1. 1 Away-side Modification and Near-side Ridge Relative to Reaction Plane at 200 GeV Au+Au Collisions 第十届全国粒子物理学术会议 （南京） Apr. 28th, 2008 Aoqi Feng, Fuqiang Wang, Yuanfang Wu (for the STAR collaboration) Institute of Particle Physics, Wuhan, China Purdue University, USA Lawrence Berkeley Lab, Berkeley, USA

2. 2 Outline  Short introduction  Motivation  Di-hadron correlation wrt reaction plane  Summary Previous key measurements of di-hadron corr. Path-length effect study via di-hadron corr. Away-side discussion. Near-side discussion.

3. 3 Short Introduction --- RHIC  A phase transition between hadronic matter and exotic quark- gluon plasma is predicted by QCD at energy density of ~ 1.  Little bang at RHIC may produce QGP.

4. 4 Short Introduction --- Jet signals Jets are good probes of the dense nuclear matter.  In PP collisions, the hard scattering of quarks and gluons early in the collision leads to the production of jets.  In AA collisions, energetic partons, resulting from initial hard scattering are predicted to lose energy (jet quenching).

5. 5 Short Introduction --- Di-hadron Corr.  Why di-hadron azimuthal correlation? Standard: jet cone method Heavy-ion collisions: di-hadron azimuthal correlation very large amount of particles are produced. It is not possible to reconstruct jets event by event due to the large background. So in heavy-ion collisions people reconstruct jet-like correlations through angular correlations in statistical basis.  What’s di-hadron correlation? Trigger particle: high p T (3<pT<4GeV/c); associate particles: lower p T.

6. 6 Motivation: the Away-side Modification  High p T di-hadron suppression partonic energy loss.  Low p T di-hadron correlations strong jet-medium interaction  High p T di-hadron correlations (w.r.t RP) path-length dependent jet quenching. PRL 90 (2003) 082302 PRL 95 (2005) 152301 PRL 93 (2004) 252301 Jet quenching: energy loss is path-length dependent.

7. 7 Motivation: the Near-side Ridge In-plane Out-of-plane 1 4 3 2 5 6 Non-central collision (20-60%): overlap region like almond. select trigger particle direction relative to reaction plane. Ridge (long range correlation in  )  is observed on the near-side. To gain more insights into the away-side modification and near-side ridge, we study RP dependence. Au+Au 0-10% STAR preliminary The underlying physics is not understood yet!

8. 8 Ref: Phys. Rev C 69, 021901, 2004 Flow Background Subtraction (1) (2), The contribution from v 4 terms is about 10%, can not be neglected! V n R is the trigger flow in the angular slice R.

9. 9 Results: Correlations v.s. Reaction Plane Away-side: Evolves from single- to double- peak. Near-side: Amplitude drops. 3<p T trig <4GeV/c, 20-60% STAR Preliminary in-plane  S =0 o out-of-plane  S =90 o φ S : the angle between trigger particle and reaction plane. 0.15 0.5 1.0 1.5 2.0 3.0 GeV Histograms: v 2 uncertainty. Red curves: dAu data

10. 10 Mid-Central v.s. Central Collisions Comparison top 5% 3<p T trig <4GeV/c & 1.0<p T asso <1.5GeV/c 20-60% in-plane  S =0out-of-plane  S =90 o In 20-60%, away-side evolves from single-peak (φ S =0) to double-peak (φ S =90 o ). In top 5%, double peak show up at a smaller φ S. At large φ S, little difference between two centrality bins. STAR Preliminary

11. 11 3<p T trig <4,1.0<p T asso <1.5GeV/c Focus On Away-side: Broadness Slice 1: similar to dAu in 20-60% broader than dAu in 5%. Slice 6: no much difference in two centrality bins. Path-length effect Slice 1: remains constant. not much broader than dAu. Slice 6: higher than slice1. increase with p T asso. Double peak: strongest when more out-of-plane and associate particle is harder. 3<p T trig <4GeV/c RMS STAR Preliminary v 2 {4} v 2 {RP} v 2 sys. error

12. 12 Focus On Near-side(1) A significant change in the near-side peak amplitude! whereas naively little modification is expected due to the minimal amount of medium that the parton transverses.

13. 13 jet ridge Focus On Near-side (2) Amplitude seems to change, whereas naively little modification is expected. 3<p T trig <4, 1.5<p T trig <2.0 GeV/c Raw(| | 0.7) Correlation in. Ridge part: | |>0.7, flow background subtracted. Jet part: acceptance factor STAR Preliminary in-plane  S =0out-of-plane  S =90 o Ridge Jet 3<p T trig <4, 1.5<p T trig <2.0 GeV/c

14. 14 Jet and Ridge Yield 20-60%top 5% jet part, near-side ridge part, near-side jet part, near-side ridge part, near-side Ridge: seem to decrease with φ s. More significant in 20-60% than top 5%. Jet: seem to slightly increase with φ s. Strong near-side jet-medium interaction in reaction plane, generating sizable ridge? Minimal near-side jet-medium interaction perpendicular to reaction plane? STAR Preliminary 3<p T trig <4, 1.5<p T trig <2.0 GeV/c

15. 15 Ridge In Two Centralities STAR Preliminary 3<p T trig <4GeV/c4<p T trig <6GeV/c Collision geometry? Gluon density? At φ S =0 o : Ridge yields are similar in two centralities.

16. 16 Summary  Both near- and away-side are modified. The modification depends on the trigger particle direction relative to RP.  Away-side: ==> path-length dependence of jet quenching.  Near-side: ==> near-side strong jet-medium interaction in-plane. collision geometry? Gluon density effect? In 20-60%, it evolves from single peak (φs =0 o ) to double peak(φs =90 o ). In top 5%, double peak shows up at a small φs. At large φs, little difference between the two centralities. Ridge drops with φs, Jet slight increase. At φs =90 o, there appears small or no ridge in 20-60%. At φs =0 o, strong ridge generation.

17. 17 Thank you!

18. 18 backup

19. 19 Flow background is suggested to be: ( Phys. Rev C 69, 021901, 2004 ) Flow Background Estimation (1) (2) (3)

20. 20 Something Relative to the Analysis  Determination of Event Plane: modified reaction plane reduce non-flow effect; associate pT range excluded avoid auto-correlations.  Corrections to raw correlation function: tracking efficiency is corrected for the associated particles; 2-particle acceptance is corrected for by the event-mixing technique.  Systematic errors: v2: average v2 as default results, v2_{4} and v2_{RP} as sys. estimation. resolutions: random sub-event and charge sign sub-event. B: from 3 different fitting methods.

21. 21 Systematics Errors  From v 2 use v 2 _{EP}, average v 2 and v 2 _{4} to estimate.  From event plane resolution it’s smaller than that from v 2.  From B 2, 4 and 6 lowest data points are used to get 3 B values.

22. 22 Fitting Method J: jet signal F: [1+2v 2 trig,R v 2 asso cos(2Δφ)] Real Flow: B*F = B* [1+2v 2 trig,R v 2 asso cos(2Δφ)] Raw: raw signal = J+B*F Define: Y= Raw/F = (J+B*F)/F = B+ J/F Find 2(4/6) continuous lowest points as the fitting range.

23. 23 2 points 6 points4 points Raw signal/(1+2*v 2 *v 2 *cos(2*dphi))

24. 24 Focus On Away-side: Amplitude top 5% πregion: drops with φ s, similar between the two centrality bins. double peak region: constant over φ s. top 5% > mid-central. 20-60% 3<p T trig <4,1.0<p T asso <1.5GeV/c STAR Preliminary π region double peak

25. 25 4<pTtrig<6 GeV/c, 20-60%

26. 26 3<p T trig <4GeV/c, top 5%

27. 27 4<p T trig <6GeV/c, top 5%

28. 28 Two Methods: Consistent

29. 29 Ridge Comparison 4<pTtrig<6, 1.5<pTasso<2.0GeV/c3<pTtrig<4, 1.5<pTasso<2.0GeV/c

30. 30 dPhi x dEta and Projection

31. 31 Jet width

32. 32 Details Near-side amplitude: |Δφ|<0.52 (-30 o,30 o ) πregion: 2.75<Δφ<3.53 (180 o -22.5 o,180 o +22.5 o ) Double-peak region: 1.44<Δφ<2.49 and 3.80< Δφ<4.84 (82.5 o,112.5 o ) and (217.5 o,277.5 o )