New results from PHENIX What’s happening at high p T ?

1. New results from PHENIXWhat’s happening at high pT? A preview of PRL covers to come Barbara V. Jacak Stony Brook October 2, 2002

2. Outline & summary • The high pT suppression is real! • Continues to higher pT • In both p0 and charged particles • High pT particles do come from jets! • Must use caution to avoid confusion with v2 • Hadronic composition at high pT • Is mysterious • Changes with centrality

3. Goals of RHIC • Collide Au + Au ions at high energy • 130 GeV/nucleon pair c.m. energy in 2000 • s = 200 GeV/nucleon pair in 2001 • Create in the laboratory high temperature and density matter • as existed ~1 msec after the Big Bang • inter-hadron distances comparable to that in neutron stars • heavy ions to achieve maximum volume • 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?

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

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

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

7. schematic view of jet production hadrons leading particle q q hadrons leading particle fast partons as probe of the plasma 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 pt particles • “jet quenching” • affect away side jet

8. Pion spectrum - p at low pT s = 200 GeV per nucleon pair

9. Pion spectrum - p0 to high pT

10. Pion spectrum – charged at very high pT Use RICH to tag high pT pions

11. Measure reference spectrum in the SAME experiment Remove extrapolation errors Reach higher pT than UA1 Agrees with NLO calculation PHENIX p0 spectrum in p-p collisions

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

13. p0 yield in AuAu vs. p-p collisions PHENIX Preliminary 70-80% Peripheral Ncoll =12.3 ±4.0 30-40% Semi-central Ncoll =220±14

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

15. Charged particle pT spectra from 200 GeV pT >2 GeV/c, slope decrease  suppression h+ + h- pT <2 GeV/c, slope increase  flow

16. Centrality dependence of change • suppression stronger with centrality & increased pT

17. Comparing different channels • Suppression to 9 GeV/c! • Factor consistent for 3 independent measurements • Difference in charged hadron ratio and neutral pion ratio accounted for by particle composition

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

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

20. Learn from xT • Hard scatterings most probably of gluons • Shadowing not a large effect as x is not very small • Beyond leading twist not as clear… • 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 • sjet falls faster than • D(z), so we probe • “kinematic limit” • with large z • hadron spectra may • be dominated by q jets From X.N. Wang Parent x for 4 GeV/c hadron

21. High pT hadrons do come from jets • Look at particle correlations for jet signature • “trigger” on leading photon with pT > 2.5 GeV/c • also look at charged-charged correlations • Jets are observed in Au + Au • v2 (nominally collectively elliptic flow) at high pT is also sensitive to jets • Bias effect: • Trigger requirement requires leading particle! • Systematic study via trigger g, hadron • use 2.5 GeV g, perhaps NOT dominated by p0…?

22. raw differential yields PHENIX Preliminary 2-4 GeV Identifying Jets - Angular Correlations • Remove soft background • by subtraction of mixed event distribution • Fit remainder: • Jet correlation in f; • shape taken from • PYTHIA • Additional v2 component • to correct flow effects

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

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

25. jets or flow correlations? fit pythia + 2v2vjcos(2) 1-2 GeV/c partner = .3-.6 GeV .6-1.0 GeV/c 2-4 GeV/c 1/Ntrig dN/d Df v2vj Jet strength See non-zero jet strength as partner pT increases!

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

27. v2 Negatives pi-&K-,pbar Positives pi+&K+,p PHENIX Preliminary PHENIX Preliminary pT (GeV/c) pT (GeV/c) v2 of identified hadrons Au+Au at sNN=200GeV r.p. |h|=3~4 min. bias v2 p cross p,K not expected from hydro p modified and p not??

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

29. hydrodynamic analysis of spectra Au+Au at s = 130 GeV Simultaneous fit to mT-m0 < 1 Gev/C PHENIX Preliminary T = 1224 MeV t = 0.72 0.01 2/dof = 30.0/40.0 200 GeV similar but T, b a bit PHENIX Preliminary

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

31. Particle composition?

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

33. Now extend to higher energy, pT • Ratio of protons to pions ~1 at high pT for central collisions • Flattens. Turnover not seen.

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

35. How do protons scale with Ncoll/Npart? Scale with Ncoll (unlike p)?!

36. High pT baryons scale with Ncoll! J. Velkovska Low pT near Npart scaling But baryons with pT > 2 GeV/c behave very differently! From jets? Unsuppressed??

37. Use pi/h to look at higher pT What’s this? protons??

38. How about electrons? • PHENIX looks for J/Y  e+e- and m+m- A needle in a haystack: find electron without mistaking a pion at the level of one in 10,000 There is the electron. Ring Imaging Cherenkov counter to tag the electrons “RICH” uses optical “boom” when vpart. > cmedium

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

40. Centrality dependence of charm

41. conclusions • The high pT suppression is real! • Continues to higher pT • In both p0 and charged particles • Charmed quarks do not show energy loss • High pT particles do come from jets! • Must use caution to avoid confusion with v2 • Hadronic composition at high pT • Is mysterious • Changes with centrality • What’s going on with protons & antiprotons??

42. Need theoretical help!! • 3 GeV/c region of spectrum is complicated • Mix of soft & hard processes • via single particle extrapolation • v2 large & not a measurement artefact • 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 g and correlations to help figure it out • Charmed quarks do not indicate large energy loss

43. Backup slides

44. pT jT Correlation width Charged hadron correlations - small Df Correlation width  jT/pT • Fit charged correlations with v2 + Gaussian (fixed pT) • Jet signal visible via s • Width of near-side Gaussian decreases with pT • No significant centrality dependence on near-side

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

46. High pT -/+ ratio • ratio ~1 at high pT in Minimum Bias data • Slightly decreasing in large Npart region?

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

48. <pt> 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

49. Can also get v2 from correlations PHENIX (and PHOBOS) measure correlation function in azimuthal angle Df from same event Df from mixed events C(Df) = ratio dN/d(Df)  [1 + 2l1cos(Df) + 2l2cos(2Df)] l2 v2 Impose pT threshold & see jet correlations

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