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Interaction between jets and dense medium in heavy-ion collisions

Interaction between jets and dense medium in heavy-ion collisions. Rudolph C. Hwa University of Oregon. TsingHua University, Beijing, China May 4, 2009. Outline. Introduction Jets at high transverse momentum p T Back-to-back jets (effect of medium on jets) Ridges

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Interaction between jets and dense medium in heavy-ion collisions

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  1. Interaction between jets and dense medium in heavy-ion collisions Rudolph C. Hwa University of Oregon TsingHua University, Beijing, China May 4, 2009

  2. Outline Introduction Jets at high transverse momentum pT Back-to-back jets (effect of medium on jets) Ridges (effect of jets on the medium) 5. Conclusion

  3. 1. Introduction Creation of hot, dense matter at RHIC T > 170 MeV ~ 1012 K  > 5 GeV/fm3 ~ 50 normal nuclear density Deconfined quarks and gluons

  4. pseudorapidity azimuthal angle transverse momentum Collision geometry

  5. pT  z y x Non-central collision py px (Npart) Azimuthal variation in non-central collisions

  6. We can’t shoot a probe through the dense medium, as in X-ray diagnostic. It must come from within. pT L ~ size of system For good resolution we need  << L In nuclear collisions the transverse size of collision zone is about 10 fm (10-12cm). For  << 1 fm, we need p = h/  >> 1 GeV At RHIC cm energy of a nucleon is 100 GeV, but it is the momentum-transfer scale that measures the small-distance resolution:

  7. Au+Au  2000 particles p+p  dijet jet parton nucleon nucleon

  8. So the pT of the detected jet in AA collision is lower than a similar jet in pp collision. pp AA That is a suppression effect pT 2. Jets at high pT Jet quenching In the transverse plane a hard scattering can occur anywhere If the hot medium is sQGP, the partons that traverse it lose energy.

  9. trigger associated particle How can we be sure that the suppression is due to parton interaction with QGP as the medium? Can it be due to some initial state interaction? A more revealing way to see its properties is to examine the azimuthal dependence of jet production Dihadron correlations

  10. STAR Striking final state effects preliminary 20-60% central trigger out-of-plane trigger in-plane Dihadron correlations in Df PRL 91, 072304

  11. absorbed undamped to detector If there is severe damping on the away side, then most observed jets are produced near the surface.

  12. away near centrality c=0.05 c=0.5 3. Back-to-back jets Hwa-Yang 0812.2205, PRC (2009) Not measurable: initial parton momenta k, k’ parton momenta at surfaces q, q’ Measurable:trigger momentum pt associated particle (same side) pa associated particle (away side) pb

  13. Away Yield per trigger Near

  14. L-t t if we fix the length L Much less energy loss on the near side Suppression factor Energy loss1- More energy loss on the away side

  15. It is only possible to fix the centrality c. Some paths are long Some are short The problem is that the path length L cannot be fixed experimentally. Data integrates over all points of interaction. Tangential jets dominate.

  16. associates “jet-axis” trigger (T2) primary trigger (T1) -2 -1 2 0 1 3 4 5 associates Dominance by tangential jets! Df STAR has recent data on Di-jets Au+Au centrality comparison T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c 1 _dN_ Ntrigd(Df ) T2A1_T1 12% Central 40-60% MB 60-80% MB 2 0 STAR Preliminary • Df projection: no significant centrality dependence • No modification of away-side jet

  17. Very hard to probe the interior of dense medium --- if the thickness cannot be controlled. That’s about the effect of dense medium on dihadron correlation in jets.

  18. Trigger Trigger • Effect of jets on medium. trigger direction   ridge A ridge is discovered on the near side. distribution of particles associated with the trigger 4. Ridges Interaction between jets and medium • Effect of medium on jets. Jet

  19. Trigger: 3 < pT < 4 GeV/c Associated: 1.5 < pT < 2 GeV/c Not hard enough for pQCD to be reliable, too hard for hydrodynamics. • Physical processes involve: • semihard parton propagating through dense medium • energy loss due to soft emission induced by medium • enhancement of thermal partons • hydro flow and hadronization • ridge formation above background We have no reliable theoretical framework in which to calculate all those subprocesses.

  20. hard parton associated particles trigger SS ST peak (J) TT ridge (R)  These wings identify the Ridge  A very quick explanation of ridge formation in the recombination model of partons Hwa-QM08 We focus below on mainly the  distribution.

  21. Trigger Trigger   irrelevant very relevant Dependence of ridge yield on the trigger azimuthal angle restrict ||<0.7 What is the direction of the trigger T?

  22. out-of-plane in-plane 6 5 4 in-plane fS=0 out-of-plane fS=90o 3 STAR Preliminary 20-60% 2 1 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. Quark Matter 2008 -- A. Feng (STAR) Dependence on trigger azimuthal angle

  23. 6 5 4 in-plane fS=0 out-of-plane fS=90o 3 2 1 Out-of-plane 3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c Ridge In-plane STAR Preliminary Jet After separating Ridge from Jet --

  24. medium probe Correlated emission model (CEM) Chiu-Hwa PRC(09) Strong ridge formation when trigger and flow directions match.

  25. R only Netrakanti QM09 s>0 Ridge: assoc pt=1-1.5 GeV/c Ridge: assoc pt=1.5-2 GeV/c Jet: assoc pt=1.5-2 GeV/c s<0 In CEM we found an asymmetry in the  distribution STAR Preliminary trigger pt=3-4 GeV/c Ridge CEM model Jet |fs|

  26.  Sound wave Heating Away side jet Trigger jet Shock wave? Recoil jet on the away-side direction Do you believe it? This is an active area of current research.

  27. Conclusion Correlation among hadrons reveals that quarks interact strongly with QGP, not weakly (as initially suspected). Interaction at intermediate pT cannot be treated by either hydrodynamics or perturbative QCD. But that is where most of the data exist, and they provide information that we need to understand.

  28. We have discussed jet-medium interaction at intermediate pT. • Effect of medium on dijets: Energy loss to medium -> strong correlation between jets. It is hard to probe the medium interior by dijets because of dominance by tangential jets --- it has been verified by data on 2jet+1 correlation. • Effect of jets on medium: Semi-hard parton -> energy loss to medium -> Ridge. Our interpretation is that the ridge is formed by the recombination of thermal partons enhanced by jet. The prediction on asymmetry has also been verified by data.

  29. Thank you!

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