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Away-side Modification and Near-side Ridge Relative to Reaction Plane

Away-side Modification and Near-side Ridge Relative to Reaction Plane at 200 GeV Au+Au Collisions. Aoqi Feng, Fuqiang Wang, Yuanfang Wu (for the STAR collaboration). Institute of Particle Physics, Wuhan, China Purdue University, USA

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Away-side Modification and Near-side Ridge Relative to Reaction Plane

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

  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 pT(3<pT<4GeV/c); associate particles: lower pT.

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

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

  8. Flow Background Subtraction Ref: Phys. Rev C 69, 021901, 2004 (1) VnR is the trigger flow in the angular slice R. , (2) The contribution from v4 terms is about 10%, can not be neglected!

  9. in-plane fS=0o out-of-plane fS=90o φS: the angle between trigger particle and reaction plane. Results: Correlations v.s. Reaction Plane 0.15 Histograms: v2 uncertainty. Red curves: dAu data 3<pTtrig<4GeV/c, 20-60% 0.5 1.0 Away-side: Evolves from single- to double-peak. Near-side: Amplitude drops. 1.5 STAR Preliminary 2.0 3.0 GeV

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

  11. 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 Focus On Away-side: Broadness 3<pTtrig<4,1.0<pTasso<1.5GeV/c v2 sys. error RMS v2{RP} v2{4} STAR Preliminary 3<pTtrig<4GeV/c Slice 1: remains constant. not much broader than dAu. Slice 6: higher than slice1. increase with pTasso. Double peak: strongest when more out-of-plane and associate particle is harder.

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

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

  15. STAR Preliminary Ridge In Two Centralities 3<pTtrig<4GeV/c 4<pTtrig<6GeV/c At φS=0o: Ridge yields are similar in two centralities. Collision geometry? Gluon density?

  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 =0o) to double peak(φs =90o). • 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 =90o, there appears small or no ridge in 20-60%. • At φs =0o, strong ridge generation.

  17. Thank you!

  18. backup

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

  20. Something Relative to the Analysis • Determination of Event Plane: • modified reaction planereduce 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 v2 use v2_{EP}, average v2 and v2_{4} to estimate. • From event plane resolution it’s smaller than that from v2. • From B 2, 4 and 6 lowest data points are used to get 3 B values.

  22. Define: Y= Raw/F = (J+B*F)/F = B+ J/F Fitting Method J: jet signal F: [1+2v2trig,Rv2assocos(2Δφ)] Real Flow: B*F = B* [1+2v2trig,Rv2assocos(2Δφ)] Raw: raw signal = J+B*F Find 2(4/6) continuous lowest points as the fitting range.

  23. Raw signal/(1+2*v2*v2*cos(2*dphi)) 2 points 4 points 6 points

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

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

  26. 3<pTtrig<4GeV/c, top 5%

  27. 4<pTtrig<6GeV/c, top 5%

  28. Two Methods: Consistent

  29. Ridge Comparison 3<pTtrig<4, 1.5<pTasso<2.0GeV/c 4<pTtrig<6, 1.5<pTasso<2.0GeV/c

  30. dPhi x dEta and Projection

  31. Jet width

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

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