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

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

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

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

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

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

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

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

9. 9 Anomaly # 2

10. 10 Anomaly #3 Jet structure Hard parton  jet {  (p 1 ) +  (p 2 ) +  (p 3 ) + ···· } trigger particleassociated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum.

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

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

13. 13 How can recombination solve the puzzles? Parton distribution (log scale) p p 1 +p 2 pq (recombine)(fragment) hadron momentum higher yieldheavy penalty

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

15. 15 Let’s look inside the black box of fragmentation.  q fragmentation z 1 gluon radiation quark pair creation Although not calculable in pQCD (especially when Q 2 gets low), gluon radiation and quark-pair creation and subsequent hadronization nevertheless take place to form pions and other hadrons.

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

17. 17 Shower parton distributions u g s s d duvalence sea L L  D  Sea K NS L  D  V G G  D  G L L s  D K Sea G G s  D K G RR RKRK 5 SPDs are determined from 5 FFs.

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

19. 19 BKK fragmentation functions

20. 20 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. D(z) h q AA Conventional approach

21. 21 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. h Now, a new component

22. 22 hard parton (u quark)

23. 23 Inclusive distribution of pions in any direction PionDistribution

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

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

26. 26 density of hard partons with p T = 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, gK=2.5

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

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

29. 29 Proton production in AuAu collisions TTS+TSS TSS

30. 30 Anomaly #1 Proton/pion ratio resolved

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

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

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

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

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

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

37. 37 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. Anomaly #3 Jet structure in Au+Au different from that in p+p collisions

38. 38 Trigger at 4 < p T < 6 GeV/c p+p:mainly SS fragmentation Au+Au: mainly TS Associated particle p 1 (trigger) p 2 (associated) k q1q1 q2q2 q3q3 q4q4 trigger associated

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

40. 40 Associated particle distribution in p 2 with trigger at p 1 trigger at p 1 integrated over 4 < p 1 < 6 GeV/c

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

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

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

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

45. 45 Molnar and Voloshin, PRL 91, 092301 (2003). Parton coalescence implies that v 2 (p T ) scales with the number of constituents STAR data

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

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

48. 48 Summary Traditional picture pTpT 0246810 hardsoft pQCD + FF More realistic picture pTpT 0246810 hardsoftsemi-hard (low)(intermediate) thermal-thermalthermal-shower (high) shower-shower

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

50. 50

51. 51 Backup slides

52. 52 Rapidity dependence of R CP in d+Au collisions BRAHMS nucl-ex/0403005 R CP < 1 at  =3.2 Central more suppressed than peripheral collisions Interpreted as possible signature of Color Glass Condensate.

53. 53  =0 R CP > 1  =3.2 R CP < 1 At forward rapidity, parton x is degraded (stopping) more for central than for peripheral x distributions at various b accounted for by recombination central < peripheral  R CP < 1 at large  BRAHMS data

54. 54 Forward R CP in d+Au collisions

55. 55

56. 56

57. 57 Forward p T spectra in d+Au collisions

58. 58 At very high p T and very high energy like at LHC, then the density of hard partons produced will be so high as to make jet-jet recombination very important. The effect should be dramatic.

59. 59 near side away side

60. 60 PHENIX

61. 61 Thermal partons No prediction from our model (without hydro input) Fit data on  spectrum for p T < 2GeV/c C = 23.2 GeV -1,T = 0.317 GeV Shower partons  = 0.07 fraction of hard partons that can get out of the dense medium to produce shower Au+Au: adjusted to fit the data at intermediate p T. d+Au: = 1