1. Hadron Production at RHIC: Where is the “soft” to “hard” transition? Julia Velkovska

2. 10 October 2002, DNP Julia Velkovska (BNL) 2 Outline l Why study hadron production at RHIC ? l Why look for “hard” processes ? l Soft and hard processes in pp collisions l What happens in AA collisions ?  Expected behavior if AA is an incoherent sum of individual pp collisions  In medium effects  Soft: Collective radial flow  Hard: Quenching of jets l The proton+anti-proton puzzle l Outlook

3. 10 October 2002, DNP Julia Velkovska (BNL) 3 And the puzzle is … Why so many protons and anti-protons at high transverse momentum ?

4. 10 October 2002, DNP Julia Velkovska (BNL) 4 RelativisticHeavyIonCollider located at Brookhaven National Laboratory Long Island, New York Where is RHIC ?

5. 10 October 2002, DNP Julia Velkovska (BNL) 5 RHIC Specifications l 3.83 km circumference l Two independent rings  120 bunches/ring  106 ns crossing time l Capable of colliding ~any nuclear species on ~any other species l Energy: è 500 GeV for p-p è 200 GeV for Au-Au (per N-N collision) l Luminosity  Au-Au: 2 x 10 26 cm -2 s -1  p-p : 2 x 10 32 cm -2 s -1

6. 10 October 2002, DNP Julia Velkovska (BNL) 6 time temperature ~ 100 s after Big Bang Nucleosynthesis begins In the beginning quark – gluon plasma ~ 10  s after Big Bang Hadron Synthesis strong force binds quarks and gluons in massive objects: protons, neutrons mass ~ 1 GeV STAR

7. 10 October 2002, DNP Julia Velkovska (BNL) 7 Hadrons in heavy ion collsions l The bulk of the particles produced in relativistic heavy ion collisions are hadrons l The collisions involve processes with in a variety of energy scales  Soft  Hard

8. 10 October 2002, DNP Julia Velkovska (BNL) 8 Soft processes l Small momentum transfer – distant “collisions” l Can’t resolve the partons in the hadron l Excite the hadrons – produce particles  How ?  How many ?  What are the spectra of the produced particles ? l QCD has strong coupling at large distances – perturbative approach doesn’t work l Only phenomenological descriptions available

9. 10 October 2002, DNP Julia Velkovska (BNL) 9 Hard processes l Large momentum transfer – or close distance l Can resolve partons: valence quarks, sea quarks and gluons l Coupling is weak - pQCD applicable parton-parton collision convolution Cross-section

10. 10 October 2002, DNP Julia Velkovska (BNL) 10 Theoretically.. l “hard” is “easy” - calculable l “soft” is “hard” – only phenomenological descriptions available

11. 10 October 2002, DNP Julia Velkovska (BNL) 11 STAR preliminary 99.5% soft (T. Ullrich) Experimentally …”hard” is hard

12. 10 October 2002, DNP Julia Velkovska (BNL) 12 Why do we want to study “hard” processes ? Freeze-out Hadron Gas Phase Transition Plasma-phase Pre-Equilibrium Hard scattering

13. 10 October 2002, DNP Julia Velkovska (BNL) 13 Gold √s NN = 130, 200 GeV Au+Au collision

14. 10 October 2002, DNP Julia Velkovska (BNL) 14 Nucleon- nucleon collisions protonanti-proton √s = 200, 546, 900 GeV UA1, 900 GeV

15. 10 October 2002, DNP Julia Velkovska (BNL) 15 Spectral shapes in pp collisions Mt-scaling for soft particles Power-law tail from hard scattering Increasing with energy

16. 10 October 2002, DNP Julia Velkovska (BNL) 16 Soft to hard transition in pp collisions Experimentally observed: above pt=2 GeV/c particles are fragments of jets Characteristic particle composition determined by fragmentation function Soft to hard transition in p/  ratio

17. 10 October 2002, DNP Julia Velkovska (BNL) 17 100% 0 % Participants Spectators AA collisions are not all the same Nuclei are extended objects  Impact parameter  Number of participants  Centrality ( % from total inelastic cross-section)

18. 10 October 2002, DNP Julia Velkovska (BNL) 18 AA as a superposition of pp l N part scaling for soft processes  Probability for a “distant” or “soft” collision is large ~ 99.5% of the inelastic cross-section.  If it happens, the nucleon is “wounded” and becomes a participant.  “Wounded” nucleon gets excited. It needs some time (~1fm/c) to react and produce particles. Insensitive to additional collisions (if any). Yields of soft particles scale from pp to AA as the number of participants (or “wounded” nucleons). l N bin scaling for hard processes  The probability for a “hard” collision for any two nucleons is small  The total probability in AA collision is multiplied by the number of times we try, e.i. – the cross-section scales with the number of binary collisions - N bin

19. 10 October 2002, DNP Julia Velkovska (BNL) 19 Identified hadron spectra in peripheral and central AuAu collisions at sqrt(s) = 200 GeV l Peripheral – similar to pp l Central  low-pt slopes increase with particle mass  proton and anti- proton yields equal the pion yield at high p T  Never observed before in pp or in AA collisions !

20. 10 October 2002, DNP Julia Velkovska (BNL) 20 (anti)proton/pion ratio: Can we determine soft to hard transition? l Ratios steeply rising to p T = 1.5 – 2 GeV/c l Above p T = 2 GeV/c – flat  ~ 1 for central Au+Au  ~ 0.3 – 0.4 for peripheral Au+Au

21. 10 October 2002, DNP Julia Velkovska (BNL) 21 Hydrodynamics description of spectra l Assume local thermal equilibrium – define EOS l Pressure drives radial flow.Use hydrodynamics to describe evolution l Collective expansion : heavier particles (K,p) shifted to larger p T l The shape is described – the absolute yields input from experiment l Deviates from data for peripheral – above 1.5 GeV/c 100% 0 %

22. 10 October 2002, DNP Julia Velkovska (BNL) 22 Picture emerging from “soft” point of view: l Both pion and (anti)proton spectra in central Au+Au collisions are described up to 3-4 GeV by hydrodynamics (soft physics) P/  ratios inconsistent with jet fragmentation – too many protons and anti-protons l 3 GeV/c pion maybe coming from hard scattering, but a 3 GeV/c (anti)proton (due to radial flow) is coming from soft processes.

23. 10 October 2002, DNP Julia Velkovska (BNL) 23 Expectations if AuAu is N bin x pp u R < 1 in regime of soft physics u R = 1 at high-p T where hard scattering dominates R= pions protons N bin ~ N part 4/3 For any centrality : N bin > N part

24. 10 October 2002, DNP Julia Velkovska (BNL) 24 Pion spectra in peripheral AuAu and pp pQCD calculation LEVAI et. al., Y. Zhang,.PRC65:034903,2002 70-80% Peripheral N bin =12.3 ±4.0 Peripheral AuAu consistent with hard-scattering R=

25. 10 October 2002, DNP Julia Velkovska (BNL) 25 Pion yield in central AuAu vs. p-p collisions R= In central AuAu Suppression ! Phenix preliminary Expectation above 2 GeV/c, If no nuclear effects

26. 10 October 2002, DNP Julia Velkovska (BNL) 26 Charged hadron suppression l suppression stronger with centrality & increased p T

27. 10 October 2002, DNP Julia Velkovska (BNL) 27 cone of hadrons “jet” p p hard-scattered parton in p+p hadron distribution softened, jets broadened? hard-scattered parton during Au+Au increased gluon-radiation within plasma? A glimpse at the ashes of plasma ? Hard scattering

28. 10 October 2002, DNP Julia Velkovska (BNL) 28 Comparison to theory --- Wang dE/dx = 0 --- dE/dx =0.25 GeV/fm Wang: X.N. Wang, Phys. Rev. C61, 064910 (2000). --- Levai L/ = 0 --- L/ = 4 Gyulassy, Levai, Vitev: P.Levai, Nuclear Physics A698 (2002) 631. --- Vitev dN g /dy = 900 GLV, Nucl. Phys. B 594, p. 371 (2001) + work in preparation.

29. 10 October 2002, DNP Julia Velkovska (BNL) 29 Scaling the proton+anti-proton spectra Radial flow Binary scaling ? N bin 975 422 135 14.8

30. 10 October 2002, DNP Julia Velkovska (BNL) 30 Are protons and anti-protons suppressed ?  protons    h

31. 10 October 2002, DNP Julia Velkovska (BNL) 31 Integrate the “hard” yield

32. 10 October 2002, DNP Julia Velkovska (BNL) 32 Compare protons and pions 

33. 10 October 2002, DNP Julia Velkovska (BNL) 33 The proton+anti-proton high-pt puzzle 1. Proton+anti-proton yields scale with N bin in the range 2-4 GeV  Hints that they come from hard scattering 2. Unlike pions they show no evidence for suppression in the measured range 3. Particle composition inconsistent with known fragmentation functions l (1) is inconsistent with “soft” and consistent with hard-scattering l (2) and (3) inconsistent with hard-scattering + in-medium effects

34. 10 October 2002, DNP Julia Velkovska (BNL) 34 What’s happening at pT >4 GeV/c ? Adding to the puzzle Are the protons ½ of the hadron yield up to 9 GeV/c ? Proton identification not available. Use pi0 and non-identified hadrons

35. 10 October 2002, DNP Julia Velkovska (BNL) 35 Conclusions l Baryon production at intermediate ( 2 – 4 GeV/c) transverse momentum is a mystery l It does not fit into “soft” or “hard” description l Novel production mechanism may be needed to explain experimental results:  N bin scaling  No evidence for in-medium suppression  Large (~ 1 ) (anti)proton/pion ratio in central Au+Au collisions

36. Extra slides

37. 10 October 2002, DNP Julia Velkovska (BNL) 37 Integrate the “soft” yield

38. 10 October 2002, DNP Julia Velkovska (BNL) 38 l Suppression to 9 GeV/c! l Factor consistent for 3 independent measurements l Difference in charged hadron ratio and neutral pion ratio accounted for by particle composition Comparing different channels

39. 10 October 2002, DNP Julia Velkovska (BNL) 39 Scaling with Npart central peripheral

40. 10 October 2002, DNP Julia Velkovska (BNL) 40 Central to peripheral ratios scaled by Npart

41. 10 October 2002, DNP Julia Velkovska (BNL) 41 Identifying Jets - Angular Correlations l Remove soft background by subtraction of mixed event distribution l Fit remainder:  Jet correlation in  ;  shape taken from PYTHIA  Additional v 2 component to correct elliptic flow effects

42. 10 October 2002, DNP Julia Velkovska (BNL) 42 Jet correlation strength vs centrality ? Same side jet Away side jet

43. 10 October 2002, DNP Julia Velkovska (BNL) 43 HBT puzzle Recent hydrodynamic calculation by U.Heinz and P. F. Kolb (hep-ph/0204061) kT dependence of R long indicates the early freeze-out? Hydro w/o Free Streaming Hydro at e crit Assuming freeze out directly at the hadronization point. (e dec = e crit ) Standard initialization and freeze out which reproduce single particle spectra. Centrality is in top 30% k T : average momentum of two particles Why duration time  =sqrt(R out 2 -R side 2 )/  of the freeze-out is so short? R out R side R long Any initial conditions in hydro. can not solve the small Rside.

44. 10 October 2002, DNP Julia Velkovska (BNL) 44 Elliptic flow – non-identified hadrons v 2 vs transverse momentum v2v2 reaction plane based analysis (r.p. |  |=3~4) p T (GeV/c) Hydro-dynamical model (*) Hydro+pQCD (dN g /dy=1000,500,200) (**) PHENIX Preliminary (*) P.Huovinen, P.F.Kolb, U.W.Heinz, P.V.Ruuskanen and S.A.Voloshin, Phys. Lett. B503, 58 (2001) dN g /dy=1000 dN g /dy=500 dN g /dy=200 (**) M.Gyulassy, I.Vitev and X.N.Wang, Phys. Rev. Lett. 86, 2537, (2001)

45. 10 October 2002, DNP Julia Velkovska (BNL) 45 Elliptic flow of identified particles 2 GeV/c

46. 10 October 2002, DNP Julia Velkovska (BNL) 46 Charged particle p T spectra from 200 GeV p T <2 GeV/c, slope increase  flow p T >2 GeV/c, slope decrease  suppression h + + h -

47. 10 October 2002, DNP Julia Velkovska (BNL) 47 Comparison to lower energy measurements