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 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 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 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 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 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) PRL 95 (2005) PRL 93 (2004) Jet quenching: energy loss is path-length dependent.

7 Motivation: the Near-side Ridge In-plane Out-of-plane 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 Ref: Phys. Rev C 69, , 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 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 GeV Histograms: v 2 uncertainty. Red curves: dAu data

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 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 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 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 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 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 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 Thank you!

18 backup

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

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

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 % 3<p T trig <4,1.0<p T asso <1.5GeV/c STAR Preliminary π region double peak

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

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

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

28 Two Methods: Consistent

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

30 dPhi x dEta and Projection

31 Jet width

32 Details Near-side amplitude: |Δφ|<0.52 (-30 o,30 o ) πregion: 2.75<Δφ<3.53 (180 o o,180 o 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 )