A Student’s Guide to Hard Scattering at RHIC

1. A Student’s Guide to Hard Scattering at RHIC Thomas K Hemmick Stony Brook University

2. Helmut Satz A Defining Moment for Me. • In 1988, Brookhaven National Lab held a school for the students in the fledgling field of Relativistic Heavy Ions. • I was one of the attendees and am still grateful for this school nearly 20 years later (I still have the Xeroxed notes) • One of my colleagues recently dug up the “class photo”. • It is simply amazing that most of the people in that photo are still in the field today…I credit the school and its teachers: • BTW: The most popular teacher at that school…

3. Goals of this Presentation • Quark Matter is one of the most exciting, current, and results-filled conferences. • Necessarily, the talks use jargon heavily and assume knowledge of the history of the field. • I hope to give a self-contained (and somewhat whirl-wind) tour over the concepts, previous measurements, and present issues in hard scattering measurements. • My goal is to help you attain something of the necessary background to fully enjoy this conference.

4. Nuclear Collision Terminology • Centrality and Reaction Plane determined on an Event-by-Event basis. • Npart= # of Participants • 2  394 • Nbinary=# of Collisions Peripheral Collision Semi-Central Collision Central Collision 100% Centrality 0% f Reaction Plane • Fourier decompose azimuthal yield:

5. The Paradigm • We accelerate nuclei to high energies with the hope and intent of utilizing the beam energy to drive a phase transition to QGP. • The created system lasts for only ~10 fm/c • The collision must not only utilize the energy effectively, but generate the signatures of the new phase for us. • I will make an artificial distinction as follows: • 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.

6. The Probes Gallery (Hard Scattering): Jet Suppression charm/bottom dynamics J/Y & U direct photonsCONTROL The importance of the control measurement(s) cannot be overstated!

7. Thermally-shaped Soft Production “Well Calibrated” Hard Scattering Calibrating the Probe(s) • Measurement from elementary collisions matches calculations. • Question: What goes into these calculations? p+p->p0 + X hep-ex/0305013 S.S. Adler et al.

8. NOTE: Only the pQCD cross sections are fundamental. PDF and Fragmentation are based upon measurement Factorization Theorem: • Nucleon is a collection of partons described by PDF. • Pair-wise interactions of partons at high Q2 can described by pQCD. • Scattered partons materialize as jets via the fragmentation function. Collins, Soper, Sterman, Nucl. Phys. B263 (1986) 37

9. Parton Distribution Functions • Parton Distribution Functions are well measured and universal (at least under the factorization theorem). • Calculations (PYTHIA) use theoretical form guided by the data: • CTEQ 5M • others… • Parton distributions in nuclei are modified as compared to nucleons. F2

10. Fragmentation Function • The fragmentation function, D(z) describes the process of by which a scattered parton materializes as a jet of particles. • A medium might be expected to modify D(z). • When the full jet is difficult to identify, z is replaced by zT referencing the leading or “trigger” particle of the jet.

11. schematic view of jet production hadrons leading particle q q hadrons leading particle q/g jets as probe of hot 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 pT particles) • “kills” jet partner on other side Jet Quenching

12. Many measurements measure at high pT(!)

13. AA AA If no “effects”: RAA < 1 in regime of soft physics RAA = 1 at high-pT where hard scattering dominates Suppression: RAA < 1 at high-pT AA RAA Normalization 1. Compare Au+Au to nucleon-nucleon cross sections 2. Compare Au+Au central/peripheral Nuclear Modification Factor: nucleon-nucleon cross section <Nbinary>/sinelp+p

14. Au-Au s = 200 GeV: high pT suppression! PRL91, 072301(2003) Effect is real…seen by ALL 4 experiments…Final or Initial State Effect?

15. An example of gluon shadowing prediction gluons in Pb / gluons in p Anti Shadowing Shadowing x More than just a bunch of nucleons • The parton distributionsin a nucleus differ fromthose of the nucleon. • Depletion at low xis called shadowing andexcess at intermediate xis called anti-shadowing. • Shadowing calculations are theoretical calculations “inspired” by experimental measurements (not fundamental).

16. probe rest frame r/ ggg • Color Glass Condensate • Gluon fusion reduces number of scattering centers in initial state. • Theoretically attractive; limits DGLAP evolution/restores unitarity

17. Proton/deuteron nucleus collision Nucleus- nucleus collision Control Experiment • Collisions of small with large nuclei quantify all cold nuclear effects. • Small + Large distinguishes all initial and final state effects. Medium? No Medium!

18. NO suppression in d+Au! PHENIX BRAHMS STAR Phobos

19. Centrality Dependence Au + Au Experiment d + Au Control Experiment • Dramatically different and opposite centrality evolution of Au+Au experiment from d+Au control. • Jet Suppression is clearly a final state effect. Final Data Preliminary Data

20. q g Second Control Experiment • The medium should be transparent to photons. • These thereby probe the initial rate of pQCD production and provide independent normalization of hard collision rates.

21. Direct Photons in Au+Au PRL 94, 232301 p0 suppression caused by medium created in Au+Au collisions Expectation for Ncoll scaling of direct photons holds for all centrality classes

22. So opaque, even a 20 GeV p0 is stopped. • Suppression is very strong (RAA=0.2!) and flat up to 20 GeV/c • Common suppression for p0 and h; it is at partonic level • e > 15 GeV/fm3; dNg/dy > 1100

23. RAA data vs GLV model Empirical energy loss from data Fractional energy loss Quantify the Energy Loss • Medium induced energy loss is the only currently known physical mechanism that can consistently explain the high pT suppression. • From GLV model, initial gluon density dng/dy~1000 is obtained. This corresponds to an initial energy density e~15 GeV/fm3.

24. e+ 0.906 <  < 1.042 p0 D*0 dN/dy = A (Ncoll) K+ m- How about a heavy probe: Charm Quark • Electon spectrum used to infer charm yield. • “Photonic” electrons measured with convertor and subtracted. • Yield scales with Nbinary • Mass alone makes for valid pQCD regime.

25. Modification of Charm M. Djordjevic, et. al. nucl-th/0507019 • Electrons from heavy quark decay have nearly same RAA as pions! • Electrons from heavy quark decay flow (“stopped in medium”)? • But how do you stop a b-quark? • Data imply small diffusion coefficient for charm.

26. Escaping Jet “Near Side” Out-plane Lost Jet “Far Side” In-plane Jet Tomography • Jets are produced as back-to-back pairs. • If one jet escapes, is the other shadowed? • Map the dynamics of Near-Side and Away-Side jets. • Vary the reaction plane vs. jet orientation. • Study the composition of the jets • Reconstruct the WHOLE jet • Find “suppressed” momentum & energy. X-ray pictures areshadows of bones Can Jet Absorption be Used to“Take an X-ray” of our Medium?

27. Back-to-back jets STAR PRL 90, 082302 (2003) Peripheral Au + Au near side Central Au + Au away side peripheral central d + Au control 0 3 Df (radians)

28. STAR STAR Out-plane In-plane Back-to-Back wrt Reaction Plane • Suppression stronger in the out-of-plane direction. • Indicates suppression depends upon length of medium traversed. • Dilemma: How to quantify “completely opaque”. • Get something to punch through. • Find the lost energy and momentum

29. Many sides of RAA • Can examine suppression at differing centrality but same medium length (via emission angle) nucl-ex/0611007

30. Au+Au collisions at 200GeV nucl-ex/0611007 10-20% 50-60% Search for the Scaling Variable • SHOCK-1! The data do not scale with rL, differing from the naïve energy loss picture. • SHOCK-2! The data do scale with L alone and show no suppression for L<2 fm

31. 1 < pT (assoc) < 2.5 GeV/c Away Jet cannot “Disappear” • Energy and momentum conservation require that the “lost” jet must be found somewhere. • “Loss” was seen for partner momenta just below the trigger particle…Search low in momentum for the remnants. PHENIX STAR

32. Correlation of soft ~1-2 GeV/c jet partners Emergence of a Volcano Shape PHENIX (nuclex/0507004) “split” of away side jet! peripheral: normal jet pattern

33. Explanations for splitting • Mach cone • Sonic (or displacement) shock wave propagating through strongly interacting medium. • Cherenkov Radiation • Color charge equivalent to high velocity electric chg • Bent Jet • Jet scatters through medium and is deflected from back-to-back

34. Explaining Modification of Jet Topology Wake Effect or “sonic boom” Cherenkov Gluon Radiation hep-ph/0411315 Casalderrey-Solana,Shuryak,Teaney nucl-th/0406018 Stoecker hep-ph/0503158 Muller,Ruppert nucl-th/0503028A. K. Chaudhuri Renk & Ruppert Phys. Rev. C73 011901 (2006) nucl-th/0507063 Koch, Majumder, X.-N. Wang Transport Theory nucl-th/0601012 Ma, Zhang, Ma, Huang, Cai, Chen, He, Long, Shen, Shi Mult. Scat. nucl-th/0605054 Chiu & Hwa Jets and Flow couple hep-ph/0411341 Armesto,Salgado,Wiedemann

35. Mach cones common in EM plasma Experimental Handle:3-particle correlations

36. near near near Medium Medium Medium away away π away di-jets 0 π 0 deflected jets mach cone Conical Flow vs Deflected Jets

37. signal obtained by subtraction of dominant backgrounds flow components, jet-related two-particle correlation clear elongation (jet deflection) off-diagonal signal related to mach cone? Three-Particle Correlations Au+Au Central 0-12% Triggered Δ2 _ _ = Raw – Jet x Bkgd – Bkgd x Bkgd (Hard-Soft) (Soft-Soft incl. Flow) Δ1 Some of both patterns

38. Hi pT Assoc. pTs D q* 3-Particle Correlations in PHENIX (3 particles from di-jet) + (2 from dijet + 1 other) Same Side Away Side PHENIX Preliminary

39. triples/trigger (A.U.) PHENIX Preliminary Renk&Ruppert: Some of both OK Correlation Topologies Normal Jet (unmodified) Azimuthal Section: Deflected Jet PHENIX Simulation (scattered jet axis) Cone Jet (medium excitation) Some of both patterns

40. Au+Au 20-30% a b b c c Near-Side Long-Range  Correlation: the Ridge Near-side jet-like corrl.+ ridge-like corrl. + v2 modulated bkg. Ridge-like corrl. + v2 modulated bkg. Away-side corrl.+ v2 modulated bkg.

41. yield of associated particles can be separated into a jet-like yield and a ridge yield jet-like yield consistent in  and  and independent of centrality ridge yield increases with centrality 3 < pt,trigger < 4 GeV and pt,assoc. > 2 GeV (J+R) method (J) method (J) method yield,) STAR preliminary Npart     Centrality Dependence of the Ridge

42. jet-like spectra harder than inclusive flatter for higher trigger pT ridge spectra similar to inclusive slightly larger slope approximately independent of trigger pT “Ridge” Particle Spectrum STAR preliminary “jet” ridge charged

43. Anomalous Composition • Large (anti)baryon to pion • Bifurcation of Rcp • One curve for mesons • One curve for baryons. • f meson proves not mass effect. • Recombination: • Coalescing constituent quarks lifts baryon “disadvantage”.

44. Recombination Models • Recombination models assume particles are formed by the coalescence of “constituent” quarks. • Explain baryon excess by simple counting of valence quark content. • Baryon vs meson scaling becomes natural consequence

45. Some Lore and My Charge to You • When Rutherford lead the Cavendish Laboratory, the scientists were thrown out and the doors padlocked promptly at 6:00 PM. • Charge to the scientists: Go Home and THINK! • When the Professor and two students shared the three wishes from the Genie of the Lamp: • Student 1: I wish to be the RICH and powerful ruler of a nation. • Student 2: I wish to live on a tropical isle with beautiful people and no cares in the world. • Professor: I want them back in the lab by nightfall. • My charges to you: • STAY OFF COMPUTER; Listen to talksand THINK. • I want you back in the lab next week.

46. 8 < pT(trig) < 15 GeV/c Emergence of dijets w/ increasing pT(assoc) pT(assoc) > 2 GeV/c pT(assoc) > 3 GeV/c pT(assoc) > 4 GeV/c pT(assoc) > 5 GeV/c pT(assoc) > 6 GeV/c pT(assoc) > 7 GeV/c pT(assoc) > 8 GeV/c • Narrow peak emerges cleanly. • Open question: Punch-through or Tangential? STAR QM2005

47. 3X dAu μμ 200 GeV AuAu μμ 200 GeV CuCu μμ 200 GeV AuAu ee 200 GeV CuCu ee 200 GeV CuCu μμ 62 GeV J/Y:Enigma wrapped in Mystery. • c-cbar produced together. • Dissolve in plasma. • Unlikely(?) to find appropriate mate. • 3X Suppression(~same as CERN) • Models: • Dissolution and recombination? • Cronin broadening? • Feed-down?

48. Enough of this Probe Business… BAM • What does the medium itself have to say?

49. y py px x y z x Pressure? “elliptic flow” barometer Almond shape overlap region incoordinate space Origin:spatial anisotropy of the system when created, followed by multiple scattering of particles in the evolving system spatial anisotropy  momentum anisotropy v2:2nd harmonic Fourier coefficient in azimuthal distribution of particles with respect to the reaction plane

50. Hydrodynamic limit exhausted at RHIC for low pT particles. Can microscopic models work as well? Flow is sensitive to thermalization time since expanding system loses spatial asymmetry over time. Hydro models require thermalization in less than t=1 fm/c Large v2 Adler et al., nucl-ex/0206006