Soumya Mohapatra for the ATLAS Collaboration Stony Brook University

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 vn carry information about the medium: initial geometry,h/s Understanding of higher order vn can shed light on the physics origin of “ridge” and “cone” seen in 2P correlations Introduction and Motivation Singles: EP method Pairs: 2PC method Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

4. Event Plane Method • Bands indicate systematic errors

5. Centrality dependence of vn • 5% Centrality bins + 0-1% centrality bin • v2 has a stronger centrality dependence. • Other vn are flatter. • In most central collisions, v3,v4 can be larger than v2 at high enough pT

6. Similar trend across all harmonics (increase till 3-4GeV then decrease) In most central collisions(0-5%): v3, v4 can be larger than v2. pT Dependence of vn vn vn Soumya Mohapatra : Stony Brook University

7. Observe scaling: vn1/n =kv21/2, where “k” is only weakly dependent on pT. R.Lacey et al. (http://arxiv.org/abs/1105.3782) vn scaling (vn 1/n /v21/2) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

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

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

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

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

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

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

15. pT Dependence of v2 • The 2PC vn for |Dh|<0.5 deviates from the EP results (for all pT) • Good agreement seen for |Dh|>2 at pT<4 GeV. • See deviations for pT>4 GeV even for |Dh|>2 due to increased away-side jet contribution (which swings along Dh). Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

16. pT Dependence of other vn We see similar trend as v2 Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

17. Recovering the correlations from EP vn From 2PC method From EP method • Chose v1,1 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. Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

18. Measured v2-v6 by both correlation and event-plane analysis. • Significant and consistent v2-v6 were observed by the two methods. • Measured in phase space much larger than at RHIC. • Each vn can act as independent cross-check for h/s. • Noted that v2 doesn’t change drastically from RHIC to LHC • Observed that the vn follow a simple scaling relation: vn1/n ∝ v21/2. • Concluded that the features in two particle correlations for |Dh|>2 at low and intermediate pT (pT<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 For more results see: • ATLAS vn analysis note : http://cdsweb.cern.ch/record/1352458 • ATLAS HI public results page : • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublicResults Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

20. Left panel: : v1(pTa) vs Dh for four fixed-pT correlations. • We see that v1 ,1(pTa ) can become negative showing eta dependence of v1 • Right panel: v1(pTb) vs for target pT in (1.4,1.6) GeV • We see that v1(pTb) depends on pTa showing the breakdown of the scaling relation Breakdown of v1,1 scaling Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

22. Similar magnitude and pT dependence in overlapping pT range v2 Comparison to RHIC |η|<1 Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

23. Charged hadrons, pT=0.5-20 GeV, mid-rapidity, |η|<1 v2 out to 20GeV central peripheral Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

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

26. pT Dependence of v3 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

27. pT Dependence of v4 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

28. pT Dependence of v5 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

29. The EP is determined with the Q-vector method using ET flow in FCal In mid-central collisions, the Q2 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

30. Two Particle Correlations • 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. Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

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

33. Soumya Mohapatra : Stony Brook University : ATLAS Collaboration