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Soumya Mohapatra Stony Brook University for the ATLAS Collaboration

Measurement of elliptic and higher order flow harmonics in 2.76 TeV Pb-Pb collisions with the ATLAS detector. Soumya Mohapatra Stony Brook University for the ATLAS Collaboration. 28 th Winter Workshop on Nuclear Dynamics April 7-14 2012.

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Soumya Mohapatra Stony Brook University for the ATLAS Collaboration

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  1. Measurement of elliptic and higher order flow harmonics in 2.76 TeVPb-Pb collisions with the ATLAS detector SoumyaMohapatra Stony Brook University for the ATLAS Collaboration 28th Winter Workshop on Nuclear Dynamics April 7-14 2012

  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 2PC method Pairs: Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  3. Tracking coverage : |h|<2.5 • FCal coverage : 3.2<|h|<4.9 (used to determine Event Planes) ATLAS Detector Inner detector |η|<2.5 Forward Calorimeter (Fcal) 3.2<|η|<4.9 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 strong centrality dependence. • Higher order vn (n>2) have weakly centrality dependence. • In most central collisions, at high pT , v3,v4 can be larger than v2.

  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 Central Peripheral vn Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  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 Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

  9. hdependence of vn vn vn vn vn • v2 has a weak dependence on h • ~5% drop within h range of 0-2.5 in central and mid-central collisions. • The h dependence is more pronounced for n>2 and in peripheral events. • For correlations: : (relation is true if the h dependence is weak). Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  10. Two-particleDh-Dfcorrelations Near-side peak is always visible Ridge seen in central and mid-central collisions, weak Dh 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 peak) 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 short range correlations. • Relatively flat afterwards, so we require a |Dh| >2 gap (to remove near-side peak). Bands indicate systematic errors Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  13. Centrality evolution of correlations • If collective flow dominates then near side peak must be larger than away side. • True till 50% centrality. • Negative v1,1 is indicator of away side jet. Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  14. pT evolution of correlations 3<pTa,pTb<4 GeV 3<pTa,pTb<4 GeV 4<pTa,pTb<6 GeV 6<pTa,pTb<8 GeV 5<pTa,pTb<6 GeV 8<pTa,pTb<20 GeV Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  15. pT dependence of 2PC 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

  16. For v1 the scaling doesn’t hold. • v1(pTb) depends on pTa showing the breakdown of the scaling relation. • Breakdown is mainly due to contributions from global momentum conservation • See talk in afternoon session by Prof. JiangyongJia. • For other harmonics the scaling holds very well (for |Dh|>2). Universality of vn 10-20% Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

  18. 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. 0-1%

  19. Measured v2-v6 by both two particle correlation and event-plane analysis. • Significant and consistent v2-v6 were observed by the two methods. • Measured in much larger phase space 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. • Double hump and ridge arise due to interplay of even and odd harmonics Summary For more results see: • ATLAS vn paper: http://arxiv.org/abs/1203.3087 • ATLAS v2 paper: Phys.Lett. B707 (2012) 330-348 • ATLAS HI public results page : • https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublicResults Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

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

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

  23. vn,n and vn vs Dhfor other centralities Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  24. pT dependence of v2, v3 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

  25. pT dependence of v4 ,v5 (2PC) Soumya Mohapatra : Stony Brook University : ATLAS Collaboration

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

  27. 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

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