News on Collectivity in PbPb Collisions at CMS

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1 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD 016 News on Collectiity in PbPb Collisions at CMS Dong Ho Moon 1,a on behalf of the CMS Collaboration 1 Chonnam National Uniersity, 61186, Gwangju, Republic of Korea Abstract. he flow anisotropies with the Fourier coefficients (n =, 3) for the charged particles produced in PbPb collisions at a nucleon-nucleon center-of-mass energy of 5.0 ev is studied with the CMS detector. In order to extract the Fourier coefficients, seeral methods were used, such as the scalar product method or multi-particle cumulant method. he results coer both of the low-p region (1 < p < 3 GeV/c) associated with hydrodynamic flow phenomena and the high-p region where anisotropic azimuthal distributions may reflect the path-length dependence of the parton energy loss in the created medium for the seen bins of collision centrality, spanning the rang of 0-60% most-central eents. 1 Introduction A Quark-Gluon-Plasma (QGP) is a state of matter which quantum chromodynamic (QCD) predicts that it will occur at extremely high temperature and density. he experiments at the Relatiistic Heay Ion Collider (RHIC) hae established the formation of QGP in ultra-relatiistic nucleus-nucleus interactions [1 4]. One of strong signatures of the existence for QGP is the suppression of high-transersemomentum (p ) hadron yield ia the nuclear modification factor, R AA, which is the ratio of the finalstate particle yields at a gien p alue to that expected based on a scaling of pp collision results in the absence of the medium [1 4]. he obseration of dijet asymmetry in PbPb collisions at the Large Hadron Collider (LHC) furnishes further proof of jet quenching, indicating a large energy loss for partons traersing the QGP medium. he measurement of R AA alone is not sufficient to differentiate among arious parton energy-loss models. In addition, it has been shown that R AA is not sensitie to sub-nucleon spatial fluctuations [5]. Additional obserables, such as the azimuthal anisotropy of high p hadrons, are needed to constrain the models [6 1]. he anisotropy can be characterized by the coefficient of the second-order Fourier harmonic,, in the azimuthal angle φ distribution of the hadron yield relatie to the reaction-plane angle Ψ RP, dn/dφ 1 + cos((φ Ψ RP )), where Ψ RP is defined by the short-axis direction of the almond -shaped interaction region created in a non-central heay-ion collision. In order to reduce the ambiguity on the measurement, scalar product or multi-particle cumulant methods can be replaced. Furthermore, 3, the coefficient of the higher harmonic, is particularly sensitie to the initial-state fluctuations. Indeed, 3 arises mainly from fluctuation in the soft sector and it remains to be seen if these fluctuations can generate triangular anisotropy at high p. In this paper, a study of the azimuthal anisotropy of ery high p particles (up to p 100GeV/c) for PbPb collisions at s NN = 5.0 ev at the LHC using the CMS detector is presented. he and a dmoon@cern.ch he Authors, published by EDP Sciences. his is an open access article distributed under the terms of the Creatie Commons Attribution License 4.0 (

2 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD coefficients are determined as a function of p and collision centrality in the pseudorapidity regions of η < 1, where η = ln[tan(θ/)] and θ is the polar angle relatie to the counterclockwise beam direction. he first precise measurement of the and 3 coefficients up to ery high p region in heay-ion collisions are represented by using scalar product method and the multi-particle cumulant analysis for seeral orders (4, 6 and 8) as a function of p. Experimental Setup.1 Eent Selection For this analysis, 015 PbPb data at s NN = 5.0 ev is utilized with an integrated luminosity of 404 μb 1. Eents were selected by using seeral trigger combinations in different p regions and centrality bins. he minimum-bias trigger coers the low p region (< 14 GeV/c), while the dedicated triggers on high p tracks are used for high p region up to 100 GeV/c. he L1 trigger seed of the triggers fired based on the highest E Regional Calorimeter rigger (RC) region in the barrel of the CMS detector ( η < 1.0). At the High Leel rigger (HL), the leading track is passing the strict quality selection criteria describes in section.. he performances of trigger for tracks used in analysis are shown in Figure 1. 1 CMS Preliminary PbPb, s NN = 5.0 ev CMS Preliminary PbPb, s NN = 5.0 ev Efficiency L1 Singlerack 1 L1 Singlerack 16 L1 Singlerack 0 L1 Singlerack 4 L1 Singlerack 8 L1+HL Efficiency HL track trigger, p >1 HL track trigger, p >18 HL track trigger, p >4 HL track trigger, p > p (GeV) Offline leading track p (GeV) Figure 1. he efficiencies of L1 trigger (left) and High leel trigger (right) as a function of p measured in η < 1.0. L1 trigger efficiencies are estimated for p of 1, 16, 0, 4 and 8 GeV/c, and HL efficiencies are done in the p regions of 1, 18, 4 and 34 GeV/c [13].. rack Selection Primary-track candidates are selected by the high-purity tracks. Additional requirements are also applied to enhance the purity of primary tracks. he significance of the separation along the beam axis (z) between the track and the best ertex, dz/σ(dz), and the significance of the impact parameter relatie to the best ertex transerse to the beam, d /σ(d ), should be less than 3. he relatie uncertainty of the transerse-momentum measurement, σ(p )/p, must be less than 10%. In order to

3 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD 016 reduce the fake rate of mis-reconstructed tracks and confirm high tracking efficiency, primary tracks with η < 1.0 and p > 1.0 GeV/c are used in the analysis. Furthermore, tracks aboe 0 GeV/c in the tracker are required to match with an energy deposit in the calorimeter to further decrease the fake rate. 3 Analysis Methods 3.1 Scalar product method he scalar-product" eent-plane method is used to study azimuthal correlations with long-range of pseudorapidity. he eent-plane is determined eent-by-eent and consists of the beam direction and the azimuthal direction of maximum particle density. Aoiding autocorrelation effects where a gien particle, or its decay products is ery important to determine the eent plane which we want to characterize. his is usually done by requiring a large gap in pseudorapidity between the particles used to establish the eent plane and those used to determine the Fourrier coefficients. he azimuthal dependence of the particle yield can be written in terms of an harmonic expansion with [14] E d3 N d 3 p = 1 π d N p dp dy 1 + n cos [ n (φ Ψ) ] (1) where φ and E are the particle s azimuthal angle and energy, respectiely. If the impact-parameter direction is known, the reference angle Ψ can be decided as the azimuthal angle of the reaction plane Ψ R as defined by the beam and impact parameter directions. o make the largest pseudorapidity gap possible, elliptic flow (n = ) and triangular flow (n = 3) eent planes are defined using calorimeter data, with the HF n planes coering the pseudorapidity range of 5 η< 3 and HF + n planes coering the range 3 η<5. Another eent plane using tracker data with 0.75 η<0.75 was also defined and used in the three-sub-eent technique for determining the resolution corrections for the HF n, HF + n. o consider asymmetries that arise from the detector acceptance and other effects correlated with the detector conditions, a two-step process is used where the eent plane is first recentered and subsequently flattened [15]. In order to measure the actual n coefficients, the current analysis utilizes the scalar product method introduced firstly by the SAR collaboration in a study of elliptic flow in AuAu collisions at s NN = 130 GeV [16]. he Q-ectors are defined as n=1 Q n M e inφ i, () i=1 where M is the number of particles in each eent. herefore, using the scalar product method, n harmonics can be expressed as Qn QnA n {SP} QnA Q nb Q na Q nc Q nb Q nc. (3) Here, the subscripts A, B, and C refer to three separate eent planes reconstructed in different regions of pseudorapidity. he particles of interest are expressed by the Q n ector and are correlated with the A eent plane. he eent planes B and C effectiely correct for the finite resolution of the A eent plane that results from finite particle multiplicities and detector effects [17]. 3

4 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD Multi-particle correlation technique with cumulants Another way to measure is the Q-Cumulant method [18, 19] from 4-, 6- and 8-particle correlations. his method supplies a fast, exact (no approximations), and unbiased (no interference between different harmonics) estimations for the cumulants. he cumulants are expressed in terms of the moments of the magnitude of the Q n flow ector. We first define the m-particle (m =, 4, 6 or 8) correlator as e in(φ 1 φ ) 4 e in(φ 1+φ φ 3 φ 4 ) 6 e in(φ 1+φ +φ 3 φ 4 φ 5 φ 6 ) 8 e in(φ 1+φ +φ 3 +φ 4 φ 5 φ 6 φ 7 φ 8 ) (4) where φ i is the azimuthal angle of the i-th particle, and indicates that the aerage is taken oer all m-particle combinations for all eents. In order to remoe self-correlations, it is required that the m particles be distinctie. he unbiased estimators of the reference m-particle cumulants, c n m, are defined as c n {4} = 4, c n {6} = , c n {8} = (5) Correlating the m-particles within the reference phase space of η <.4 and p range of 0.3 < p < 3.0 GeV/c, we then obtain the reference flow {m}, with 4 4 {4} = cn 4, {6} = 4 c n6, 8 8 {8} = 1 33 c n8. With respect to the reference flow, the differential m(p,η) can then be expressed as (6) {4}(p,η) = d {4}/ 3/4 c n {4}, (7) {6}(p,η) = d {6}/ 5/6 cn {6}/4 1/6, (8) {8}(p,η) = d {8}/ 7/8 c n {8}/33 1/8, (9) where the differential cumulants d {m} are estimated by replacing one of the m-particle cumulants in Eq. (5) with a particle from a certain particle of interest (POI) phase space in p or η [17]. 4 Results he and 3 results are shown in Figure. hose are measured by using the scalar product (SP) method as a function of p ( 100 GeV/c) in 7 centrality ranges (0 60%) of PbPb collisions at s NN = 5.0 ev. he statistical uncertainties are represented in the bars and the systematic uncertainties are denoted in the shaded boxes. 4

5 n EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD % 5-10% 10-0% 0-30% % 40-50% 50-60% CMS Preliminary PbPb s = 5.0 ev NN n {SP} {SP} 3 Figure. he and 3 results from SP method as a function of p in seen centrality ranges of PbPb collisions at s NN = 5.0 ev. Shaded boxes represent systematic uncertainties [17]. he significant positie alues are obsered in the most of centrality ranges up to p 100 GeV/c. he alue first rises and then falls off around 3 GeV/c quickly from low p to high p region. In the low p region, alues are first increasing from most central to mid-peripheral eents (30-40%), and then decreasing in the more peripheral centrality ranges. At the higher p region (>10 GeV/C), the alues are slowly decreasing as p increases. he 3 results show similar tendency as that for.atlowp, the obsered 3 alues don t depend on centrality. Significant positie 3 are found to continue up to p of 0 GeV/c for all centrality ranges. Aboe p = 0 GeV/c, the 3 alues are consistent with zero within gien uncertainties. hese results of and 3 coer the widest p range eer achieed. Such a result at high p is sensitie to the path length dependence of hard partons in the QGP and will be expected to put more constrain on the energy loss models. Figure 3 shows the comparison between results at s NN = 5.0 ev and results obtained with the Eent Plane (EP) at s NN =.76 ev [0] and theoretical calculations from CUJE3.0 [1, ] at 5.0 ev. he results from SP and EP are similar up to ery high p. he difference do not exceed 5%. he model (CUJE3.0) calculations are compatible with the results at high p (> 40 GeV/c) within systematic uncertainties in all oer centrality ranges except for the most peripheral one where the model doesn t reproduce the data. In order to inestigate the origin of this correlation obsered at ery high p further, we obtained alues from multi-particle cumulant analysis (4, 6 and 8) and compared with the SP results as shown in Figure 4. At low p (p < 3 GeV/c), the results follow the expectation from hydrodynamic models meaning {SP} > {4} {6} {8}. At high p (10 GeV/c), scalar product and multiparticle correlation results tend to conerge to the same alue. his obseration suggests strong eidence that collectie anisotropic particle emission in heay ion collisions extend to ery high p region. Although the nature of collectiity at high p is likely to be related to jet quenching phenomena, the obsered anisotropies at both high and low p are related to initial-state geometry and its eent-by-eent fluctuations. 5

6 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD % 5-10% 10-0% 0-30% % 40-50% 50-60% CMS Preliminary PbPb s = 5.0 ev NN {EP}.76 ev {SP} CUJE3 5.0 ev Figure 3. he results from SP and EP method as a function of p in seen centrality ranges of PbPb collisions at s NN = 5.0 ev and.76 ev respectiely. Shaded boxes represent systematic uncertainties [17] and dashed lines are predictions from CUJE3.0 model [1, ]. 0. CMS Preliminary PbPb s = 5.0 ev NN 5-10% 10-0% 0-30% n n % 40-50% {SP} {4} {6} {8} 50-60% (GeV/c) p (GeV/c) p (GeV/c) p Figure 4. Comparison between the results from SP and cumulant methods as a function of p in six centrality ranges of PbPb collisions at s NN = 5.0 ev. Shaded boxes represent systematic uncertainties [17]. 6

7 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD CMS Preliminary PbPb s NN = 5.0 ev {SP} {4} low 1.0 < p < 1.5 GeV/c Slope {SP} 0.5±4 {4} 0.47±6 Slope {SP} 0.39±3 {4} 0.47±6 Slope {SP} 0.7±3 {4} 0.4±7 high 5 14 < p < 0 GeV/c 0 < p < 6 GeV/c 6 < p < 35 GeV/c low low low Figure 5. he alues from different centrality ranges at high p :1< p < 14 GeV/c, 0 < p < 6 GeV/c and 6 < p < 35 GeV/c s. low p (1 < p < 1.5 GeV/c) obtained from SP (closed circles) and cumulant (open squares) methods in PbPb collisions at s NN = 5.0 ev [17]. o find possible connection of azimuthal anisotropies between low-p (dominated by hydrodynamics) and high p (dominated by jet quenching) regions, the correlation between the results at low and high p was studied. Figure 5 shows from 1 < p < 14 GeV/c, 0 < p < 6 GeV/c and 6 < p < 35 GeV/c as a function of from 1.0 < p < 1.5 GeV/c extracted from each centrality range of PbPb collisions at s NN = 5.0 ev. Each point represents a different centrality range starting from the most central eents on the left toward peripheral eents on the right. he first four points hae a centrality bin width of 5% (from 0 to 0%) while, for the others, the bin width is 10%. Full circles are the results obtained from SP method and open square are extracted from 4-particle correlation analysis. A strong correlation of alues between high and low p regions is obsered oer the full centrality range. While low p correlations can be explained by the collectie expansion of the medium, high p correlations come from the path-length dependence of the hard-scattered partons undergo while traersing the medium. his indicates that the initial geometry and its fluctuations may be responsible for the correlations obsered both at low and high p. 5 Summary In summary, the azimuthal anisotropy of charged particles with respect to the eent plane has been studied in PbPb collisions at s NN = 5.0 ev using the CMS detector. he and 3 alues are measured oer a wide p range from 1 GeV/c to 100 GeV/c indifferent collision centralities using scalar product method. he results coer for the first time the ery high p region beyond 60 GeV/c. Moreoer, these results proide a 3 measurement up to a p region that has neer before been studied (up to 100 GeV/c). he obsered (p ) shows a tendency of rapid increasing to a maximum at p of 3 GeV/c and a steep fall for all of centrality ranges. he 3 (p ) behaior is ery similar to at low p but its alue is consistent with zero oer the full centrality range for p > 0 GeV/c. he obsered trend in the centrality dependence of oer a wide p range suggests a potential connection to the initial-state geometry. o further inestigate the origin of the correlation at high p, a multi-particle correlation analysis was performed to extract from 1 GeV/c to, at maximum, 100 GeV/c. At low p, {4}, {6} and {8} hae similar alues and smaller than {SP}. his follows hydrodynamic 7

8 EPJ Web of Conferences 141, 0100 (017) DOI: 1051/ epjconf/ ISMD 016 expectations and is a sign of collectiity. he same behaior is obsered at high p and reflects the collectie nature of these correlations. As the last, the correlations between alues at low and high p as a function of centrality hae been studied. he results shows the strong correlations. he low and high p correlations are interpreted by the collectie expansion and by the path-length dependence of hard-scattered partons in the medium, respectiely. hese results indicate that the initial geometry and its fluctuations are responsible for the low and high p correlations. Acknowledgement his paper was financially supported by Chonnam National Uniersity (Grant number: ). References [1] BRAHMS Collaboration, Nucl. Phys. A 757, 1 (005) [] PHENIX Collaboration, Nucl. Phys. A 757, 184 (005) [3] PHOBOS Collaboration, Nucl. Phys. A 757, 8 (005) [4] SAR Collaboration, Nucl. Phys. A 757, 10 (005) [5] Noronha-Hostler, Jacquelyn and Betz, Barbara and Noronha, Jorge and Gyulassy, Miklos, arxi: [6] Peigne, S. and Smilga, A. V., arxi: [7] Wicks, Simon and others, Nucl. Phys. A 784, 46 (007) [8] Jia, Jiangyong and Wei, Rui, arxi: [9] Jia, Jiangyong and Horowitz, W.A. and Liao, Jinfeng, Phys. Re. C 84, (011) [10] Renk, horsten, Phys. Re. C 83, (011) [11] Betz, Barbara and Gyulassy, Miklos and orrieri, Giorgio, Phys. Re. C 84, (011) [1] Betz, Barbara and Gyulassy, Miklos, arxi: [13] CMS Collaboration, [14] Poskanzer, Arthur M. and Voloshin, S. A., Phys.Re. C 58, 1671 (1998) [15] Barrette, J. and others, Phys.Re. C 56, (1997) [16] Adler, C. and others, Phys.Re. C 66, (00) [17] CMS Collaboration, CMS PAS-HIN (015) [18] Bilandzic, Ante and Snellings, Raimond and Voloshin Sergei, Phys.Re. C 83, (011) [19] Bilandzic, Ante and Christensen, Christian Holm and Gulbrandsen, Kristjan and Hansen, Alexander and Zhou, You, Phys.Re. C 89, (014) [0] Chatrchyan, Serguei and others, Phys. Re. Lett. 109, 0301 (01) [1] Xu Jiechen and Liao Jinfeng and Miklos Gyulassy, Chinese Physics Letters 3, (015) [] Xu, Jiechen and Liao, Jinfeng and Gyulassy, Miklos, Journal of High Energy Physics 016, 1 50 (016) 8