High-p T Physics at RHIC and Evidences of Recombination

1. International Symposium on Multiparticle Dynamics Sonoma, CA, July 2004 High-pT Physics at RHIC and Evidences of Recombination Rudolph C. Hwa University of Oregon

2. In collaboration with Chunbin Yang Hua-zhong Normal University Wuhan, China

3. Anomalies at high pT according to the “standard model” Alternative to the “standard model” at high pT • Recombination in fragmentation • Shower partons • Inclusive distributions at all pT • Dihadron correlations Outline All anomalies can be understood in terms of parton recombination

4. h D(z) q A A Conventional approach to hadron production at high pT Hard scattering near the surface because of energy loss in medium --- jet quenching.

5. If hard parton fragments in vacuum, then the fragmentation products should be independent of the medium. h Particle ratio should depend on the FF D(z) only. D(z) q The observed data reveal several anomalies according to that picture.

6. Rp/π Not possible in fragmentation model: u Anomaly #1 Rp/π  1

7. Anomaly #2 in pA or dA collisions Cronin Effect Cronin et al, Phys.Rev.D (1975) h q p kT broadening by multiple scattering in the initial state. A p >  RHIC expt (2003) Unchallenged for ~30 years. If the medium effect is before fragmentation, then  should be independent of h=  or p

8. PHENIX and STAR experiments found (2002) Can’t be explained by fragmentation. RHIC data from dAu collisions at 200 GeV per NN pair Ratio of central to peripheral collisions: RCP

9. Anomaly # 2

10. trigger particle associated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum. Anomaly #3 Jet structure Hard parton  jet { (p1) + (p2) + (p3) + ···· }

11. pp Anomaly #3 Jet structure for Au+Au collisions is different from that for p+p collisions Fuqiang Wang (STAR) nucl-ex/0404010

12. Anomaly #4 Azimuthal anisotropy Anomaly #5 Forward-backward asymmetry at intermediate pT Come back later when there is time. Resolution of the anomalies

13. How can recombination solve the puzzles? hadron momentum Parton distribution (log scale) p p q p1+p2 (recombine) (fragment) higher yield heavy penalty

14. The black box of fragmentation q p 1 z A QCD process from quark to pion, not calculable in pQCD Momentum fraction z < 1 Dp/q Phenomenological fragmentation function z 1

15. Let’s look inside the black box of fragmentation. q p 1 z fragmentation gluon radiation quark pair creation Although not calculable in pQCD (especially when Q2 gets low), gluon radiation and quark-pair creation and subsequent hadronization nevertheless take place to form pions and other hadrons.

16. hard parton meson shower partons fragmentation recombination can be determined known from recombination model known from data (e+e-, p, … ) Description of fragmentation by recombination

17. Shower parton distributions valence u d s sea u d L L DSeaKNS L  DVG G  DGL Ls  DKSeaG Gs  DKG s R g 5 SPDs are determined from5 FFs. RK

18. Shower Parton Distributions u -> u valence g -> u u -> d u -> s g -> s Hwa & CB Yang, hep-ph/0312271

19. BKK fragmentation functions

20. h Conventional approach D(z) q A A Once the shower parton distributions are known, they can be applied to heavy-ion collisions. The recombination of thermal partons with shower partons becomes conceptually unavoidable.

21. h Now, a new component Once the shower parton distributions are known, they can be applied to heavy-ion collisions. The recombination of thermal partons with shower partons becomes conceptually unavoidable.

22. hard parton (u quark)

23. Pion Distribution Inclusive distribution of pions in any direction

24. Proton formation: uud distribution usual fragmentation soft component soft semi-hard components (by means of recombination) Pion formation: distribution thermal shower

25. Shower distribution in AuAu collisions SPD of parton j in shower of hard parton i hard parton momentum distribution of hard parton i in AuAu collisions fraction of hard partons that get out of medium to produce shower calculable Thermal distribution Contains hydrodynamical properties, not included in our model. Fit low-pT data to determine C & T.

26. density of hard partons with pT = k Input: parton distributions CTEQ5L nuclear shadowing EKS98 hard scattering pQCD Srivastava, Gale, Fries, PRC 67, 034903 (2003) C, B,  are tabulated for i=u, d, s, u, d, g K=2.5

27. 2-shower partons in 2 jets: 2-shower partons in 1 jet: Negligible at RHIC, but can be important at LHC.

28.  production in AuAu central collision at 200 GeV Hwa & CB Yang, nucl-th/0401001, PRC(2004)

29. TSS Proton production in AuAu collisions TTS+TSS

30. Anomaly #1 Proton/pion ratio resolved

31. Monte Carlo Calculation in Coalescence model Greco, Ko & Levai, PRL (2003)

32. Production of  and  in central Au+Au Hwa & Yang, nucl-th/0406072

33. Anomaly #2 d+Au collisions(to study the Cronin Effect) peripheral central d d more T more TS less T less TS

34. No pT broadening by multiple scattering in the initial state. Medium effect due to thermal-shower recombination. d+Au collisions Pions Hwa & CB Yang, nucl-th/0403001, PRL(2004)

35. Thermal-shower recombination is negligible. Proton nucl-th/0404066

36. because 3q  p, 2q   Anomaly #2 resolved Nuclear Modification Factor

37. Anomaly #3 Jet structure in Au+Au different from that in p+p collisions Jet Structure Since TS recombination is more important in Au+Au than in p+p collisions, we expect jets in Au+Au to be different from those in p+p. Consider dihadron correlation in the same jet on the near side.

38. p1(trigger) q3 q1 k q2 q4 p2(associated) trigger associated Trigger at 4 < pT < 6 GeV/c p+p: mainly SS fragmentation Au+Au: mainly TS Associated particle

39. (TS) + [SS] (SS) + [TS] (SS) + [SS] too small for Au+Au but the only term for p+p There are other contributions as well. trigger associated (TS) + [TS]

40. Associated particledistribution in p2with trigger at p1 trigger at p1integrated over 4 < p1 < 6 GeV/c

41. Central Au+Au collisions (TS)[TS] + (TS)[SS]+(SS)[TS] (SS)[SS] for p+p collisions much lower

42. Central Au+Au collisions Hwa & Yang, nucl-th/0407081

43. Au+Au central d+Au peripheral p+p d+Au central d+Au peripheral resolved Anomaly #3 Jets in Au+Au and in p+p are very different. Centrality dependence of associated-particle dist. Ratios Distributions

44. v2(p) > v2() at pT > 2.5 GeV/c Anomaly #4 Azimuthal anisotropy v2: coeff. of 2nd harmonic of  distribution PHENIX, PRL 91 (2003)

45. STAR data Molnar and Voloshin, PRL 91, 092301 (2003). Parton coalescence implies that v2(pT) scales with the number of constituents

46. Anomaly #5 Forward-backward asymmetry at intermed. pT in d+Au collisions STAR preliminary data

47. Less soft partons in forward (d) direction than in backward (Au) direction. Less TS recombination in forward than in backward direction. It is natural for parton recombination to result in forward-backward asymmetry More interesting behavior found in large pTand large pL region.

48. pT 0 2 4 6 8 10 soft hard More realistic picture pT 0 2 4 6 8 10 soft semi-hard hard (low) (intermediate) (high) shower-shower thermal-thermal thermal-shower Summary Traditional picture pQCD + FF

49. All anomalies at intermediate pT can be understood in terms of recombination of thermaland shower partons Recombination is the hadronization process. At pT > 9 GeV/c fragmentation dominates, but it can still be expressed as shower-showerrecombination. Thus we need not consider fragmentation separately any more.

50. The End for now