2. New results from PHENIX What’s happening at high p T ? A preview of PRL covers to come

3. Outline & summary l The high p T suppression is real! Continues to higher p T In both  0 and charged particles l High p T particles do come from jets! Must use caution to avoid confusion with v2 l Hadronic composition at high p T Is mysterious Changes with centrality

4. Goals of RHIC l Collide Au + Au ions at high energy 130 GeV/nucleon pair c.m. energy in 2000  s = 200 GeV/nucleon pair in 2001 l Create in the laboratory high temperature and density matter as existed ~1  sec after the Big Bang inter-hadron distances comparable to that in neutron stars heavy ions to achieve maximum volume l Study the hot, dense system thermal equilibrium? do the nuclei dissolve into a quark gluon plasma? characteristics of the phase transition? transport properties of plasma? equation of state?

5. QCD Phase Transition l transition affects evolution of early universe latent heat & surface tension  matter inhomogeneity in evolving universe? l equation of state of nuclear matter  compression in stellar explosions l we don’t understand how process of quark confinement works how symmetries are broken by nature  massive particles from ~ massless quarks

6. did something new happen at RHIC? l Study collision dynamics (via final state) l Probe the early (hot) phase Equilibrium? hadron spectra, yields Collective behavior i.e. pressure and expansion? elliptic, radial flow vacuum QGP Particles created early in predictable quantity interact differently in QGP vs. hadron matter fast quarks, J/ , strange quark content, quark content, thermal radiation

7. PHENIX at RHIC 2 Central spectrometers 2 Forward spectrometers 3 Global detectors PHENIX philosophy: optimize for signals / sample soft physics

8. fast partons as probe of the plasma hadrons q q leading particle leading particle schematic view of jet production Jets in heavy ion collisions: observed via fast leading particles or azimuthal correlations between the leading particles But, before they create jets, the scattered quarks radiate energy in the colored medium  decreases their momentum  fewer high p t particles  “jet quenching”  affect away side jet

9. Pion spectrum -   at low p T  s = 200 GeV per nucleon pair

10. Pion spectrum -  0 to high p T

11. Pion spectrum – charged at very high p T Use RICH to tag high p T pions

12. PHENIX  0 spectrum in p-p collisions l Measure reference spectrum in the SAME experiment l Remove extrapolation errors l Reach higher p T than UA1 l Agrees with NLO calculation

13. Compare Au+Au to p+p l Use measured p+p to predict rate of plasma probe in Au+Au l Hard scattering probability scales with # of binary nucleon-nucleon collisions l Construct R AA = R AA should be 1 if nothing happens to the probe

14.  0 yield in AuAu vs. p-p collisions 70-80% Peripheral Ncoll =12.3 ±4.0 30-40% Semi-central Ncoll =220±14 PHENIX Preliminary

15. Suppression due to parton energy loss? l Data not consistent with predictions including no energy loss l GLV L/ =4 somewhat better agreement l Both predictions including energy loss consistent with data up to 5 or 6 GeV/c but maybe not quite… l P.Levai, Nuclear Physics A698 (2002) 631. l X.N. Wang, Phys. Rev. C61, 064910 (2000).

16. 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 -

17. Centrality dependence of change l suppression stronger with centrality & increased p T

18. 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

19. How do high p T yields scale? l vs. binary collisions: continuous decrease as function of centrality factor ~ 3.5 from peripheral to central l vs. participants: first increase, then decrease as function of centrality for N part > 100 have 3  change (scaling or no?) surface emission? re-interactions? accident? 18% scaling uncertainty from corrections

20. x of struck parton if p T(had) / p T(jet) ~ 1 then x T ~ x(parton) at y=0 SPS and RHIC at different x! RHIC: ~1.6 x 10 -2 at 200 GeV still not very small… x T =

21. Learn from x T l Hard scatterings most probably of gluons l Shadowing not a large effect as x is not very small Beyond leading twist not as clear… l Natural to compare with gluon jets studied in e + e -, BUT Our leading hadrons are very soft (<10 GeV/c) We mostly see just part of the fragmentation function  jet falls faster than D(z), so we probe “kinematic limit” with large z hadron spectra may be dominated by q jets Parent x for 4 GeV/c hadron From X.N. Wang

22. High p T hadrons do come from jets l Look at particle correlations for jet signature “trigger” on leading photon with p T > 2.5 GeV/c also look at charged-charged correlations Jets are observed in Au + Au l v2 (nominally collectively elliptic flow) at high p T is also sensitive to jets l Bias effect: Trigger requirement requires leading particle! Systematic study via trigger , hadron use 2.5 GeV , perhaps NOT dominated by  0 …?

23. 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 flow effects PHENIX Preliminary raw differential yields 2-4 GeV

24. Verify PYTHIA using p+p collisions  (neutral E>2.5 GeV + 1-2 GeV/c charged partner) |  |<.35 |  |>.35  ake cuts in  to enhance near or far-side correlations Blue = PYTHIA

25. In Au+Au collisions 1-2 GeV partner  (neutral E>2.5 GeV + charged partner) |  |<.35 |  |>.35 1/N trig dN/d  Correlation after mixed event background subtraction Clear jet signal in Au + Au Different away side effect than in p+p

26.  Jet strength See non-zero jet strength as partner p T increases! jets or flow correlations? fit pythia + 2v 2 v j cos(2  ) partner =.3-.6 GeV.6-1.0 GeV/c2-4 GeV/c 1/N trig dN/d  v2vj 1-2 GeV/c

27. min bias 200 GeV Au+ Au v2 at high p T v2 via reaction plane at  =3-4 and via 2-particle correlations similar No jet contamination of reaction plane Diverge at p T > 4 GeV/c? l Low p T as expected from hydrodynamics l v2 > 0.15 at p T >3 GeV/c interpretation? 15% jets per STAR flow vs. hard processes contribution unclear

28. Au+Au at  s NN =200GeV v2v2 r.p. |  |=3~4 min. bias v 2 of identified hadrons Negatives pi-&K-,pbar Positives pi+&K+,p PHENIX Preliminary p T (GeV/c) v2v2 p cross ,K not expected from hydro  modified and p not??

29. Look at charged particle spectra 0 – 5 % 5 -10 % 10- 15 % 15 – 20 % 20 – 30 % 30 – 40 % 40 – 50 % 50 – 60 % 60 – 70 % 70 – 80 % 80 – 93 % Au+Au at  s = 200 GeV PHENIX preliminary

30. PHENIX Preliminary hydrodynamic analysis of spectra PHENIX Preliminary T = 122  4 MeV  t = 0.72  0.01  2 /dof = 30.0/40.0 Simultaneous fit to m T -m 0 < 1 Gev/C Au+Au at  s = 130 GeV 200 GeV similar but T ,  a bit

31. Extrapolate soft component using hydrodynamics l Hydrodynamic flow modifies p t threshold where hard physics starts to dominate l physics has soft (thermal) contributions until p t  3 GeV/c Calculate spectra using hydro parameters h + + h - =  , K, p Compare sum to measured Charged particle p T spectrum J. Burward-Hoy

32. Particle composition?

33. Dynamics affect p/pion ratios l hydro boosts baryons to higher pt Jet quenching should reduce  yield (by ~3-5) baryons less depleted as less likely to be jet leading particles Vitev & Gyulassy nucl-th/0104066 pbar/ pi-

34. l Ratio of protons to pions ~1 at high p T for central collisions Flattens. Turnover not seen. Now extend to higher energy, p T

35. Centrality dependence of p/pi + - Ratios reach ~1 for central collisions Peripheral collisions lower, but still above gluon jet ratios at high p T Maybe not so surprising 1)“peripheral” means 60- 91.4% of  total 2) p/pi = 0.3 at ISR

36. How do protons scale with N coll /N part ? Scale with N coll (unlike  )?!

37. High p T baryons scale with N coll ! Low p T near N part scaling But baryons with p T > 2 GeV/c behave very differently! From jets? Unsuppressed?? J. Velkovska

38. Use pi/h to look at higher p T What’s this? protons??

39. How about electrons? PHENIX looks for J/  e+e - and  There is the electron. A needle in a haystack: find electron without mistaking a pion at the level of one in 10,000 Ring Imaging Cherenkov counter to tag the electrons “RICH” uses optical “boom” when v part. > c medium

40. We do find the electrons Energy/Momentum Electron enriched sample (using RICH) All tracks And J/ 

41. Centrality dependence of charm

42. conclusions l The high p T suppression is real! Continues to higher p T In both  0 and charged particles Charmed quarks do not show energy loss l High p T particles do come from jets! Must use caution to avoid confusion with v2 l Hadronic composition at high p T Is mysterious Changes with centrality What’s going on with protons & antiprotons??

43. Need theoretical help!! l 3 GeV/c region of spectrum is complicated Mix of soft & hard processes via single particle extrapolation v2 large & not a measurement artefact l Large proton contribution to spectrum flow seems a reasonable explanation BUT – why is pi/h so low out to 8 GeV/c???? Suppression of pions, but not leading baryons? PHENIX has  and correlations to help figure it out l Charmed quarks do not indicate large energy loss

44. Backup slides

45. Charged hadron correlations - small  Fit charged correlations with v2 + Gaussian (fixed p T) Jet signal visible via  Width of near-side Gaussian decreases with p T No significant centrality dependence on near-side Correlation width jTjT pTpT Correlation width  j T /p T

46. Note pbar/p behavior Centrality dependence only for p T > 3 GeV/c Peripheral collisions have quite a few protons at mid-y Considerable baryon stopping still! Caution for high p T physics interpretation!!

47. High p T  - /  + ratio l ratio ~1 at high p T in Minimum Bias data l Slightly decreasing in large N part region?

48. Hydrodynamics-inspired fit After Schnedermann, et al. Phys. Rev. C48, 2462 (1993)

49. increases with centrality Expect such a trend from radial flow but also from partonic multiple scattering and gluon saturation don’t know whether final or initial state effect

50. Can also get v 2 from correlations PHENIX (and PHOBOS) measure correlation function in azimuthal angle  from same event  from mixed events C(  ) = ratio dN/d(  )  [1 + 2 1 cos(  ) + 2 2 cos(2  )] 2  v 2 Impose p T threshold & see jet correlations

51. At high p T jet correlations weak or missing! Reaction plane results a mystery... Hydrodynamics no longer dominates Correlation method on HIJING picks out back-to-back particles from jets For data correlation & reaction plane methods agree J. Rak

52. hard/soft competition as probe Above p t ~ 1.5 GeV/c, hydrodynamic flow in the reaction plane has competition from hard processes, which are not correlated with that plane so look for disappearance of elliptic flow depends on amount of energy loss!

53. Nuclear effects in initial stage l Structure functions are modified in nuclei l Shadowing in small-x region due to high parton density from superposition of all the nucleons F 2 A (x) --------- AF 2 N (x) Accessible at RHIC

54. Measure radial expansion T eff = T fo + m 2 T fo = 140 - 150 MeV  radial = 0.5 - 0.6 (higher for central collisions) was 0.4 at lower energy less flow in peripheral collisions!

55. baryon yields PHENIX preliminary antiproton dN/dy = 20! Was 0.18 at SPS

56. strangeness production PHENIX preliminary  s=17 GeV Pb+Pb Phys.Lett.B 471, 6 (1999) Both K+/  and K-/  increase with Npart Peripheral collisions near pp value K+/  and K-/  do not diverge as at SPS,AGS K/  at  s=200 p+p Z.Phys. C41,179 (1988) (UA5)

57. For RHIC and CERN Thanks to Xin-Nian Wang! For p t = 4 GeV/c hadron from jet fragmentation, what is x distribution of parent parton? Does not include kt broadening

58. From Xin-Nian’s calculation l For CERN energy ~ 0.87 distribution is fairly symmetric l for RHIC peak z ~ 1 long tail to higher x parent parton i.e. tail to smaller z estimate with z range 0.7-1.0 l remake x plot

59. more realistic fragmentation function z range indicated by horizontal bar: answer is not very different to exclude known soft physics regime P T = 3 GeV/c (is a safer boundary for hard processes) Thanks to X.N. Wang