ATLAS measurement of reactionplane correlations in PbPb collisions

ATLAS measurement of reaction-plane correlations in Pb-Pb collisions at √s. NN =2. 76 Te. V Jiangyong Jia for the ATLAS Collaboration Conference Note at: https: //cdsweb. cern. ch/record/1451882 v 1 -v 6 measurements, see ar. Xiv: 1203. 3087 and talk by S. Mohapatra Tuesday, plenary 2 B Hard Probe 2012 May 27 th- June 1 st

2 Motivation Alver, Roland etc ~400 nucleons n n ~20000 particles Initial geometry has multiple shape components due to fluctuations. They are transferred by flow into momentum space with global phases Φn. Additional information can be inferred from correlations of Φn!

How to measure the reaction plane correlations n Two-plane correlations in n Dictated by invariance under , k = least common multiple of n & m and any global rotation ar. Xiv: 1104. 4740 3

How to measure the reaction plane correlations n Two-plane correlations in n n Dictated by invariance under 4 , k = least common multiple of n & m and any global rotation ar. Xiv: 1104. 4740 Measure correlation between EP, followed by a simple resolution correction. Desired correlator Observed correlator ar. Xiv: 1105. 3928 PHENIX but no corrections for reso. Resolution for individual planes

Two-plane correlators n n Sensitivity limit is set by the raw signal (n<=6) and the values of Res{} List of two-plane correlators: 5

6 Two-plane correlators n n n Sensitivity limit is set by the raw signal (n<=6) and the values of Res{} List of two-plane correlators: Can generalize into multi-plane correlations n Variable: satisfying ar. Xiv: 1104. 4740 ar. Xiv: 1203. 5095 1205. 3585 Introduce notation

Three-plane correlators “ 2 -3 -5” “ 2 -4 -6” “ 2 -3 -4” n Constructed as linear combination of two-plane correlators, for example: Reflects correlation of two Φn relative to the third angle 7

Expectations from Glauber Model n n 8 Plane directions in configuration space Expected to be strongly modified by medium evolution in the final state ar. Xiv: 1011. 1853, 1203. 5095 1205. 3585

ATLAS detectors for Event Plane: Ψn ECal+FCal |η|<4. 9 Divide into subevents with flexible η ranges! 9

ATLAS detectors for Event Plane: Ψn ECal+FCal |η|<4. 9 Inner detector Tracks |η|<2. 5 Divide into subevents with flexible η ranges! 10

Obtaining the event plane correlations 11 So just combine into one measurement A & C are symmetric so only 3 combined measurements: Type 1, 2, 3 n n Gap is required to remove autocorrelation, more important for Res. Event-mixing to check acceptance effect (planes taken from different events)

Observed two-plane correlation signal 4(Ψ 2 -Ψ 4) 6(Ψ 3 -Ψ 6) 6(Ψ 2 -Ψ 3) 6(Ψ 2 -Ψ 6) 12(Ψ 3 -Ψ 4) 10(Ψ 2 -Ψ 5) Significant correlations beyond detector systematics 12

Correlation between Φ 2 and Φ 3: “ 2 -3” corr Observed correlation n Combined Res Corrected result Small observed signal, good resolution small corrected signal (<0. 02) 13

Compare with ALICE n Overall magnitudes are similar, but our data show slight increase towards peripheral collisions. 14

Correlation between Φ 2 and Φ 4: “ 2 -4” corr n Coefficients decrease slowly with j, imply a narrow correlation. n n Φ 2 and Φ 4 correlation due to average geometry? Note: lowest-order coefficient is the projection of v 4 onto Ψ 2 plane e. g. ar. Xiv: 1205. 5761 15

Correlation of Φ 2 or Φ 3 with Φ 6 n n Φ 2 and Φ 3 weakly correlated, but both strongly correlated with Φ 6. They show opposite centrality dependence n n Φ 2 Φ 6 correlation may due to average geometry, but Φ 3 Φ 6 correlation? v 6 dominated by non-linear contribution: v 23, v 32 ? Teaney and Yan 16

Φ 3 vs Φ 4 and Φ 2 vs Φ 5 correlations are weak (< few %) 17

Observed three-plane correlation signal Significant correlations signals observed 18

Three-plane correlation results “ 2 -3 -5” correlation “ 2 -4 -6” correlation “ 2 -3 -4” correlation Rich centrality dependence patterns are observed 19

Deriving the sine and cosine product 20

Summary n We measured correlations between two and three reaction planes n n Significant correlations are observed for Correlation is very small but nonzero for Correlation is negative for Some correlators qualitatively similar to Glauber model, others do not n n 21 Non-linear response of the medium to initial fluctuations i. e. correlations can be generated dynamically via hydrodynamic evolution? ar. Xiv: 1104. 0650 This measurement provides new constraints for models.

Cross-checks: two-plane correlations n Despite different acceptance and detector technology (EM energy vs charged tracks), results are consistent 22

Cross-checks: three-plane correlations 23

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25 From observed to final signal: cos (2Φ 2+3Φ 3 -5Φ 5) Raw signal n Combined reso Corrected signal Do each of the three types of combination separately and then combine The three types are consistent!

Selection of correlation observables n Can generalize into multi-plane correlations Introduce notation n Sensitivity limit is set by the resolution Res{} n n χn, thus Res{}, decreases fast with n Decrease slower with j, especially if χn is large ar. Xiv: 1205. 3585 26