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Experimental Status of Parton Saturation at RHIC

Experimental Status of Parton Saturation at RHIC. Peter Steinberg Brookhaven National Laboratory Forward Physics @ RHIC October 9, 2003. Color Glass Condensate. Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc. Implementation of low-x QCD Color

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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 Forward Physics @ RHIC October 9, 2003

  2. 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 Particle Spectra Multiplicity “Soft Physics” Geometry & Energy

  3. RHIC Phenomena 19.6 GeV 130 GeV 200 GeV dN/dh h • What controls thesoft physics? • Saturation scale? • QCD Scale? • Kinematic effects?

  4. HERA Phenomena Saturation predictsthat a single scale dominates low-x gluon structurein DIS G-BW model predicts “Geometrical Scaling” DIS Data Stasto, Golec-Biernat, Kwiecinski (2001)

  5. Saturation Phenomenology • Qs controls low-x physics: applies universally! • Golec-Biernat-Wusthoff energy scaling of g*p cross section • Rapidity • Centrality – Npart scaling (sources) modified by thickness • McLerran-VenugopalanMuellerKharzeev/Nardi Geometry QCD InitialFinal

  6. Geometrical Scaling @ RHIC? • RHIC data shows evidence of similar “geometric scaling”: • NB: Scaling factors needed • Strangeness x 2 • Baryons / 2 Schaffner-Bielich, McLerran,Venugopalan, Kharzeev (2001)

  7. mT scaling 2003 New PHENIX data Large weak decay corrections (~30-40%) at low pT Scaling factors ~20% Harder to claim unambiguous mT scaling Expected from radial expansion? Can we still make claim for geometrical scaling at RHIC? PHENIX data, nucl-ex/0307022

  8. Centrality Dependence Accardi & Gyulassy (2003) Many modelscan incorporatenuclear thickness “Two-component”:Hard + Soft “One-component”:CGC + DGLAP(Kharzeev & Nardi) Maybe saturation scale too small @ y=0?

  9. Forward Physics • Geometrical scaling • y – ybeam ~ log(x) • Qs2(y)~ Qo2exp(ly) • High-x probes low-x • Projectile partons put “on shell” by target partons • What do we know about physics at y>0 q g “kick”~Qs

  10. Pseudorapidity Distributions KLN: Final state from 21 gluon scattering Overall scale Jacobian Quark counting Kharzeev, Levin, Nardi (2001) (LPHD) Energy, Rapidity, Centrality

  11. Au+Au vs. Elementary Systems Very differentQ2 e+e- ~ A+A despite different Q (Qs vs. s)

  12. Spectral Modification? BRAHMS BRAHMS Qs2 should increase by exp(ly)=e(.15*2.2)~2i.e. from 2 to 4 GeV2 No obvious hardening of spectrum seen

  13. Status of CGC @ RHIC • At present, RHIC data does not seem to require saturation • mT scaling may not hold • Rapidity shapes may be universal, and not very sensitive to Qs • Spectra do not harden at accessible h • And yet, CGC is an intriguing approach • Universality of strong interactions • Coherence of low-x gluons  Npart scaling

  14. Npart Scaling in d+A? STAR Data(PAS Representation) Au+Au d+Au

  15. Further tests of CGC • How can we push the CGC phenomenology? • Far-forward (or far-backwards…) • New detectors at small angles, e.g. in BRAHMS

  16. Shattering the CGC with p+A Predictions from Dumitru, Jalilian-Marian, etc. A. Dumitru, hep-ph/0210412 • p+A collisions: • Cleaner signal • Two scales • Calculations should be more reliable • Systematics ofmay indicate saturation effects

  17. Forward Physics is “Simple”! PHOBOS Au+Au 200 GeV 130 GeV Scaling with x Limiting Fragmentation: h’ may be therelevant variablefor the physics 19.6 GeV h = h - ybeam

  18. Energy Scan Normalized here KLN, l=.3 Landau Hydro Change RHIC energiesand any rapidity will trend towards smaller h’ or y’ Parameter-free Landau Hydro(Cooper & Schonberg)

  19. Fixed Target RHIC Au+Au100+100 RHIC100+1 Test y’/h’ Universality y~0 y’~yb MRS FS In a RHIC fixed-target mode, BRAHMS FS & MRS flip roles!!

  20. Fixed Target Energy Scan 100 50 25 Scan fixed target energyto scan out to smallerrapidities relative to yb 10 Of course, will eventuallyhave to worry about remnant baryon density.

  21. Conclusions • Saturation offers an intriguing approach to understanding RHIC data • Universal properties of strong interaction allow interplay between RHIC & HERA • While saturation can describe parts of RHIC data, other models can too • No dramatic unique features seen • Forward physics may offer a test • Nicely dovetails with energy scan and fixed-target efforts, which may allow substantial reach in h’ or y’ in BRAHMS

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