8, 0903 (0), [6] B. Schenke, S. Jeon, C. Gale, J. Phys. G38, 69 (0) 5

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