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The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration

The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration Yale University. Weisshorn (4505m), Switzerland. Near-side  x  correlations. Ridge/Jet characteristics in Au+Au. Current scenarios. Can we test these scenarios?. Summary (and Outlook). Outline.

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The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration

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  1. The “ridge“ in high energy nuclear collisions Jörn Putschke for the STAR Collaboration Yale University Weisshorn (4505m), Switzerland

  2. Near-side x correlations Ridge/Jet characteristics in Au+Au Current scenarios Can we test these scenarios? Summary (and Outlook) Outline

  3. Ridge observation: near-side  component picture Di-hadron correlations associated Dj trigger Au+Au 0-10% STAR preliminary Components: • Near-side jet peakcomparable to d+Au • Near-side independent ridge • Away-side (and v2) 3 < pt,trigger < 4 GeV pt,assoc. > 2 GeV New phenomenon inAu+Au “The ridge”: • What is it ? ‘something’ coupling to long flow ? Can this quantify E-loss ? • How to deal with it?Need to subtract for near-side studies?

  4. Near-side  (ridge) shape in central Au+Au pt,assoc. > 2 GeV 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV Jet  STAR preliminary yield(in  window Ridge   window center STAR preliminary • Ridge approx. independent of and pt,trig • Jet part increasing with pt,trig

  5. Jet-like peak width in central Au+Au pt,assoc. > 2 GeV 4 < pt,trigger < 5 GeV and pt,assoc. > 2 GeV STAR preliminary • Jet peak symmetric in and for pt,trig > 4 GeV and comparable to d+Au • Jet peak asymmetric in  for pt,trig < 4 GeV and significantly broader than d+Au

  6. Ridge yield vs. pt,trig in Au+Au pt,assoc. > 2 GeV STAR preliminary Ridge yield persists to highest trigger pt correlated to jet production Ridge only in Au+Au (not present in p+p or d+Au or peripheral Au+Au)

  7. Ridge/Jet pt,assoc spectrum in central Au+Au Ridge, 4<pt,trig<6 inclusive Jet, 4<pt,trig<6 Ridge, 6<pt,trig<10 Ridge/Jet yield Jet, 6<pt,trig<10 STAR preliminary STAR preliminary • Jet pt-spectra harder and increasing with pt,trig, as expected from jet fragmentation • Ridge pt-spectra are ‘bulk-like’ and approx. independent on pt,trig

  8. Subtracting the ridge: vacuum fragmentation (?) Jet+Ridge “Jet” in Au+Au Jet in d+Au STAR preliminary STAR preliminary zT = pT,assoc/pT,trig zT = pT,assoc/pT,trig Subtraction of -independent ‘ridge-yield’ recovers centrality-independent jet yield p+p like (vacuum) fragmentation after energy loss?

  9. “Jet”/Ridge energy • Applying this “2-component picture” to lower pt,assoc measurements: zt,jet(Au+Au) ~ zt,jet(d+Au)  subtracting p+p jet energy from Au+Au • upper estimate of the energy deposit in the ridge ~ few GeV “Direct” measure of energy loss ? } “Ridge energy” } “Ridge energy” STAR, Phys. Rev. Lett. 95 (2005) 15230 0.15 < pt,assoc < 4 GeV 4 < pt,trigger < 6 GeV 6 < pt,trigger < 10 GeV

  10. Particle composition in Ridge/Jet vs. inclusive Ridge: B/M ratio closer to bulk Jet : B/M ratio ~ p+p

  11. Ridge characteristics Weisshorn (4505m), Switzerland • ridge approx. independent on  • ridge persists up to highest trigger pt correlated to jet production • ridge spectrum ~ “bulk-like” • ridge energy roughly a few GeV • no significant PID trigger dependence (not shown) • /K0S ratio in ridge ~ inclusive B/M ratio • jet di-hadron fragmentation function after subtracting the ridge contributions comparable to d+Au STAR Au+Au 0-10%, RHIC, US (~0m) preliminary Are we seeing vacuum fragmentation after energy loss on the near-side in central Au+Au collisions with the lost energy deposited in the ridge ?

  12. What is the ridge? A. Majumder, B. Muller, S. Bass hep-ph/0611135 Armesto et al, nucl-ex/0405301 nucl-th/0706.3531 E. Shuryak • Radiated gluons, broadened by • Longitudinal flow, Armesto et al, PRL 93 (2004) • QCD magnetic fields, Majumder et al, hep-ph/0611035 • Anisotropic plasma, P. Romantschke, PRC,75014901 (2007) • Medium heating + recombinationChiu & Hwa, PRC72, 034903 • Radial flow + trigger biasS. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287E. Shuryak, nucl-th/0706.3531 • Momentum-kick modelC.-Y. Wong , hep-ph/0707.2385 Proposed explanations so far:

  13. Discussion ridge/jet yield increasing pt,trig h+,- ridge jet pt,assoc. • ridge spectrum slightly harder (?) than inclusive h+,- (tens of MeV) consistent with medium heatingparton recombination (T~15 MeV) ? • agreement with radial flow + jet quenching ? • ridge spectrum qualitatively in agreement with parton energy loss and coupling to longitudinal flow Unfortunately all models are in qualitative agreement  quantitative calculations needed !

  14. What else can/have we measure(d) to test these scenarios … • 3-particle  near-side correlations  test longitudinal flow picture • Is there a ridge on the away-side ?  di-hadron triggered correlations (3-particle x correlations) • Geometry effects (pathlength)  ridge vs. reaction plane / Cu+Cu • Energy dependence: 62 vs. 200 GeV  similar medium but different partonic spectrum • How far does the ridge extend ?  forward rapidity measurements • Full jet reconstruction  E-by-E jet-shape modification (radiation spectrum) More ? Predictions ? Ideas ? …

  15. 3-particle  near-side correlations trigger  random  13 associated  12 enhancement long. flow picture 13 asymmetry in 13 x 12 suppression 12 trigger  associated   Long. flow pictureArmesto et al, PRL 93 (2004) Radial flow + trigger bias S. Voloshin, nucl-th/0312065, Nucl. Phys. A749, 287 Toy model

  16. 3-particle  near-side correlation measurements 3<pTTrig<10 1<pTAsso<3 ||<0.7 STAR preliminary d+Au Au+Au 0-12% STAR Preliminary STAR Preliminary The ridge appears to be uniform event by event within the STAR acceptance Caveats: STAR acceptance limits sensitivity in Dh regions needed to distinguish between radial and long. flow picture. Non-trivial bkg. subtraction in 3-particle correlations …

  17. Is there a ridge on the away-side ? 8<pt,trigger<15 GeV T1: “near-side trigger” > 6 (8) GeV A: assoc. particles > 2 GeV disappearing bkg. T2: “away-side trigger” > 4 (6) GeV (and < near-side trigger) within 180deg.+- 30 deg. Use “away-side trigger” for 2-particle  correlations Caveat: probably favor small energy loss of the surviving di-hadron pair

  18. Di-jets in Au+Au STAR preliminary Df projection STAR preliminary 1 _dN_ Ntrigd(Df ) T1: pT>5GeV/c T2: pT>4GeV/c A : pT>1.5GeV/c STAR Preliminary Au+Au d+Au 3 2 T1A & T2 T2A T1A & T2 T2A 1 0 Dh projection -2 -1 2 0 1 3 4 5 Df D Df Di-jet measurements suggest that neither the widths in  and (ridge/mach cone) are modified nor the yields are suppressed and comparable to d+Au Caveat: Non-trivial bkg. subtraction Surviving (di-jet) pairs at high pt seem to favor conditions with small energy loss  ridge correlated with energy loss !(?)

  19. Jet/ridge w.r.t. reaction plane 6 5 4 3 2 1 in-plane fS=0 out-of-plane fS=90o STAR preliminary Ridge 20-60% jet part, near-side ridge part, near-side 20-60% 3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c Jet Out-of-plane • Ridge yield decreases with φS. Smaller ridge yield at larger φS • Jet yield approx. independent of φS and comparable with d+Au STAR preliminary Jet yield independent of φS, consistent with vacuum fragmentation after energy loss and lost energy deposited in ridge, if medium is “black” out-of-plane and more “gray” in-plane for surviving jets. In-plane

  20. Jet/Ridge in Cu+Cu 3.0 GeV/c < pTtrigger 6.0 GeV/c; 1.5 GeV/c < pTassociated < pTtrigger Ridge Jet • Jet yield comparable in Cu+Cu and Au+Au at similar Npart • Ridge yield comparable in Cu+Cu and Au+Au at similar Npart(consistent with other jet-quenching variables like RAA)

  21. Jet/Ridge 62 vs. 200 GeV 3.0 GeV/c < pTtrigger 6.0 GeV/c; 1.5 GeV/c < pTassociated < pTtrigger Ridge 200 GeV 200 GeV 62 GeV Jet 62 GeV • Jet yield significantly smaller in 62 GeV vs. 200 GeV “trivial” kinematic effect due to steeper jet spectrum in 62 GeV (pQCD/Pythia) • Ridge yield also suppressed in 62 GeV vs. 200 GeV (Ridge/Jet ratio comparable) Ridge yield correlated with pQCD jet properties (?); radial flow and v2 ≈ in 62 and 200 GeV (Does the ridge yield scale with RAA ? Less suppression in RAA at similar pt ?)

  22. Is there a ridge at forward rapidity ? PHOBOS (QM08) preliminary; central Au+Au |trig| <1 and 2.7 < |assoc| < 3.9 pTassoc = 0.2-2.0 GeV/c: no near-side peak within systematic errors pTassoc > 1 GeV/c : non-zero correlation at near side (?) STAR and PHOBOS measurements suggestthe presence of a finite ridge yield at  ~ 3 (?) Caveat: no momentum measurement at high rapidity in PHOBOS, STAR data are not conclusive

  23. Summary Is the “ridge” due to energy loss ?If so, do we have a direct measurement of energy loss and medium response ? Many measurements characterizing the ridge properties are already available. Quantitative calculations are needed now to understand the underlying mechanism of the ridge ! Additional measurements possible (on the way) to furtheraddress the origin of the ridge. Extend sensitivity withfull jet-reconstruction.

  24. Outlook: Ridge/Jet v2; different event classes? characteristics of the events yielding a “ridge pair” appear to be very different from those yielding a “jet pair” “jet” “ridge” STAR Preliminary • the “ridge” is calculated by projecting ||>0.7 correlation to ||<0.7 • the “jet” is the remaining correlation at ||<0.7 after subtracting the “ridge” • inferred v2 for events associated with “ridge” pairs is large • inferred v2 for events associated with “jet” pairs is small • this conclusion is a direct consequence of: zero-yield at minimum assumption and the 3-component model: • (v2 modulated background + ridge + jet)

  25. Outlook: Jet reconstruction in heavy ion collisions Cone radius R=sqrt(f2+h2) pT Unbiased jet reconstruction: parton kinematics  hadrons Heavy ions: large background from underlying event • Control background by limiting jet cone radius R, track pT cut •  measure a fraction of partonic energy

  26. Outlook: Full jet reconstruction in HI @ RHIC ? STAR, hep-ex/0608030 Central Cu+Cu 200 GeV Armesto et al, nucl-ex/0405301 Full jet reconstruction in p+p up to 40 GeV Back of the envelope calculation … • Jets > 10-15 GeV should be reconstructable in Cu+Cu @ RHIC (should be sufficient statistic on tape …)

  27. Backup slides

  28. Particle composition in Ridge/Jet |Δη|<0.7 |Δη|<2.0 STAR preliminary STAR preliminary /K0S ratio: in the ridge: ~ 1.0  similar to that from inclusive pT spectra in the jet: ~ 0.5  consistent with p+p Outlook: similar measurement will be pursued using dE/dx ID to look at p/π ratio in ridge/jet

  29. Ridge “observation” 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV d+Au minbias Au+Au 0-10% STAR preliminary STAR preliminary Additional near-side long range corrl. in  in central Au+Au (“ridge like” corrl.) observedDan Magestro, Hard Probes 2004, STAR, nucl-ex/0509030, Phys. Rev. C73 (2006) 064907 and P. Jacobs, nucl-ex/0503022

  30. Ridge phenomenology 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV pt,assoc. > 2 GeV Jet+Ridge () Jet () Jet) STAR preliminary yield,) Npart Jet STAR preliminary yield(in  window Ridge  window center • Ridge approx. independent of  • Jet yield in  and  independent on centrality and comparable to d+Au • Jet peak symmetric in  and  for pt,trig > 4 GeV Definition of “ridge yield”: ridge yield := Jet+Ridge( Jet()

  31. Extracting the ridge yield     3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV Jet+Ridge () Jet () Jet) STAR preliminary yield,) Npart Jet yield independent of Npart and consistent with d+Au reference measurements ! Definition of “ridge yield”: ridge yield := Jet+Ridge( Jet() relative ridge yield := ridge yield / Jet()

  32. Fundamental quantity P(DE) Salgado and Wiedemann, Phys. Rev. D68, 014008 ~15 GeV Renk, Eskola, hep-ph/0610059 Energy loss distribution P(DE) depends on Radiation spectrum Geometry, time evolution of matter Can we constrain this by experiment? High-pt di-hadron correlations: favor cases with small/no energy loss Low-pt di-hadron correlations: modification of near-side jet-part and complex away-side structure Need an unbiased measure of the parton energy  full jet reconstruction

  33. Ridge/Jet ratio 62 and 200 GeV 3.0 GeV/c < pTtrigger 6.0 GeV/c; 1.5 GeV/c < pTassociated < pTtrigger Ridge/Jet ratio independent of energy, system Au+Au √sNN=200 GeV from J. Bielcikova (STAR), J.Phys.G34:S929-930,2007 Cu+Cu √sNN=200 GeV from C. Nattrass (STAR), SQM2007

  34. Ridge w.r.t reaction plane cont. STAR Preliminary 3<pTtrig<4GeV/c 4<pTtrig<6GeV/c At φS=0o: Ridge yields are similar in two centralities. Collision geometry? Gluon density?

  35. Trigger PID Ridge/Jet Jet Ridge J. Bielcikova (STAR), QM’2006 • ridge yield increases with centrality • (ridge for K0S trigger < ridge for Λ trigger ?) • jet yield is independent of centrality and agrees with d+Au

  36. Trigger PID ridge/jet pt,assoc spectrum J. Bielcikova (STAR), QM’2006 • Trigger PID ridge spectra comparable to h correlations and ~ bulk • Trigger PID Jet spectra comparable to h correlationsand slightly harder ~ 50 MeV

  37. Ridge yield in Au+Au and Cu+Cu relative ridge yield := ridge yield / Jet() pt,assoc. > 2 GeV relative ridge yield relative ridge yield Au+Au 200 GeV Cu+Cu 200 GeV STAR preliminary Au+Au 200 GeV (30-40 %) Cu+Cu 200 GeV (0-10 %) relative ridge yield relative ridge yield 3<pt,trigger<4 GeV STAR preliminary Relative ridge yield comparable at same Npart in Au+Au and Cu+Cu

  38. Extracting near-side “jet-like” yields J = near-side jet-like corrl. R = “ridge”-like corrl. (J) ||<0.7 2 (J) ||<0.7 1 2 const bkg. subtracted const bkg. subtracted  (J+R) - (R) (J) flow (v2) corrected (J+R) ||<1.7 (J+R) ||<1.7 no bkg. subtraction v2 modulated bkg. subtracted Au+Au 20-30%

  39. Analysis methods cont. preliminary • Use event-mixing to account for pair acceptance and use eff. correction for ass. particles • Background: • Subtract constant background for (J) • Subtract v2 modulated background for (J+R)(ZYAM method) • Assume Gaussian correlation shape:yield() = gaus integral / bin counting () = gaus width 2.5 < pt,trigger < 3 GeV and 0.3 < pt,assoc. < 0.8 GeV v2{RP} • v2 and systematic error estimation in Au+Au: a) Used v2 values = mean between v2 RP and v2{4} b) Systematic errors mainly due to uncertainties in v2; use v2 RP and v2{4} as upper and lower limit ZYAM norm. v2{4} Df <v2{RP}+v2{4}>

  40. Di-hadron azimuthal correlations increasing pt,assoc. high pt : 8 < pt,trigger < 15 low pt: 4 < pt,trigger < 6 and pt,assoc > 0.15 GeV Away-side (and near-side) yield is enhanced for low pt,assoc in Au+Au Away-side yield is suppressed but finite and measurable at high pt,trigger and pt,assoc in central Au+Au collisions Recoil distribution soft and broad, angular substructure, Mach cone? (not topic of this talk)

  41. Di-hadron associated spectra high pt low pt STAR, Phys. Rev. Lett. 97 (2006) 162301 STAR, Phys. Rev. Lett. 95 (2005) 15230 Low pt : Strong modification of near-side and away-side observed High pt : Near-side unmodified, away-side suppressed but shape unmodified • Surviving pairs at high pt seems to favor conditions with small energy loss: • tangential (halo) emission, finite probability for no energy loss (energy loss fluctuations) or dilution due to the expanding system(T. Renk, Hard Probes 06 and PQM: Dainese, Loizides and Paic) • limited sensitivity to energy loss in di-hadron fragmentation on the away-side ? • study near-side modification

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