1. Measurement of elliptic and higher order flow at Pb+Pb Collisions with the ATLAS detector Soumya Mohapatra for the ATLAS Collaboration Stony Brook University

2. Initial spatial fluctuations of nucleons lead to higher moments of deformations in the fireball, each with its own orientation. The spatial anisotropy is transferred to momentum space by collective flow The harmonics v n carry information about the medium: initial geometry, /s Understanding of higher order v n can shed light on the physics origin of ridge andcone seen in 2P correlations Soumya Mohapatra : Stony Brook University : ATLAS Collaboration Introduction and Motivation Singles: Pairs: EP method2PC method 2

3. Tracking coverage : | |<2.5 FCal coverage : 3.3<| |<4.8 (used to determine Event Planes) ATLAS Detector Soumya Mohapatra : Stony Brook University : ATLAS Collaboration |η|<2.5 3.3<|η|<4.8 3

4. Event Plane Method Bands indicate systematic errors 4

5. Centrality dependence of v n 5% Centrality bins + 0-1% centrality bin v 2 has a stronger centrality dependence. Other v n are flatter. In most central collisions, v 3,v 4 can be larger than v 2 at high enough p T 5

6. Similar trend across all harmonics (increase till 3-4GeV then decrease) In most central collisions(0-5%): v 3, v 4 can be larger than v 2. p T Dependence of v n Soumya Mohapatra : Stony Brook University vnvn vnvn 6

7. Observe scaling: v n 1/n =kv 2 1/2, where k is only weakly dependent on p T. R.Lacey et al. (http://arxiv.org/abs/1105.3782) v n scaling Soumya Mohapatra : Stony Brook University : ATLAS Collaboration (v n 1/n /v 2 1/2 ) 7

8. v n from RHIC to LHC Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 8 From Xiaoyang Gong Quark Matter 2011 Talk vn(Ψn)vn(Ψn)vn(Ψn)vn(Ψn)

9. Dependence of v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration weak dependence on ~5% drop within acceptance) For Correlations: relation is true only if the dependence is weak 9

10. Two Particle correlations Near-side jet peak is always visible Ridge seen in central and mid- central collisions, weak dependence Away side has double hump structure in most central events Peripheral events have near side peak truncated Ridge strength first increases then decreases with centrality Peripheral events have jet related peaks only 10

11. Obtaining harmonics from correlations Soumya Mohapatra : Stony Brook University : ATLAS Collaboration a)The 2D correlation function in. b)The corresponding 1D correlation function in for 2<| |<5 ( the | | cut removes near side jet) c)The v n,n obtained using a Discrete Fourier Transformation(DFT) d)Corresponding v n values Bands indicate systematic errors 11

12. dependence of v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration Repeat procedure in narrow slices to obtain v n vs v n values peak at low, due to jet bias. Relatively flat afterwards, so we require a | >2 gap (to remove near-side jet). Bands indicate systematic errors 12

13. Universality of v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 13 v n,n is expected to factorize into single v n for flow Obtain v n using fixed p T correlations cross-check via mixed-p T correlation Indeed, v n,n factorizes! (above certain )

14. Comparison between the two methods Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 14 Centrality Dependence

15. p T Dependence of v 2 Soumya Mohapatra : Stony Brook University : ATLAS Collaboration The 2PC v n for | |<0.5 deviates from the EP results (for all p T ) Good agreement seen for | |>2 at p T <4 GeV. See deviations for p T >4 GeV even for | |>2 due to increased away-side jet contribution (which swings along. 15

16. p T Dependence of other v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration We see similar trend as v 2 16

17. Recovering the correlations from EP v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration From 2PC method From EP method Chose v 1,1 and normalization to be same as original correlation function, but all other harmonics are from EP analysis. Correlation function is well reproduced, ridge and cone are recovered! Common physics origin for the near and away- side long range structures. 17

18. Measured v 2 -v 6 by both correlation and event-plane analysis. – Significant and consistent v 2 -v 6 were observed by the two methods. – Measured in phase space much larger than at RHIC. – Each v n can act as independent cross-check for /s. Noted that v 2 doesnt change drastically from RHIC to LHC Observed that the v n follow a simple scaling relation: v n 1/n v 2 1/2. Concluded that the features in two particle correlations for | at low and intermediate p T (p T <4.0GeV) can be accounted for by the collective flow of the medium. – Double hump and ridge arise due to interplay of even and odd harmonics Summary Soumya Mohapatra : Stony Brook University : ATLAS Collaboration ATLAS v n analysis note : http://cdsweb.cern.ch/record/1352458http://cdsweb.cern.ch/record/1352458 ATLAS HI public results page : https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublicResults For more results see: 18

19. BACKUP SLIDES Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 19

20. Left panel: : v 1 (p T a ) vs for four fixed-p T correlations. – We see that v 1,1 (p T a ) can become negative showing eta dependence of v 1 Right panel: v 1 (p T b ) vs for target p T in (1.4,1.6) GeV – We see that v 1 (p T b ) depends on p T a showing the breakdown of the scaling relation Breakdown of v 1,1 scaling Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 20

21. Universality of v n Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 21

22. Similar magnitude and p T dependence in overlapping p T range v 2 Comparison to RHIC Soumya Mohapatra : Stony Brook University : ATLAS Collaboration |η|<1 22

23. Charged hadrons, p T =0.5-20 GeV, mid-rapidity, |η|<1 v 2 out to 20GeV Soumya Mohapatra : Stony Brook University : ATLAS Collaboration central peripheral 23

24. p T evolution of two-particle correlations for 0-10% centrality selection, with a large rapidity gap (| |>2) to suppress the near-side jets and select only the long range components. p T evolution of correlations Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 24

25. v n,n and v n vs for other centralities Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 25

26. p T Dependence of v 3 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 26

27. p T Dependence of v 4 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 27

28. p T Dependence of v 5 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 28

29. The EP is determined with the Q-vector method using E T flow in FCal In mid-central collisions, the Q 2 vector is distributed in a ring-like structure indicating the excellent ability of the FCal in determining the reaction plane In Central and mid-central collisions and for higher harmonics, the ring blurs out Q vector example Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 29

30. The correlations are constructed by dividing foreground pairs by mixed background pairs. Mixed background pairs account for detector acceptance. Final correlation contains only physical effects. The detector acceptance causes fluctuations ~ 0.001 in the foreground pairs, which mostly cancels out in the ratio. Two Particle Correlations Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 30

31. Recovering 0-1% correlation Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 31

32. Recovering 0-5% correlation Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 32

33. Soumya Mohapatra : Stony Brook University : ATLAS Collaboration 33