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Experimental Status of Parton Saturation at RHIC

Experimental Status of Parton Saturation at RHIC. Peter Steinberg Brookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September 2003. What we do @ RHIC. bang. Freezeout into known hadrons. Hydrodynamic Phase (QGP?). Energy Deposition. Colliding Nuclei.

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Experimental Status of Parton Saturation at RHIC

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  1. Experimental Status ofParton Saturation at RHIC Peter SteinbergBrookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September 2003

  2. What we do @ RHIC bang Freezeout intoknown hadrons HydrodynamicPhase (QGP?) EnergyDeposition Colliding Nuclei We measure the “final” state, we are most interested in the “intermediate” state, so clearly we need to understand the “initial” state…

  3. Nuclear Geometry Au+AuRHIC “Glauber Model” BinaryCollisions b Participant Participants:Long distance,Coherent Collisions: Short distance,Incoherent

  4. Particle Density at 90o in pp & AA Evidence forcollective behavior? PHOBOS @ RHIC (PRL 2001)

  5. dN/dh: Theory vs. Experiment Why is the multiplicity so low? Where is the dramatic risein hard processes expectedat RHIC energies? Eskola, QM2001

  6. Color Glass Condensate Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc. • Implementation of low-x QCD • Color • Integrates (freezes) out the hard scales (time dilation) • Glass • Coherent multi-gluon state • Condensate • Universal • Same for all hadrons Multiplicity Particle Spectra “Soft Physics” controlled by scale Qs2 Geometry & Energy

  7. Geometric Scaling Saturation predictsthat a single scale dominates low-x gluon structure Predicts “geometrical scaling” Stasto, Golec-Biernat, Kwiecinski (2001)

  8. Geometrical Scaling @ RHIC • RHIC data can be said to also show “geometric scaling”: • NB: Corrections are needed • Strangeness x 2 • Baryons / 2 Schaffner-Bielich, McLerran,Venugopalan, Kharzeev (2001)

  9. Saturation Phenomenology • Qs reflects density of partons in transverse plane • Golec-Biernat-Wusthoff energy scaling of g*p cross section • Rapidity • Centrality – Npart scaling (sources) modified by thickness • McLerran-VenugopalanMuellerKharzeev/Nardi HERAG-BW Geometry QCD InitialFinal

  10. Centrality Dependence Accardi & Gyulassy (2003) Many modelscan incorporatenuclear thickness “Two-component”:Hard + Soft “One-component”:CGC + DGLAP(Kharzeev & Nardi)

  11. Saturation vs. Real Data Basic CGC process: 21 scattering Overall scale Jacobian Quark counting Antoni Szczurek, Sunday (LPHD) Energy, Rapidity, Centrality

  12. BRAHMS dN/dy BRAHMSrapiditydistribution BRAHMS Preliminary 2003 Central Au+Au

  13. Limiting Fragmentation in A+A PHOBOS Au+Au 200 GeV 130 GeV 19.6 GeV • Away from y=0, low-xgluons scatter from high-x • “Free” forward structure Limiting Fragmentation h = h - ybeam

  14. Limiting Fragmentation in p+p 900 GeV 546 GeV 200 GeV 53 GeV UA5 inelastic  3 Data from pbar + p alsoshows limiting fragmentationHow essential is partonsaturation to this effect? 2 1 0 -2 0 -4 -6

  15. Limiting Behavior in e+e- DELPHI, PLB459 (1999)

  16. Saturation vs. pp data Can saturation describe elementary collisions? Kharzeev, Levin, Nardi (2002) PHOBOS vs. UA5 Success in Au+Au is helped by similar shape with p+p

  17. Is Soft Physics Universal? PHOBOS(submitted to PRL) e+e- ~ A+A despite different Q (Qs vs. s): pT, flow, etc. “Simple” but a puzzle for CGC  is e+e- a “dense” state?

  18. “Soft Scaling” in Au+Au PHOBOS Total charged multiplicityreflects “soft scaling”(i.e. participants)  not much room for“Hard + Soft” Slight modificationof original questions: Why is the multiplicity so low,and why is it so close to e+e-? Where is the contributionfrom hard processes expectedin RHIC central collisions?

  19. Violation of Ncoll scaling PHENIX Collisions Expectation ifall Ncoll contribute at given pT x4-5 Ncoll=1 Participant b

  20. Soft Scaling of Hard Processes PHOBOS studied thisin detail. Npart scaling seen at low and high pT After first showing ofthis effect in July 2002,Kharzeev, Levin, McLerranoffered a theoreticaldescription

  21. A New Phase Diagram? A A ln 1/x Quantum evolutionretains correlationscharacteristic ofsoft physics QuantumColor Fluid(ExtendedScaling) CGC PartonGas NPQCD D. Kharzeev ln Q2

  22. A “Control” Experiment To rule out saturation scenario, RHIC devoteda large fraction of Run 3 to d+Au collisions Non-saturateddeuteronwave function d “Cronin” Saturatednuclearwave function A “Suppressed”

  23. First RHIC d+Au Results

  24. Search for suppression in d+Au STAR PHENIX Striking absence of suppression claimed by allexperiments, especially relative to central Au+Au Dominant physics seems to be “Cronin Effect” (R>1)

  25. Centrality Dependence h~1 PHOBOS 70-100% 0-20% Centrality dependence rulesout an “onset” of saturationin central d+Au

  26. Is CGC @ RHIC Dead? • This has been a major set-back for CGC-based phenomenology • Lessons from Au+Au not applicable to d+Au • Was success in Au+Au fortuitous? • However, we seem to observe dominance of soft degrees of freedom • Saturation provides a natural framework • A problematic model should not invalidate a compelling theory

  27. Npart Scaling in d+A? STAR Data(PAS Representation) Can perform sameanalysis for A+A & d+A Is this a similar structurewith different parameters? Au+Au d+Au

  28. Summary & Conclusions • Saturation physics offers a compelling perspective on nuclear collisions • Dominance of soft degrees of freedom due to initial state gluon coherence • A single scale controlling various physics • Diminished importance of “final state” effects • Regularities in data supportive of CGC • Multiplicities, limiting fragmentation, mT scaling, Npart scaling at high pT • However, not unique to saturation (or even heavy ions…) • d+Au failure may not be the end of the story

  29. Extra Slides

  30. Update to mT scaling With new PHENIX data,scaling plot is somewhatmodified: • Weak corrections toprotons • Scaling is somewhatdifferent (20% vs. 100%)

  31. mT scaling in p+p

  32. Is Saturation Unique?

  33. Au+Au collisions at s=19.6, 130, 200 GeV • dN/dh for |h|<5.4 over full azimuth • Centrality from paddles (130/200) & Nhits (19.6) • Top 50% of total cross section (Npart~65-360) 130 GeV 200 GeV 19.6 GeV PHOBOS Preliminary dN/dh Most Central Npart h h h

  34. Centrality Dependence of dN/dh Are these effects related? Long-range correlations? Energy conservation? Stopping? Other collision systems? 200 GeV 130 GeV dN/dh/Npart/2 19.6 GeV h = h - ybeam CentralityDependence Location Interpretation

  35. Mid-rapidity Revisited (dN/dyT )

  36. Geometric Scaling Revisited Amusing repeat of hardsoft “duality” seen in geometrical scaling

  37. KLN in y and h PHOBOS Data: 200 GeV Central Au+AuKLN Implementation by P.A.S.

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