High p T Physics in Heavy Ion Collisions

1. High pT Physics in Heavy Ion Collisions Rudolph C. Hwa University of Oregon CIAE, Beijing June 13, 2005

2. particle High pT Physics of Nuclear Collisions at High Energy Well studied for 20 years ---- pQCD What was a discovery yesterday is now used for calibration today. Instead of being concerned with 5% discrepancy in pp collisions, there are problems involving factors of 10 differences to understand in nuclear collisions.

4. Work done in separate collaborations with Chunbin Yang (HZNU, Wuhan; UO) Rainer Fries (Univ. of Minnesota) Zhiquang Tan (HZNU, Wuhan; UO) Charles Chiu (Univ. of Texas, Austin)

5. Anomalies at high pT according to the “standard model of hadronization” --- parton fragmentation The resolution: parton recombination • Recombination in fragmentation • Shower partons • Inclusive distributions at all pT • Cronin effect • Hadron correlations in jets Outline

6. 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.

7. 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.

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

9. cm energy cm energy

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

11. 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

12. Anomaly # 2 STAR

13. v2(p) > v2() at pT > 2.5 GeV/c Anomaly #3 Azimuthal anisotropy v2: coeff. of 2nd harmonic of  distribution PHENIX, PRL 91 (2003)

14. Anomaly #4 Forward-backward asymmetry at intermed. pT in d+Au collisions (STAR) B/F

15. forward has more transverse broadening • backward has no broadening Forward-backward asymmetry in d+Au collisions If initial transverse broadening of parton gives hadrons at high pT, then Expects more forward particles at high pT than backward particles

16. Rapidity dependence of RCP in d+Au collisions BRAHMS PRL 93, 242303(2004) Central more suppressed than peripheral collisions RCP < 1 at =3.2 Interpreted as possible signature of Color Glass Condensate.

17. trigger particle associated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum. Anomaly #5 Jet structure Hard parton  jet { (p1) + (p2) + (p3) + ···· }

18. pp Anomaly #5 Jet structure for Au+Au collisions is different from that for p+p collisions Fuqiang Wang (STAR) nucl-ex/0404010

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

20. 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

21. 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.

22. 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

23. 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

24. Shower Parton Distributions Hwa & CB Yang, PRC 70, 024904 (04)

25. BKK fragmentation functions

26. 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.

27. 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.

28. hard parton (u quark)

29. Pion Distribution Inclusive distribution of pions in any direction

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

31. 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.

32. soft TT TS hard SS thermal Pion distribution (log scale) fragmentation Transverse momentum Now, we go to REAL DATA, and real theoretical results.

33. fragmentation thermal  production in AuAu central collision at 200 GeV Hwa & CB Yang, PRC70, 024905 (2004)

34. TSS Proton production in AuAu collisions TTS+TSS

35. Anomaly #1 Proton/pion ratio resolved

36. All in recombination/ coalescence model Compilation of Rp/ by R. Seto (UCR)

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

38. soft-soft No pT broadening by multiple scattering in the initial state. Medium effect is due to thermal (soft)-shower recombination in the final state. d+Au collisions Pions Hwa & CB Yang, PRL 93, 082302 (2004)

39. Thermal-shower recombination is negligible. Proton Hwa & Yang, PRC 70, 037901 (2004)

40. Anomaly #2 because 3q  p, 2q   Nuclear Modification Factor This is the most important result that validates parton recombination.

41. STAR data Anomaly #3 Azimuthal anisotropy Molnar and Voloshin, PRL 91, 092301 (2003). Parton coalescence implies that v2(pT) scales with the number of constituents

42. Anomaly #4 Forward-backward asymmetry Less soft partons in forward (d) direction than 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.

43. Forward production in d+Au collisions BRAHMS data Hwa, Yang, Fries, PRC 71, 024902 (2005) Underlying physics for hadron production is not changed from backward to forward rapidity.

44. Anomaly #5 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.

45. Correlations 1. Correlation in jets: trigger, associated particle, background subtraction, etc. 2. Two-particle correlation with the two particles treated on equal footing.

46. Normalized correlation function In-between correlation function Correlation function

47. Correlation of partons in jets A. Two shower partons in a jet in vacuum k Fixed hard parton momentum k (as in e+e- annihilation) x1 x2 The two shower partons are correlated.

48. no correlation Hwa & Tan, nucl-th/0503052

49. B. Two shower partons in a jet in HIC Hard parton momentum k is not fixed. fi(k) fi(k) fi(k) fi(k) is small for 0-10%, smaller for 80-92%

50. Hwa & Tan, nucl-th/0503052