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

Outline 1.Introduction 2.Jets at high transverse momentum p T 3.Back-to-back jets (effect of medium on jets) 4.Ridges (effect of jets on the medium) 5. Conclusion

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

Collision geometry azimuthal angle transverse momentum pseudorapidity

Non-central collision x y z (N part ) Azimuthal variation in non-central collisions pxpx pypy pTpT 

For good resolution we need << L L ~ size of system In nuclear collisions the transverse size of collision zone is about 10 fm ( cm). For > 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: We can’t shoot a probe through the dense medium, as in X-ray diagnostic. pTpT It must come from within.

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

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. pTpT pp AA So the p T of the detected jet in AA collision is lower than a similar jet in pp collision. That is a suppression effect 2. Jets at high p T

A more revealing way to see its properties is to examine the azimuthal dependence of jet production 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? Dihadron correlations

PRL 91, trigger in-plane trigger out-of-plane STAR preliminary 20-60% central Striking final state effects Dihadron correlations in 

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

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

Yield per trigger Near Away

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

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

Au+Au centrality comparison 12% Central 40-60% MB 60-80% MB  _dN_ N trig d  ) 2 STAR Preliminary 0 T1: p T >5 GeV/c, T2: p T >4 GeV/c, A: p T >1.5 GeV/c  projection: no significant centrality dependence No modification of away-side jet T2A1_T1 STAR has recent data on Di-jets associates primary trigger (T1) “jet-axis” trigger (T2) Dominance by tangential jets!

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.

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

24 Trigger: 3 < p T < 4 GeV/c Associated: 1.5 < p T < 2 GeV/c Not hard enough for pQCD to be reliable, too hard for hydrodynamics. We have no reliable theoretical framework in which to calculate all those subprocesses. 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

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

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

Quark Matter A. Feng (STAR) Dependence on trigger azimuthal angle in- plane out-of-plane top 5% 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

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 In-plane Out-of- plane After separating Ridge from Jet --

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

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

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

Correlation among hadrons reveals that quarks interact strongly with QGP, not weakly (as initially suspected). Interaction at intermediate p T 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. Conclusion

We have discussed jet-medium interaction at intermediate p T. 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. 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.

Thank you!