Peter SteinbergISMD2003 Experimental Status of Parton Saturation at RHIC Peter Steinberg Brookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September 2003

Peter SteinbergISMD2003 What we RHIC bang Hydrodynamic Phase (QGP?) Freezeout into known hadrons Energy Deposition Colliding Nuclei We measure the “final” state, we are most interested in the “intermediate” state, so clearly we need to understand the “initial” state…

Peter SteinbergISMD2003 Nuclear Geometry b Participant Binary Collisions “Glauber Model” Collisions: Short distance, Incoherent Participants: Long distance, Coherent Au+Au RHIC

Peter SteinbergISMD2003 Particle Density at 90 o in pp & AA Evidence for collective behavior? RHIC (PRL 2001)

Peter SteinbergISMD2003 dN/d  : Theory vs. Experiment Eskola, QM2001 Why is the multiplicity so low? Where is the dramatic rise in hard processes expected at RHIC energies?

Peter SteinbergISMD2003 Color Glass Condensate 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 MultiplicityParticle Spectra Geometry & Energy Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc. “Soft Physics” controlled by scale Q s 2

Peter SteinbergISMD2003 Geometric Scaling Saturation predicts that a single scale dominates low-x gluon structure Predicts “geometrical scaling” Stasto, Golec-Biernat, Kwiecinski (2001)

Peter SteinbergISMD2003 Geometrical 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)

Peter SteinbergISMD2003 Saturation Phenomenology Q s reflects density of partons in transverse plane Golec-Biernat-Wusthoff energy scaling of   p cross section Rapidity Centrality – N part scaling (sources) modified by thickness McLerran-Venugopalan  Mueller  Kharzeev/Nardi GeometryQCD Initial  Final HERA G-BW

Peter SteinbergISMD2003 Centrality Dependence Many models can incorporate nuclear thickness “Two-component”: Hard + Soft “One-component”: CGC + DGLAP (Kharzeev & Nardi) Accardi & Gyulassy (2003)

Peter SteinbergISMD2003 Saturation vs. Real Data Basic CGC process: 2  1 scattering Overall scale Jacobian Quark counting (LPHD) Energy, Rapidity, Centrality Antoni Szczurek, Sunday

Peter SteinbergISMD2003 BRAHMS dN/dy BRAHMS Preliminary 2003 Central Au+Au BRAHMS rapidity distribution

Peter SteinbergISMD2003  =  - y beam PHOBOS Au+Au 19.6 GeV 130 GeV 200 GeV Limiting Fragmentation in A+A Away from y=0, low-x gluons scatter from high-x  “Free” forward structure Limiting Fragmentation

Peter SteinbergISMD UA5 inelastic 0 Limiting Fragmentation in p+p 53 GeV 200 GeV 546 GeV 900 GeV Data from pbar + p also shows limiting fragmentation How essential is parton saturation to this effect?

Peter SteinbergISMD2003 Limiting Behavior in e + e - DELPHI, PLB459 (1999)

Peter SteinbergISMD2003 Saturation vs. pp data Can saturation describe elementary collisions? PHOBOS vs. UA5 Success in Au+Au is helped by similar shape with p+p Kharzeev, Levin, Nardi (2002)

Peter SteinbergISMD2003 Is Soft Physics Universal? e+e- ~ A+A despite different Q (Q s vs.  s): p T, flow, etc. “Simple” but a puzzle for CGC  is e+e- a “dense” state? PHOBOS (submitted to PRL)

Peter SteinbergISMD2003 “Soft Scaling” in Au+Au Why is the multiplicity so low, and why is it so close to e+e-? Where is the contribution from hard processes expected in RHIC central collisions? Slight modification of original questions: Total charged multiplicity reflects “soft scaling” (i.e. participants)  not much room for “Hard + Soft” PHOBOS

Peter SteinbergISMD2003 Violation of N coll scaling N coll =1 b Participant Collisions x4-5 PHENIX Expectation if all N coll contribute at given p T

Peter SteinbergISMD2003 Soft Scaling of Hard Processes PHOBOS studied this in detail. N part scaling seen at low and high pT After first showing of this effect in July 2002, Kharzeev, Levin, McLerran offered a theoretical description

Peter SteinbergISMD2003 A New Phase Diagram? ln Q 2 ln 1/x Parton Gas Quantum Color Fluid (Extended Scaling) CGC NP QCD D. Kharzeev Quantum evolution retains correlations characteristic of soft physics A A

Peter SteinbergISMD2003 A “Control” Experiment To rule out saturation scenario, RHIC devoted a large fraction of Run 3 to d+Au collisions d A Saturated nuclear wave function Non-saturated deuteron wave function “Cronin” “Suppressed”

Peter SteinbergISMD2003 First RHIC d+Au Results

Peter SteinbergISMD2003 Search for suppression in d+Au Striking absence of suppression claimed by all experiments, especially relative to central Au+Au Dominant physics seems to be “Cronin Effect” (R>1) STAR PHENIX

Peter SteinbergISMD2003 Centrality Dependence Centrality dependence rules out an “onset” of saturation in central d+Au % 0-20% PHOBOS  ~1

Peter SteinbergISMD2003 Is 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

Peter SteinbergISMD2003 Can perform same analysis for A+A & d+A Is this a similar structure with different parameters? N part Scaling in d+A? Au+Au d+Au STAR Data (PAS Representation)

Peter SteinbergISMD2003 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, m T scaling, N part scaling at high pT However, not unique to saturation (or even heavy ions…) d+Au failure may not be the end of the story

Peter SteinbergISMD2003 Extra Slides

Peter SteinbergISMD2003 Update to m T scaling With new PHENIX data, scaling plot is somewhat modified: 1.Weak corrections to protons 2.Scaling is somewhat different (20% vs. 100%)

Peter SteinbergISMD2003 m T scaling in p+p

Peter SteinbergISMD2003 Is Saturation Unique?

Peter SteinbergISMD2003 Au+Au collisions at  s=19.6, 130, 200 GeV dN/d  for |  |<5.4 over full azimuth Centrality from paddles (130/200) & N hits (19.6) Top 50% of total cross section (N part ~65-360) dN/d     19.6 GeV 130 GeV200 GeV PHOBOS Preliminary N part Most Central

Peter SteinbergISMD2003 Centrality Dependence of dN/d  Are these effects related? Long-range correlations? Energy conservation? Stopping? Other collision systems?  ’ ~ -5Rises Saturation? 2- component ?  ’ ~ -1.5Stable Scaling  ’~ 1.5Falls Cascading in spectators? Centrality Dependence InterpretationLocation 200 GeV 130 GeV 19.6 GeV dN/d  /  N part /2   =  - y beam

Peter SteinbergISMD2003 Mid-rapidity Revisited (dN/dy T )

Peter SteinbergISMD2003 Geometric Scaling Revisited Amusing repeat of hard  soft “duality” seen in geometrical scaling

Peter SteinbergISMD2003 KLN in y and  PHOBOS Data: 200 GeV Central Au+Au KLN Implementation by P.A.S.