1. Peter SteinbergISMD2003 Experimental Status of Parton Saturation at RHIC Peter Steinberg Brookhaven National Laboratory Forward Physics @ RHIC October 9, 2003

2. 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 Multiplicity Particle Spectra Geometry & Energy Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc. “Soft Physics”

3. Peter SteinbergISMD2003 dN/d   19.6 GeV130 GeV200 GeV RHIC Phenomena What controls the soft physics? Saturation scale? QCD Scale? Kinematic effects?

4. Peter SteinbergISMD2003 HERA Phenomena Saturation predicts that a single scale dominates low-x gluon structure in DIS G-BW model predicts “Geometrical Scaling” Stasto, Golec-Biernat, Kwiecinski (2001) DIS Data

5. Peter SteinbergISMD2003 Saturation Phenomenology Q s controls low-x physics: applies universally! 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

6. Peter SteinbergISMD2003 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. Peter SteinbergISMD2003 m T scaling 2003 New PHENIX data Large weak decay corrections (~30-40%) at low p T Scaling factors ~20% Harder to claim unambiguous m T scaling Expected from radial expansion? Can we still make claim for geometrical scaling at RHIC? PHENIX data, nucl-ex/0307022

8. Peter SteinbergISMD2003 Centrality Dependence Many models can incorporate nuclear thickness “Two-component”: Hard + Soft “One-component”: CGC + DGLAP (Kharzeev & Nardi) Accardi & Gyulassy (2003) Maybe saturation scale too small @ y=0?

9. Peter SteinbergISMD2003 Forward Physics Geometrical scaling y – y beam ~ log(x) Q s 2 (y)~ Q o 2 exp( y) 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”~Q s

10. Peter SteinbergISMD2003 Pseudorapidity Distributions KLN: Final state from 2  1 gluon scattering Overall scale Jacobian Quark counting (LPHD) Energy, Rapidity, Centrality Kharzeev, Levin, Nardi (2001)

11. Peter SteinbergISMD2003 Au+Au vs. Elementary Systems Very different Q 2 e + e - ~ A+A despite different Q (Q s vs.  s)

12. Peter SteinbergISMD2003 Spectral Modification? Q s 2 should increase by exp( y)=e (.15*2.2) ~2 i.e. from 2 to 4 GeV 2 No obvious hardening of spectrum seen BRAHMS

13. Peter SteinbergISMD2003 Status of CGC @ RHIC At present, RHIC data does not seem to require saturation m T scaling may not hold Rapidity shapes may be universal, and not very sensitive to Q s Spectra do not harden at accessible  And yet, CGC is an intriguing approach Universality of strong interactions Coherence of low-x gluons  N part scaling

14. Peter SteinbergISMD2003 N part Scaling in d+A? Au+Au d+Au STAR Data (PAS Representation)

15. Peter SteinbergISMD2003 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. Peter SteinbergISMD2003 Shattering the CGC with p+A Predictions from Dumitru, Jalilian-Marian, etc. p+A collisions: Cleaner signal Two scales Calculations should be more reliable Systematics of may indicate saturation effects A. Dumitru, hep-ph/0210412

17. Peter SteinbergISMD2003  =  - y beam PHOBOS Au+Au 19.6 GeV 130 GeV 200 GeV Forward Physics is “Simple”! Scaling with x  Limiting Fragmentation:  ’ may be the relevant variable for the physics

18. Peter SteinbergISMD2003 Energy Scan KLN, =.3 Landau Hydro Parameter-free Landau Hydro (Cooper & Schonberg) Change RHIC energies and any rapidity will trend towards smaller  ’ or y’ Normalized here

19. Peter SteinbergISMD2003 Fixed Target RHIC Au+Au 100+100 RHIC 100+1 MRSFS In a RHIC fixed-target mode, BRAHMS FS & MRS flip roles!! Test y’/  ’ Universality y~0  y’~y b

20. Peter SteinbergISMD2003 Fixed Target Energy Scan 100 50 25 10 Scan fixed target energy to scan out to smaller rapidities relative to y b Of course, will eventually have to worry about remnant baryon density.

21. Peter SteinbergISMD2003 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  ’ or y’ in BRAHMS