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The first two years of RHIC: predictions vs. reality

The first two years of RHIC: predictions vs. reality. Summary of the workshop: Who wins the wine, and why? And, by the way, What did we learn from the exercise?. Barbara V. Jacak Stony Brook December 15, 2002. Particle yields and spectra. global quantities hadron distributions.

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The first two years of RHIC: predictions vs. reality

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  1. The first two years of RHIC: predictions vs. reality Summary of the workshop: Who wins the wine, and why? And, by the way, What did we learn from the exercise? Barbara V. Jacak Stony Brook December 15, 2002

  2. Particle yields and spectra global quantities hadron distributions

  3. What do the data say? G. Roland dNch/dh = 640 Rises somewhat faster than Npart

  4. Rapidity distribution 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

  6. Anti-particle/particle ratios I. Bearden Ratios similar to those in p+p! p+p collisions BRAHMS 200 GeV At mid-rapidity: Net-protons: dN/dy  7 proton yield: dN/dy  29  ¾ from pair-production ISR extrapolation

  7. What model can reproduce the net baryons? Net baryon central plateau (y=0 to ~ y=2) Cannot (yet) differentiate AMPT vs. HIJING/BJ

  8. AMPT - CheMing Ko Degree of difficulty = 3.5 • Ingredients: • HIJING, ZPC parton cascade, ART hadronic rescatting • ET = 750 GeV at y=0 (50% off  *) • data say: 3.3 GeV x (300/2) = 495 GeV • 80 baryons at y= 3.9 (data say 34 at y=3.5) • at y=0: 14 p, 10 pbar; pbar/p = 0.6 (data say 29, 22, 0.74) • (ratio is within 25% of data  ***) • dNch/dh ~ 800 (dNch/dh within 25%  ***) • 430 p, 60 K per unit y at mid-y (data say 640 ,230, 40) • Central plateau |y|<1.5 for mesons (pion data says 1.5  *****) Total score: 3.5 + 10.5 + 10.5 + 17.5= 42

  9. What did we learn? • To get proper particle yields must tweak model so it no longer agrees with pp collisions • Changed fragmentation function to match lower s data, rationale: fragmentation in dense matter • Must add a partonic phase with large scattering cross sections to reproduce v2 and HBT • To reproduce K-/K+ need additional hadronic rescattering channels • Then get f K+K- correct in s = 130 GeV/A data

  10. LEXUS – Joe Kapusta Degree of difficulty = 2 • Ingredients: parameterized p+p collision results, Glauber, NN hard collision probability parameter l = 0.6 • Minimalist approach, which works at SPS • Net proton density = 13 (data say 7  *) • dNch/dy = 1200, but should have been 950 using p+p at proper s • Correcting by 15% for yh, get 1020 or 800 • (800 is within 25% of data, but –1 for p+p oops  **) • Particle spectra are too steep, but missing power law tail • proton <pT> ~ 0.925 GeV/c (data say 0.94  *****) Total score: 2 + 4 + 10 = 16 (so their next model will be a Bentley…?)

  11. What did we learn? • Create more hadrons in LEXUS than in wounded nucleon model, since wounded nucleons are not sterile in LEXUS. Overprediction  some destructive interference among stopped nucleons at mid-y? • Total multiplicity is fixed by energy conservation • Baryon density fixed by Dy in each collision • Minimalist picture works ~ OK for the simplest observables, but not for more complex ones • Caution in interpreting scaling with Ncoll or Npart !

  12. Particle Spectra @ 200 GeV BRAHMS: 10% central PHOBOS: 10% PHENIX: 5% STAR: 5% QM2002 summary slide (Ullrich) Feed-down matters !!!

  13. <pT> vs. Npart J. Velkovska <pT> [GeV/c] <pT> [GeV/c] • Systematic error on • 200 GeV data • p (10 %), K (15 %), • p (14 %) open symbol : 130 GeV data • Increase of <pT> as a function of Npart and tends to saturate • p < K < proton (pbar) • Consistent with hydrodynamic expansion picture.

  14. Radial flow <pT> prediction with Tth and <b> obtained from blastwave fit (green line) STAR <pT> prediction for Tch = 170 MeV and <b>=0 pp no rescattering, no flow no thermal equilibrium preliminary F. Wang <pT> of X and W from exponential fits in mT Do they flow ? Or is <pT> lower due to different fit function?

  15. Does it flow? Fits to Omega mT spectra M. van Leeuwen (NA49) C. Suire (STAR) STAR preliminary RHIC SPS/NA49 bT is not well constrained ! • At SPS  and  are now found to be consistent with common freeze-out • Maybe  and  are consistent with a blastwave fit at RHIC • Need to constrain further  more data & much more for v2 of 

  16. UrQMD - Bleicher Degree of difficulty = 2 • Ingredients: excitation and fragmentation of color strings, formation and decay of hadronic resonances, hadronic rescattering • dET/dh = 600, dNch/dh = 750, ET/Nch = 0.85 GeV • Data say 495, 640, 0.9 • Get ET to 20%, Nch to 17%  *** and *** • y=0: 12 net protons, 400 p-, 45 K+ • Data: 7, 230, 40  *, *, and **** • <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… Score = 32

  17. We learned that • Need QGP-type equation of state to get the v2 and radial flow correctly • UrQMD has insufficient initial pressure as the strings don’t scatter. • Mass shifts of resonances very sensitive to breakup dynamics. Resonances are not dissolved  implies fast freeze-out

  18. 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 model summary - Magestro Degree of difficulty = 1 • Johanna: chemical equilibrium with T=170 MeV, mB = 10 MeV • Johann: sudden freezeout with incomplete chemical equilibrium T=177 MeV mB = 29 MeV Scores: Johanna – within ~15% **** Johann - within ~ 40% **

  19. Lessons from statistical analyses • See chemical equilibrium populations at RHIC as at SPS • mB is lower, but not as low as predicted • No anomalous strangeness enhancement • Simple thermal emission produces proton spectra flatter than pion spectra, so they must cross someplace! • Of course the big question is where and why there??

  20. Elliptic flow

  21. 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)

  22. Still flowing at pT = 8 GeV/c? Unlikely!! A puzzle at high pT Nu Xu Adler et al., nucl-ex/0206006

  23. 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) v2

  24. Hydrodynamics – Ulrich Heinz, Peter Kolb Predictions of major importance! • Ingredients: thermal with some initial conditions, QGP EOS early with transition to resonance gas, geometry + Glauber, hydrodynamics • Predictions: • Thermalization by 0.6 fm/c at RHIC • v2 as function of pion multiplicity density (to fix initial cond.) • v2 has a dip (~5%) due to phase transition softening EOS • RHIC is near this point (data says v2 ~ 6%) • v2 vs. pT increases to 2 GeV/c • v2(mesons) > v2 (baryons) • spectra (once initial condition is fixed) • Lessons: v2 requires early rescattering! Hadronization follows thermalization by 5-7 fm/c. But, final state decoupling needs work (get HBT wrong)

  25. 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. Maybe small viscosity early, but increases in hadron gas phase?

  26. Parton transport theory – Denes Molnar Degree of difficulty = 5 • Next step beyond hydro – calculate parton transport, fixing s (i.e. transport opacity c) • Predictions & insights: • ET loss due to pdV work so (ET)cent < (ET)peripheral • ET results 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! • pQCD fixes dNgluon/dy at large pT • pQCD fixes parton s at large Q2 •  Picture doesn’t want to hang together!

  27. Next, jets and high pT summary from Thomas Peitzmann, QM2002

  28. 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

  29. 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

  30. 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 Degree of difficulty = 5 • 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

  31. Nbinary ? 2003 ? PHENIX 130 BRAHMS PRL88(02) STAR 130 Npart/2 hch 15% too many particles, baryons over-quenched, but predicted the suppression BUT: dE/dx =2 GeV/fm or 0.5 GeV/fm or not linear with x?

  32. 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)

  33. Other penetrating probes • Open Charm • J/Y • Dileptons Need (a lot) more statistics in the data But getting a first sniff of physics already

  34. 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)

  35. Open charm - Lin about x2 within predicted curves with or w/o energy loss no x4 suppression from peripheral to central, as predicted for dE/dx=-0.5GeV/fm But - Is 40-70% peripheral enough? error bars still big!

  36. Some old things and some new things • HBT • High pT baryons • Dijets vs. monojets • Well, there was a prediction but for 10x the pT • Parton saturation

  37. HBT – lots of questions Panitkin, Pratt • How to increase R without increasing Rout/Rside? • EOS, initial T and rprofiles (Csőrgó), emissivity? • Why entropy looks low? • Low entropy implies equilibrated QGP ruled out

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

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

  40. 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  CGC • Saturation at y=0, and even more so at forward y • affects QCD evolution, even at Q2 > Qs2 • causes multiplicity to scale with Npart, even at high pT • hard parton scattering suppressed by CGC  monojets • does saturation set in already at s ~ 20GeV? I doubt this! • Should measure in forward y in p+A, where Qs is larger and CGC is magnified. • This should clarify initial vs. final state effect in AA!

  41. conclusions • Have early pressure buildup – high dNg/dy & they scatter! •  success of hydro, need for string melting, large s… • 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 where does the collectivity 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 Experiments: homework to allow quantitative comparisons (multiple 15% factors = sloppy interpretations!)

  42. And the winners are… • 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 “inventing” v2 analysis) • Much promise for understanding properties of QGP • Jet energy loss (Gyulassy,Wang, Vitev, Levai)

  43. 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

  44. 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

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

  46. 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?

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

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

  49. 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

  50. 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

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