1. Results from the Experiment at RHIC Abigail Bickley University of Maryland For the Collaboration Topics in Heavy Ion Collisions June 25-28, 2003, Montreal

2. Collaboration (May 2003) Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts, Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll, Patrick Decowski, Edmundo Garcia, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell, Joshua Hamblen, Adam Harrington, Conor Henderson, David Hofman, Richard Hollis, Roman Holynski, Burt Holzman, Aneta Iordanova, Erik Johnson, Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Jang Woo Lee, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Aaron Noell, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Louis Remsberg, Christof Roland, Gunther Roland, Joe Sagerer, Pradeep Sarin, Pawel Sawicki, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Ray Teng, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Bernard Wadsworth, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch, Jinlong Zhang ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

3. PHOBOS Detector in 2002 (12m)

4. Broad Capabilities of the Phobos Spectrometer Low p T : PID –Stopping in Si (p T <0.2 GeV/c) –dE/dx versus p (p T <1GeV/c) Particle Ratios (Au+Au) High p T : Non-PID –Charged particle spectra Au+Au compared to d+Au

5. Particle Identification Based on dE/dx measurements in Si sensors Au+Au p k π

6. Particle Ratios: Energy Dependence p/p K – /K + p T = 0.07 – 1.0 GeV/c  PHOBOS 130 GeV PRL 87, 102301, 2001  PHOBOS 200 GeV PRC 67, 021901(R), 2003  At RHIC see a rapidly decreasing net baryon density near midrapidity. A+A central collisions K - /K +

7. Energy evolution of  B Using measured /, / and statistical model of F. Becattini et al., (PRC64,024901,2001) with T ch  160 – 170 MeV Estimated baryochemical potential at  s NN = 200 GeV:  B = 26  2 MeV Fit: P.Braun-Munzinger NP. A697, 902, 2002.

8. Centrality and p T dependence  Ratios are independent of p T and N part PHOBOS preliminary Au+Au 200 GeV 10% most central  p/p K – /K +   /  + PHOBOS preliminary  p/p K – /K +   /  +

9. Systematic Errors not shown Phobos Preliminary P T Distribution of Charged Particles Particle ratios dominated by particles with p T <1GeV/c; “Soft” part of spectrum What about high p T particle production? Look at the “Hard” part of the spectrum.

10. 65 ± 4 93 ± 5 138 ± 6 200 ± 8 276 ± 9 344 ± 12 45-50% 35-45% 25-35% 15-25% 6-15% 0-6% Centrality Transverse Momentum Distributions for Au+Au @ 200 GeV 0.2<y   <1.4 nucl-ex/0302015: Submitted to Phys Lett B

11. Ratio of Au+Au and pp Spectra at 200 GeV Observe a significant change in the spectral shape between pp and Au+Au collisions already for mid-peripheral events. Central Au+Au collisions show a strong violation of (expected) collision scaling at high p T. mid-peripheral central N part = 65 ± 4 N part = 344 ± 12 pp from UA1: Nucl.Phys.B335, (1990) 261.

12. Transverse Momentum Distributions for d+Au @ 200 GeV 0.2<  <1.4 Centrality Determination: Energy in Silicon Ring Counters Energy (arb. units ) Counts 3.0<|  |<5.4 Transverse Momentum Distributions 0 – 20% 20 – 40% 40 – 70% 70 – 100% 3.3 ± 1.0 6.7 ± 0.9 10.9 ± 0.9 15.5 ± 1.0 70-100% 40-70% 20-40% 0-20% Centrality nucl-ex/0306025: Submitted to PRL

13. pp from UA1: Nucl.Phys.B335, (1990) 261 Phobos d+Au Results PHOBOS d+Au: nucl-ex/0306025 central Au+Au

14. Spectrometer Results Low Pt –Au+Au particle ratios: show that a net baryon free region is being approached at RHIC High Pt –Au+Au spectra: central spectrum shows strong suppression relative to collision scaling –d+Au spectra: suppression observed in Au+Au spectra is not due to initial state effects