1 Probing dense matter at extremely high temperature Rudolph C. Hwa University of Oregon Jiao Tong University, Shanghai, China April 20, 2009

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3 1.High temperature. How high? 1000K?10 6 ?10 9 ? K 2.How do we get there? 3.How do we probe it? 4.What do we know so far? 5.What are new very recently? 6.What can be expected in the future? Outline

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8 RHIC Relativistic Heavy Ion Collider GeV (Au+Au) LHC Large Hadron Collider TeV (Pb+Pb)

9 STAR PHOBOS PHENIX BRAHMS

10 STAR detector

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15 lumpy initial conditions and a QGP expansion collision evolution particle detectors collision overlap zone QGP phase quark and gluon degrees of freedom  ~ 10 fm/c hadronization kinetic freeze-out lumpy initial energy density  0 ~1 fm/c  ~ 0 fm/c distributions and correlations of produced particles quantum fluctuations expansion and cooling  ~ fm/c

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17 Collision geometry azimuthal angle transverse momentum pseudorapidity

18 Collision geometry centrality Au + Au  s NN = 200 GeV very central c= very peripheral

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

20 Non-central collision x y z How can we probe such a medium? We need a penetrating probe. Example: X-ray

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22 For good resolution we need << L L ~ size of biological molecules 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.

23 pTpT 26 lowintermediatehigh softhardsemi-hard relativistic hydrodynamics perturbative Quantum Chromo Dynamics (quarks, gluons) -- partons Jet production in pp collision nucleon parton jet no reliable theory

24 What do we see at high p T ? p T (GeV/c) Au+Au   0 + anything

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28 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

29 A more revealing way to see its properties is to examine the azimuthal dependence of jet production trigger associated particle How do we know that the suppression is due to parton interaction with QGP as the medium? Dihadron correlations

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

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

32 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

33 Yield per trigger Near Away

34 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

35 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.

36 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 Dijets associates primary trigger (T1) “jet-axis” trigger (T2) Dominance by tangential jets!

37 Very hard to probe the interior of dense medium --- if the thickness cannot be controlled. That’s the problem with jet-jet correlation. So let’s move on to the medium response to jets.

38 Jet-medium interaction 1. Effect of medium on jets. Δφ Δη Trigger Assoc. trigger direction distribution of particles associated with the trigger A ridge is discovered on the near side. ridge Jet 2. Effect of jets on medium.

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

40 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 assoc New data presented at QM08 A. Feng (STAR): Dependence of ridge yield on In-plane Out-of- plane

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

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

43 At LHC, cm energy is increased over RHIC by factor of 27. Energy density is expected to increase by < 10. T initial ………………………………………………………. < 2. It is hard to hold the dense matter together for long to thermalize. Large p T range will increase by > 20. Good ground to test pQCD. There are wide variations in extrapolation to higher energies. Ex. Most people predict p/  < 0.5 for 10<p T <20 GeV/c. We (RH & CBYang) predict 5 < p/  < 20.

44 Most significant advance will be either to confirm conventional wisdom Thank you! or to validate unconventional ideas. I hope that I can tell you which next time.