1. Strange hard probes to characterize the partonic medium at RHIC Rene Bellwied Wayne State University Phase Transition In Strongly Interacting Matter, NPDC18 Prague, Czech Republic, August 23- 29, 2004

2. dNg/dy ~ 200 (HIJING) dNg/dy ~ 1000 (CGC) Gluon density in proper model Equals final state hadron density: dN ch /dy ~ 1000 (measured) Parton – hadron duality ?? Signs of partonic ‘hydro’ matter Constituent quark scaling First time in Heavy-Ion Collisions a system created which, at low p t,is in quantitative agreement with ideal hydrodynamic model (for mid-central to central collisions)

3. How do we determine medium properties ? (by producing probe and medium in the same collision) We are producing ‘soft’ and ‘hard’ matter. An arbitrary distinction is coming from the applicability of pQCD which is generally set to p T > 2 GeV/c (hard). Below 2 GeV/c we expect thermal bulk matter production. –Medium: The bulk of the particles; dominantly soft production and possibly exhibiting some phase. –Probe: Particles whose production is calculable, measurable, and thermally incompatible with (distinct from) the medium (hard production) Measure bulk matter properties to determine global properties (collectivity, equilibration, timescales) (Talks by Boris Hippolyte and Magali Estienne) Measure the modification of high pt probes to determine specific properties of the matter produced (jet tomography)

4. Understanding ‘jet and bulk’ properties in the same experiment 99.5% Dominant feature: order of magnitude increase at high p T

5. Behavior of hard probes when traversing an opaque medium Jets from hard scattered quarks observed via fast leading particles or azimuthal correlations between the leading particles However, before they create jets, the scattered quarks radiate energy (~ GeV/fm) in the colored medium:  decreases their momentum (fewer high p T particles)  “kills” jet partner on other side

6. pp is well described by fragmentation Ingredients: –pQCD –Parton distribution functions –Fragmentation functions –Next-to-leading (NLO) calculations p+p->  0 + X Hard Scattering Thermally- shaped Soft Production hep-ex/0305013 S.S. Adler et al. “Well Calibrated”

7. We measure two predicted QGP signatures The ‘quenching’ of high pt particles due to radiative partonic energy loss The disappearance of the away-side jet in dijet events traversing the apparently opaque medium ?

8. Test the matter with high pt probes Is jet quenching an initial or final state effect ? Measure dA

9. Nuclear suppression factors (AA/pp) vs (dA/pp) Striking difference of d+Au and Au+Au results. Enhancement vs. suppression. (Cronin effect in cold nuclear matter). Final state effect confirmed by back-to-back correlations Energy loss depends on the size of medium traversed Cronin Effect: Multiple Collisions broaden high P T spectrum Pedestal&flow subtracted

10. AA quenching: where is the energy ? On the away side: energy loss in medium has been converted to lower pt particles Leading hadrons Medium Away syst. error Near STAR Preliminary pTrigger = 4 -6GeV/c in cone is still higher than in medium but is approaching equilibration with medium Statistical distribution of momentum conservation describes the correlation function at all centralities

11. Summary of measured experimental observations At RHIC we showed that Au+Au collisions create a medium that is dense, dissipative and exhibits strong collective behavior –We observe suppression phenomena in single particle observables and very importantly also in the correlations (large acceptance) –We observe constituent quark scaling in v 2 and R cp at ~ 2-5 GeV/c and gluon density scaling in the energy production –We observe strong collective behavior (flow) in all bulk matter observables (nucl-th/0403032)

12. What else can be measured ? We have the unique opportunity to measure the fragmentation of a parton into hadrons in the vacuum and in the medium. We can learn about hadronization, and therefore learn how particles acquire mass, by measuring medium modifications to the fragmentation process in an opaque medium and compare to the behavior in the vacuum. This is a fundamental question of physics that also connects Nuclear Physics to Elementary Particle Physics !

13. Modification of fragmentation functions (e.g.Gyulassy et al.,nucl-th/0302077) Induced Gluon Radiation  ~collinear gluons in cone  “Softened” fragmentation (quite generic, but attributable to radiative rather than collisional energy)

14. Different partons lose different amounts of energy Examples: 1.) dead cone effect for heavy quarks: Heavy quarks in the vacuum and in the medium (Dokshitzer and Kharzeev (PLB 519 (2001) 199)) the radiation at small angles is suppressed 2.) gluon vs. quark energy loss: Gluons should lose more energy and have higher particle multiplicities due to the color factor effect.

15. Quark vs. gluon jet measurements Gluon bremsstrahlung is expected to be higher in gluon than in quark jets by C A /C F (= 2.25). Jet multiplicities in elementary collisions already higher by color factor due to softer fragmentation function. Measurement shows ratio slightly lower at lower pt and expected value at higher pt (higher order corrections ?)

16. 1.) anti-s softer than anti-light at low x (hep-ph/9303255) 2.) s to anti-s asymmetry in the q to  fragmentation ? (hep-ph/0005210) Flavor Dependence of Parton Distributions

17. Fragmentation functions for different baryons Bourelly & Soffer (hep-ph/0305070) Statistical approach based on production cross section measurements in e+e-

18. Do we understand fragmentation ? statistical approach based on measured inclusive cross sections of unpolarized octet baryons in e+e- annihilation: D u  ~ 0.07 D s ,and D u  / D s  ratio about constant as a function of x but: de Florian et al., (1998): u,d,s contribution to  is about the same J.J.Yang (2001,2002): D u  / D s  drops by factor 5 with increasing X.

19. Is RHIC the right place to study fragmentation as a function of x ? There couldn’t be a better place !!

20. The goal of particle identified fragmentation in the medium 1.) we need to understand fragmentation (hadronization) in the elementary binary process 2.) In addition to the statistical approach we can use the medium modified fragmentation functions in AA collisions 3.) the claim is that by having different contributions to the elementary fragmentation function at different x and by having these different contributions lose different amounts of energy (z) in the opaque medium, we learn about the basic hadronization process by measuring particle identified fragmentation and correlation functions

21. Identified particles at intermediate to high-p t Two groups, baryons and mesons, which seem to approach each other around 5 GeV/c Suggesting relevance of constituent quarks for hadron production Coalescence/recombination provides a description ~1.5 - 5 GeV/c

22. The ‘intermediate’ pt region pTpT pQCD Hydro 2-3 GeV/c6-7 GeV/c ? Soft Fragmentation and quenching of jets 0 p T independence of pbar/p ratio. p/  and  /K ratio increases with p T to > 1 at p T ~ 3-4 GeV/c in central collisions. Suppression factors of p,  different to that of , K 0 s in the intermediate p T region. Parton recombination and coalescence

23. Recombination + Fragmentation at mid pt Recombination at moderate P T Parton pt shifts to higher hadron p T. Fragmentation at high P T: Parton pt shifts to lower hadron p T recombining partons: p 1 +p 2 =p h fragmenting parton: p h = z p, z<1 Recomb. Frag.

24.  -charged hadron correlations in pp and AA 0-5% 10-30%5-10% 30-50%50-70% pp

25. Two jets or a monojet plus momentum conservation ? pp: Au + Au: Gaussian Fit of back side: Cosine Fit of back side (momentum balance): Same Side Back Side Background Parameters are compared for different fits for two different p T cuts as a function of centralities  Nicolas Borghini et al. Phys. Rev. C 62, 034902(2000).

26. Λ +h correlations AuAu (cosine fit) Conclusion: fit quality is equally good.

27. 1.5<p T,trigger <3.0, 1.5<p T,asso <3.0 Width and associated particle yields(Λ+h) Gaussian fit Cosine fit

28. Comparison of STAR/PHENIX for same side difference for baryon and meson trigger particles as a function of centrality Different methods, similar result: STAR integrated Gaussian fit, PHENIX bin counting in fixed  bin

29. Comparison of STAR/PHENIX for away side difference for baryon and meson trigger particles as a function of centrality Different methods, similar result: STAR integrated cos-fit, PHENIX bin counting in fixed  bin

30. STAR comparison for different particle species No significant difference for different trigger particle species as a function of centrality at intermediate trigger pt.

31. Asymmetry seems to develop as a function of trigger pt (increased ‘jettiness’) (Same side – away side) two particle correlation strength in central Au-Au Collisions at RHIC Strange trigger, charged associated particles (associated pt > 2 GeV/c) trigger pt (GeV/c) STAR preliminary

32. But it may provide answers to 3 out of the 11 greatest unanswered questions of Physics !! A few thoughts for your way home The fundamental question of parton to hadron conversion can be tackled though through systematic studies of particle identified fragmentation processes inside and outside the produced medium. This topic is far reaching, it is challenging, it will require systematic studies, and it might require more dedicated equipment than the existing RHIC/LHC detectors. From my point of view that in itself is tantalizing, but it does not lead to a larger physics payoff per se. Many wise men, in particular theorists, claim that the evidence for QGP formation is overwhelming and indeed the signatures for the creation of strongly interacting, collective partonic matter formation is strong (sQGP).

33. A few thoughts for your way home The matter produced is an almost perfect fluid ! A strongly interacting parton liquid is not what we expected. (sQGP is the new theory label) Maximum opacity (Gyulassy 01)Navier-Stokes (Teaney 03)

34. Where is the weakly interacting plasma ? Shuryak, QM04 deconfinement  restoration parton fluid (pre-hadrons) Cassing, priv.comm.

35. England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt India: Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC Netherlands: NIKHEF Poland: Warsaw University of Technology Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino Switzerland: University of Bern U.S. Labs: Argonne, Lawrence Berkeley, and Brookhaven National Labs U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale Brazil: Universidade de Sao Paolo China: IHEP - Beijing, IPP - Wuhan, USTC, Tsinghua, SINR, IMP Lanzhou Croatia: Zagreb University Czech Republic: Nuclear Physics Institute STAR: 51 Institutions, ~ 500 People

36. Consequences of a strong v 2 and final state jet quenching at RHIC 1.) v 2 is strong and has to come from very early time after collision. Hadronic v 2 is not sufficient in terms of magnitude and timescale. 2.) v 2 is very well described by hydrodynamics (fluid dynamics). 3.) if the phase producing the flow is partonic then we have partonic fluid (dissipative, strongly interacting, small correlation length) rather than a plasma (large correlation length, weakly interacting quasi-particle gas).

37. v2 in Au+Au 62GeV

38. Elliptic flow v 2 scaling at intermediate to high-p t two groups, baryons and mesons suggesting relevance of constituent quarks in hadron production S.A. Voloshin, Nucl. Phys. A715, 379 (2003). D. Molnar and S.A. Voloshin, PRL 91, 092301(2003).  Further tests: ,  0, K*, pentaquarks  scaling could be seen as a signature of deconfinement !

39. Hadron suppression prevails at 62 GeV 2  bins, driven by p+p –  = 0: p T <~6 GeV –  = 0.7: p T <~10 GeV Significant suppression seen at 62 and 200 GeV 1/3 of dataset: quantitative treatment awaits full analysis R CP

40. Charged particle correlations: AA/pp ratio for Gaussian fit to same side peak (trig-pt =1.5-3.0 GeV/c, assoc-pt = 1.5-3.0 Gev/c )

41.  +h vs. Anti-  +h (most central)

42. The compelling global questions Could there be evidence for a different phase of matter at even lower x ? Are the quarks and gluons weakly interacting, as expected from a plasma, or strongly interacting as expected from an ideal fluid description ? Is this phase thermally and chemically equilibrated ? Is there evidence for a phase transition to a deconfined and chirally symmetric phase of quarks and gluons at high T ?

43. Modification of fragmentation functions (e.g.hep-ph/0005044)

44. What is there to measure ? The series of measurements is very big, but at a minimum any particle identified measurement at high pt will lead to a quantification of the energy loss process. STAR and PHENIX have started to measure identified particle yields and azimuthal two particle correlations at high pt. Particle species, trigger pt and associated pt cuts can be varied. The problem is to distinguish between jet properties and bulk matter background and the intermediate pt coalescence production.

45. Time scales according to STAR data hadronization initial state pre-equilibrium QGP and hydrodynamic expansion hadronic phase and freeze-out dN/dt 1 fm/c 5 fm/c 10 fm/c20 fm/c time Chemical freeze out Kinetic freeze out Balance function (require flow) Resonance survival Rlong (and HBT wrt reaction plane) Rout, Rside

46. AA/pp for (Λ+h) and (h+h) 1.5<p T,trigger <3.0, 1.5<p T,asso <3.0 Gaussian fit Cosine fit Large AA/pp ratio for the same side  Trigger Bias? X N Wang, nucl-th/0405017

47. Is suppression of high pt particles in RHIC AA collisions an initial state (due to gluon saturation) or final state (due to jet quenching) effect? Initial state? Final state? partonic energy loss in dense medium generated in collision strong modification of Au wavefunction (gluon saturation) Ultimate test: dA collisions