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Lessons from RHIC: predictions vs. reality

Lessons from RHIC: predictions vs. reality. Summary of the Institute for Nuclear Theory workshop on the first two years of RHIC December 2002 not just about strangeness And who won the wine??. Barbara V. Jacak Stony Brook March 15, 2003. Organization of this talk.

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Lessons from RHIC: predictions vs. reality

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  1. Lessons from RHIC: predictions vs. reality Summary of the Institute for Nuclear Theory workshop on the first two years of RHIC December 2002 not just about strangeness And who won the wine?? Barbara V. Jacak Stony Brook March 15, 2003

  2. Organization of this talk • MANY results and MANY predictions! • I’ll follow the data (being an experimentalist) • Start with yields, y distributions, spectra which were predicted by event generators • Compare the predictions from the competition • Draw some physics conclusions • Confront, also, a few newer theoretical ideas…

  3. Charged particle yields dNch/dh = 640 Rises somewhat faster than Npart

  4. Rapidity distribution Longitudinal dynamics PHOBOS dNp/dy ~ 220-230 per charge dNK+/dy ~ 40 dNp/dy ~ 28 Net baryon density at mid-y small, but not 0 mB small

  5. Transverse energy PHENIX preliminary ET/particle ~ 0.9 GeV Similar cent. dependence as <pt> But <pt> goes up with s by 20% while ET is constant  particle mix is changing PHENIX preliminary STAR

  6. Can models reproduce the net baryons? Net baryon central plateau (y=0 to ~ y=2) Cannot (yet) differentiate AMPT vs. HIJING/BJ Fritiof 1.7 Venus 4.02 RQMD Fritiof 7.2

  7. Event generator entry #1: AMPT (C.M. Ko, et al.) • Ingredients: • HIJING, Zhang’s parton cascade, ART hadronic rescatting • Get dNch/dh within 25%, meson & net baryon central plateau but spectra, ET off by 50% & baryon y loss insufficient • NOTE! • To get reasonable particle yields must tweak model so it no longer agrees with pp collisions. • Modified fragmentation function to match lower s data, rationale: fragmentation in dense matter NOT like pp • Must add a partonic phase with large scattering cross sections to reproduce v2 and HBT • To reproduce K-/K+, f need additional hadronic rescattering channels

  8. v2 from AMPT

  9. “Minimalist” event generator entry: LEXUS J. Kapusta and collaborators • Ingredients: parameterized p+p collision results + Glauber NN hard collision probability l = 0.6 (works OK at SPS) • Total multiplicity is fixed by energy conservation • Baryon density fixed by Dy in each collision • Create more hadrons in LEXUS than in wounded nucleon model, since wounded nucleons are not sterile in LEXUS. Some evidence for destructive interference among stopped nucleons at mid-y • Minimalist picture works ~ OK for the simple observables (dN/dy, <pt>) but not for more complex ones • energy conservation  Ncoll or Npart scaling of yields?

  10. Particle Spectra @ 200 GeV BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% QM2002 summary slide (T. Ullrich)

  11. <pT> vs. Npart <pT> [GeV/c] <pT> [GeV/c] • Systematic error on • 200 GeV data • p (10 %), K (15 %), • p (14 %) open symbol : 130 GeV data • <pT> increases with Npart ; tends to saturate • p < K < proton (pbar): consistent with radial expansion

  12. Next event generator contestant: UrQMD Bleicher, et al. • Ingredients: excitation and fragmentation of color strings, formation and decay of hadronic resonances, hadronic rescattering • Predict dET/dh = 600, dNch/dh = 750, ET/Nch = 0.85 GeV • Data say 495, 640, 0.9 • Get ET to within 20%, Nch to 17% • At y=0 expect: 12 net protons, 400 p-, 45 K+ • Data: 7, 230, 40 • Predict <pT> = 375, 500, 780 for p, K, p • Data: 400, 650, 940 not enough radial flow! • v2 ~ 1% (way too low as the strings don’t collide) • Dense set of non-interacting strings… a problem…

  13. We learned that • Must have QGP-type equation of state to get the v2 and radial flow correctly! • UrQMD has insufficient initial pressure as the strings don’t scatter. • AMPT “fixed” this by letting strings interact. • Mass shifts of resonances are very sensitive to breakup dynamics. • Resonances are not dissolved  implies fast freeze-out

  14. Preliminary STAR STAR Preliminary Centrality dependence of v2 Note possible dependence on low pt cut 200 GeV: 0.2< pt < 2.0 130 GeV: 0.075< pt < 2.0 200 GeV: 0.150< pt < 2.0 4-part cumulants v2=0.05 200 GeV: Preliminary - Consistent results - At 200 GeV better pronounced decrease of v2 for the most peripheral collisions. QM2002 summary slide (Voloshin)

  15. v2 of mesons & baryons Au+Au at sNN=200GeV 1) High quality M.B. data!!! 2) Consistent between PHENIX and STAR pT < 2 GeV/c v2(light) > v2(heavy) pT > 2.5 GeV/c v2(light) < v2(heavy) Model: P.Huovinen, et al., Phys. Lett. B503, 58 (2001)

  16. Hydrodynamics – Ulrich Heinz, Peter Kolb • Ingredients: initial conditions, hydrodynamics • QGP EOS with transition to resonance gas • Predictions: • Thermalization in 0.6 fm/c at RHIC • Get v2(pion multiplicity density) - must fix initial conditions • v2 value ~5% at RHIC due to phase transition softening EOS • (data say v2 ~ 6%) • v2 vs. pT increases to 2 GeV/c • v2(mesons) > v2 (baryons) • spectra come out OK (once initial condition is fixed) • Lessons: v2 requires early rescattering! Hadronization follows thermalization by 5-7 fm/c. But, final state decoupling needs work (hydro gets HBT wrong)

  17. Hydrodynamics –Teaney & Shuryak • Ingredients: hydrodynamics + RQMD for hadronic state and freeze-out • Predictions: • RHIC should be near softest point in EOS • s dependence of v2 correctly predicted for b=6 fm • fixed initial conditions, then got spectra correct • Predict particle yields without rescaling • Initial entropy too high, HBT radii too large! • Lessons: hydro good to pT ~ 1.5 GeV/c • Viscosity corrections may be important; cause v2 to bend over at 1 GeV/c pT (compared to ideal gas). Also helps reduce HBT radii. • Does viscosity increase in hadron gas phase?

  18. HBT – lots of questions Panitkin, Pratt • Data on HBT seem to prefer fast freezeout • How to increase R without increasing Rout/Rside? • EOS, initial T and rprofiles (Csőrgó), emissivity?

  19. How do the initial conditions come about? Denes Molnar, et al • calculate parton transport, fixing s (i.e. transport opacity c) • Predictions & insights: • ET loss due to pdV work so (ET)cent < (ET)peripheral • the ET data require small s (3 mb) • can’t easily fix up with inelastic collisions • (need parton subdivision to avoid numerical “viscosity”) • Can reproduce v2 if dNgluon/dy very large or sel= 45 mb • But large opacity underpredicts HBT spectra! • And the inputs are not free for the choosing… • pQCD fixes dNgluon/dy at large pT • pQCD fixes parton s at large Q2 •  Picture doesn’t want to hang together!

  20. No Shadow, No Quench No Shadow, dEg/dx=0.5 GeV/fm GLV “Thin” Plasma Limit Default: Shadow, dEg/dx=2.0 Jet Quenching – Gyulassy, Wang, Vitev, Levai • HIJING: Beam jets @ pt<2 GeV (LUND), pQCD mini jets @ pt>2 GeV (PYTHIA), geometry (Glauber), 1D expansion, conservation laws; tuned to pp data 10-103 GeV • + nuclear shadowing and parton energy loss “knobs” BDMS “Thick” Plasma Limit

  21. protons p0, h Baryons at high pT Baryon yields scale with Ncoll near pT = 2 – 3 GeV/c Then start to fall Meaning of Ncoll scaling? Accident? Complex hard/soft interplay? Quark coalescence? Medium modified jet fragmentation function?

  22. Other penetrating probes • J/Y • Open Charm • Di-jets vs. mono-jets Need (a lot) more statistics in the data But getting a first sniff of physics already

  23. J/Y Energy/Momentum Data consistent with: Hadronic comover breakup (Ramona Vogt) w/o QGP Limiting suppression via surface emission (C.Y. Wong) Dissociation + thermal regeneration (R. Rapp)

  24. Open charm – Ziwei Lin Data & predictions within factor 2 (with or w/o energy loss) no x4 suppression seen from periph. to central, as predicted for dE/dx=-0.5GeV/fm But - Is 40-70% peripheral enough? error bars still big!

  25. trigger-jet not much modification (the trigger particles from jets!) Away side: strong jet suppression Away-side Jet Suppression D. Hardtke • jet quenching  surface emission of jets? • Color glass back-to-back jets simply not created…

  26. Au+Au /pp Mini-Jets Parton saturation Dima Kharzeev, Jamal Jalilian-Marian • Hadron multiplicities imply a coherent initial state • Initial NN interactions are NOT independent! • High parton density  weak coupling  Color Glass • hard parton scattering suppressed  • Nch scales with Npart, even at high pT ;monojets • saturation already • at s ~ 20GeV? • I doubt this! Measure forward y in p+A (Qs larger, CGC is magnified) clarify initial vs. final state effect in AA!

  27. conclusions • Have early pressure buildup – high dNg/dy & they scatter! •  success of hydro, need for string melting, large s… • Freeze-out is fast • High pT, high mass data look like pQCD + something • Jet quenching works; surface emission?? • Baryon flow is a nuclear effect! • Color glass is intriguing, but if right where does the collectivity (v2, bT) come from? • Event generators (still) a valuable tool to investigate sensitivity of observables to physics ingredients • Integrated quantities are simple (conservation laws!) • Caution in interpreting scaling with Npart or Ncoll • e+e- scaling with Npart is arbitrary, agreement irrelevant

  28. So, are we seeing quark gluon plasma? • If it looks like a duck, walks like a duck…. • BUT • Serious conclusion should await • results from the “control” experiment d+Au • theoretical description(s) which hangs together

  29. And the winners of the wine … • Best predictions of general features by event generator • AMPT (Ko, Lin, Zhang) • Novel approach, theoretically intriguing (+ agrees with data) • Baryon junctions (Kharzeev, Vance, Gyulassy, Wang) • Important prediction with potential great insights to QGP • Hydrodynamics (Heinz & Kolb, Teaney & Shuryak, Bass & Dumitru, Ollitrault for teaching us v2 analysis) • Much promise for understanding properties of QGP • Jet energy loss (Gyulassy,Wang, Vitev, Levai) The wine is history…

  30. Predictions (200 GeV) Exptl. (130 GeV) Exptl. (200 GeV) 0.75 0.66 0.076 0.074 0.95 0.90 0.15 0.15 0.75 PHENIX STAR 0.89 0.58 0.95 0.66 0.021 0.19 0.0015 Statistical models • Johanna: chemical equilibrium with T=170 MeV, mB = 10 MeV • Johann: sudden freezeout with incomplete chemical equilib. T=177 MeV mB = 29 MeV mB lower than SPS, but not as low as predicted No anomalous strangeness enhancement…

  31. Anti-particle/particle ratios vs. y vs. p+p Au + Au chemistry, stopping… p+p collisions BRAHMS 200 GeV Yields at mid-rapidity: Net-protons: dN/dy  7 Protons : dN/dy  29  ¾ from pair-production ISR extrapolation Ratios similar to those in p+p!

  32. Nbinary ? 2003 ? PHENIX 130 BRAHMS PRL88(02) STAR 130 Npart/2 hch is dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x? have a definite prediction for d+Au!

  33. Preliminary sNN = 200 GeV Preliminary sNN = 200 GeV Charged Hadron Spectra 200 GeV results from all experiments Shape changes from peripheral  central C. Roland, PHOBOS Parallel Saturday

  34. p/p at high pT Higher than in p+p collisions or fragmentation of gluon jets in e+e- collisions Vitev & Gyulassy nucl-th/0104066 Can explain by combination of hydro expansion at low pT with jet quenching at high pT

  35. Vitev: they can get v2 right • There is a quantitative difference • Calculations/fits with flat • or continuously growing Check against high-pT data (200 AGeV) b=7 fm b~7 fm C. Adler et al. [STAR Collab.], arXiv: nucl-ex/0206006 Same for 0-50% • The decrease with pT is now • supported by data • For minimum bias this rate is • slightly slower K. Filimonov [STAR Collab.], arXiv: nucl-ex/0210027 See: N.Borghini, P.Dinh, J-Y.Ollitrault, Phys.Rev. C 64 (2001)

  36. yield in AuAu vs. p-p collisions D. d’Enterria Yield ratio s=200/130 GeV Consistent at at high pT with pQCD predictions (STAR) PHENIX Preliminary 70-80% Peripheral Ncoll =12.3 ±4.0

  37. kT dependence of R Centrality is in top 30% • Broad <kT> range : 0.2 - 1.2 GeV/c • All R parameters decrease as a function of kT •  consistent with collective expansion picture. • Stronger kT dependent in Rlong have been observed. kT : average momentum of pair

  38. Comparison of kaon to pion In the most 30% central

  39. Comparison with hydrodynamic model Centrality is in top 30% Recent hydrodynamic calculation by U.Heinz and P. F. Kolb (hep-ph/0204061) Hydro w/o FS • Standard initialization and freeze out which reproduce single particle spectra. Hydro at ecrit • Assuming freeze out directly at the hadronization point. (edec = ecrit) kT dependence of Rlong indicates the early freeze-out?

  40. kT dependence of Rout/Rside A. Enikizono QM2002 C.M. Kuo, QM2002 poster (PHOBOS) 200 GeV: @0.25 GeV/c

  41. HBT PUZZLE Small Rout implies small Dt P.Kolb Small Rbeam implies small breakup t, ~10 fm/c Large Rside implies large R

  42. near-side correlation of charged tracks (STAR) trigger particle pT = 4-6 GeV/c Df distribution for pT > 2 GeV/c signature of jets also seen in g (p0) triggered events (PHENIX) trigger particle pT > 2.5 GeV/c Df distribution for pT = 2-4 GeV/c Jet Evidence in Azimuthal Correlations at RHIC QM2002 summary slide (Peitzmann) M. Chiu, PHENIX Parallel Saturday

  43. 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

  44. 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

  45. 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

  46. 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!

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

  48. 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??

  49. Charm cross section at RHIC

  50. Centrality dependence of charm

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