1. Elementary interactions and cold nuclear matter at RHIC Bjørn H. Samset Dr. student., UiO (In melting the nucleus, did we break some stained glass windows?)

2. RHIC Goal: To study and understand hot and dense nuclear matter. “In a 200AGeV Au+Au collision we produce 4630 ± 370 charged particles.” What does that mean? We need a reference: proton gold deuteron Elementary interactions and cold nuclear matter

3. Discovery in Au+Au: High-p t suppression “How many particles did we produce relative to the equivalent number of p+p collisions?” BRAHMS PRL 91 (2003)072305 Nuclear modification factors at midrapidity have been extensively studied by all four RHIC expermients.

4. Discovery in Au+Au: High-p t suppression BRAHMS PRL 91 (2003)072305 BRAHMS can deliver measurements of identified charged particle production over the range 0 < y < 3 (y b =5.3). This allows us to study e.g. the evolution of nuclear modification factors as we move away from the well studied midrapidity zone.

5. BRAHMS discovery: Forward suppression in R dA Relative to p+p, we see fewer particles at high pt at forward rapidities. Unexpected – what's the cause? One explanation: Nuclear shadowing. The nucleons at the front of the gold ion “shadow” the ones at the back, reducing the cross section for high-p t particle production. But there's also another theory: nucl-ex/0403005 To be publ. in PRL

6. D. Kharzeev et al. Phys.Rev.D68:094013,2003 y=0 y=3 ? Another possibility: Color Glass Condensate? Proton at low energies Proton at high energies Probe the proton with higher and higher energy... This saturated state, known as a Color Glass Condensate, may be the initial state in Au+Au collisions at RHIC. If so, it may allow for more controlled theoretical predictions for RHIC results. CGC prediction for R dAu :

7. Next step: Identified particles BRAHMS will deliver transverse momentum spectra for , K, p from p+p and d+Au, as a function of rapidity. Example of PID at high y and p: This will address the origin of forward suppression in d+Au, baryon stopping and strangeness production as a function of system size etc., and give a reference for Au+Au results where systematic errors cancel out.  K p K p 

8. What we can get from ID'd particles: p+p: Charged particle ratios Energy dependence Ratios vs. rapidity Limiting fragmentation Isospin conservation Baryon junctions Overall similarity to Au+Au nucl-ex/0409002, Subm. PLB

9. Currently addressing origin of suppression in d+Au: BRAHMS PRELIMINARY Suppression seems to follow mesons (pions), not baryons (protons). ● Interesting physics, possibly related to the CGC ● Relevant for understanding Au+Au collisions at full RHIC energy (QGP?) Consistent with midrapidity results reported by PHENIX. R. Debbe, BNL

10. In the near future: Identified particle spectra and assoc. results from d+Au: ● Hongyan Yang, PhD student, UiB ● Svein Lindal, Master student, UiO Identified particle spectra and assoc. results from p+p: ● Bjørn H. Samset, PhD student, UiO plus results from the other BRAHMS institutions in Denmark, Poland, Romania, France and U.S.A. (New York, Texas, Kansas). BRAHMS results have already affected the possible spacetime evolution of a heavy ion collision once. Stay tuned for more:

11. BRAHMS: Forward rapidities! BRAHMS can deliver measurements of identified charged particle production over the range 0 < y < 3 (y b =5.3). This allows us to study e.g. the evolution of nuclear modification factors as we move away from the well studied midrapidity zone.