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Experimental Status of Parton Saturation at RHIC Peter Steinberg Brookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September 2003 Peter Steinberg ISMD2003 What we do @ RHIC bang Colliding Nuclei Energy Deposition Hydrodynamic Phase (QGP?) Freezeout into known hadrons We measure the “final” state, we are most interested in the “intermediate” state, so clearly we need to understand the “initial” state… Peter Steinberg ISMD2003 Nuclear Geometry “Glauber Model” Au+Au RHIC Binary Collisions b i A1/ 3 Participant Collisions: Short distance, Incoherent Peter Steinberg N coll 1 2 N part 1 i N 4/3 part Participants: Long distance, Coherent ISMD2003 Particle Density at 90o in pp & AA PHOBOS @ RHIC (PRL 2001) Evidence for collective behavior? Peter Steinberg ISMD2003 dN/dh: Theory vs. Experiment Why is the multiplicity so low? Where is the dramatic rise in hard processes expected at RHIC energies? Eskola, QM2001 Peter Steinberg ISMD2003 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 Peter Steinberg ISMD2003 Geometric Scaling Saturation predicts that a single scale dominates low-x gluon structure Predicts “geometrical scaling” Q x Q 2 ~ 2 Q0 x0 Qs 2 2 Stasto, Golec-Biernat, Kwiecinski (2001) Peter Steinberg ISMD2003 Geometrical Scaling @ RHIC • RHIC data can be said to also show “geometric scaling”: mT mT d N ~ f ~ 1 s 3 dp p p s s 3 E Schaffner-Bielich, McLerran, Venugopalan, Kharzeev (2001) n • NB: Corrections are needed • Strangeness x 2 • Baryons / 2 Peter Steinberg ISMD2003 Saturation Phenomenology • Qs reflects density of partons in transverse plane • Golec-Biernat-Wusthoff energy scaling of g*p cross section ~ .25 3 Qs2 W • Rapidity x Q x Q x0 2 s 2 0 HERA G-BW Q02 ~ 2 GeV @130 GeV ( RHIC ) • Centrality – Npart scaling (sources) modified by thickness Qs2 s xG part ~ Ncoll / N part ~ A1/ 3 • McLerran-VenugopalanMuellerKharzeev/Nardi 2 S Q dN cN part xG x, Qs2 A s2 dy s Qs InitialFinal Peter Steinberg Geometry QCD ISMD2003 Centrality Dependence Accardi & Gyulassy (2003) Many models can incorporate nuclear thickness “Two-component”: Hard + Soft dN n0 1 x N part dh n0 xN coll “One-component”: CGC + DGLAP (Kharzeev & Nardi) Qs2 dN cN part ln 2 dh QCD Peter Steinberg ISMD2003 Saturation vs. Real Data • Basic CGC process: 21 scattering Antoni Szczurek, Sunday • Overall scale N cN g (LPHD) • Jacobian dN dN ~ dh dy • Quark counting Energy, Rapidity, Centrality Peter Steinberg 1 x 4 ISMD2003 BRAHMS dN/dy BRAHMS rapidity distribution BRAHMS Preliminary 2003 Central Au+Au Peter Steinberg ISMD2003 Limiting Fragmentation in A+A dN dh N part 2 PHOBOS Au+Au 200 GeV h ' h ybeam ~ ln xF ln pT / M p 130 GeV 19.6 GeV 1 x 4 Away from y=0, low-x gluons scatter from high-x “Free” forward structure h = h - ybeam Peter Steinberg Limiting Fragmentation ISMD2003 Limiting Fragmentation in p+p dN dh UA5 inelastic 900 GeV 3 Data from pbar + p also shows limiting fragmentation 546 GeV 2 How essential is parton saturation to this effect? 200 GeV 53 GeV 1 0 -6 Peter Steinberg -4 -2 0 h ybeam ISMD2003 Limiting Behavior in e+eDELPHI, PLB459 (1999) Peter Steinberg ISMD2003 Saturation vs. pp data Can saturation describe elementary collisions? Kharzeev, Levin, Nardi (2002) PHOBOS vs. UA5 AA pp 4 / 3 Qs2 ~ 0.5 GeV 2 Success in Au+Au is helped by similar shape with p+p Peter Steinberg ISMD2003 Is Soft Physics Universal? s 200 GeV 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? Peter Steinberg ISMD2003 “Soft Scaling” in Au+Au PHOBOS Total charged multiplicity reflects “soft scaling” (i.e. participants) not much room for “Hard + Soft” Slight modification of original questions: 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? Peter Steinberg ISMD2003 Violation of Ncoll scaling Collisions b PHENIX i 4 Participant Expectation if all Ncoll contribute at given pT x4-5 N coll RAA Ncoll=1 1 N pT A A N coll N pT p p Peter Steinberg ISMD2003 Soft Scaling of Hard Processes PHOBOS studied this in detail. Npart scaling seen at low and high pT After first showing of this effect in July 2002, Kharzeev, Levin, McLerran offered a theoretical description Peter Steinberg ISMD2003 A New Phase Diagram? ln 1/x CGC Quantum evolution retains correlations characteristic of soft physics Quantum Color Fluid (Extended Scaling) A NP QCD D. Kharzeev Peter Steinberg Parton Gas A ln Q2 ISMD2003 A “Control” Experiment To rule out saturation scenario, RHIC devoted a large fraction of Run 3 to d+Au collisions d Non-saturated deuteron wave function “Cronin” pT A Saturated nuclear wave function “Suppressed” pT Peter Steinberg ISMD2003 First RHIC d+Au Results Peter Steinberg ISMD2003 Search for suppression in d+Au STAR 1 dN / dpT RdA T b d / dpT dA p p PHENIX Striking absence of suppression claimed by all experiments, especially relative to central Au+Au Dominant physics seems to be “Cronin Effect” (R>1) Peter Steinberg ISMD2003 Centrality Dependence h~1 PHOBOS 70-100% 0-20% Centrality dependence rules out an “onset” of saturation in central d+Au Peter Steinberg ISMD2003 Is CGC @ RHIC Dead? • This has been a major set-back for CGCbased 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 Steinberg ISMD2003 Npart Scaling in d+A? R XA Can perform same analysis for A+A & d+A R N part XA N coll 1 dN / dpT N part / 2 T b d / dpT STAR Data (PAS Representation) Au+Au X A p p d+Au Is this a similar structure with different parameters? Peter Steinberg ISMD2003 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 Peter Steinberg ISMD2003 Extra Slides Peter Steinberg ISMD2003 Update to mT scaling With new PHENIX data, scaling plot is somewhat modified: 1. Weak corrections to protons 2. Scaling is somewhat different (20% vs. 100%) Peter Steinberg ISMD2003 mT scaling in p+p Peter Steinberg ISMD2003 Is Saturation Unique? Peter Steinberg ISMD2003 dN/dh 130 GeV 19.6 GeV 200 GeV PHOBOS Preliminary Most Central h h Npart h • 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) Peter Steinberg ISMD2003 2 N part dN h vs. N part dh Location Centrality Dependence Interpretation h’ ~ -5 Rises Saturation? 2component ? h’ ~ -1.5 Stable Scaling Falls Cascading in spectators? h’~ 1.5 dN/dh/Npart/2 Centrality Dependence of dN/dh 200 GeV 130 GeV 19.6 GeV • Are these effects related? • Long-range correlations? • Energy conservation? • Stopping? • Other collision Peter Steinberg h = h - ybeam ISMD2003 Mid-rapidity Revisited (dN/dyT ) Peter Steinberg ISMD2003 Geometric Scaling Revisited Amusing repeat of hardsoft “duality” seen in geometrical scaling Peter Steinberg ISMD2003 KLN in y and h PHOBOS Data: 200 GeV Central Au+Au KLN Implementation by P.A.S. Peter Steinberg ISMD2003