PHOBOS Discoveries at RHIC

1. RBRC Workshop 14/15 May 2004 PHOBOS Discoveries at RHIC Wit Busza on behalf of the PHOBOS Collaboration

2. Collaboration (May 2004) Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley, Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis, Roman Holynski, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

3. To date, in Heavy Ion Collisions, there is no evidencefor the weakly interactingQGP, as naively imagined by a large segment of the community before RHIC turn-on,… • …however, • We have discovered a strongly interacting medium with extremely high energy density whose description in terms of simple hadronic degrees of freedom makes no sense; • Furthermore, we have discovered that much of the data can be expressed in terms of simple scaling rules which suggest the existence of strong global constraints.

4. …Key earlyPHOBOS observation…

5. Particle Density near Mid-Rapidity Models prior to RHIC PRL 85, 3100 (2000) PRL 88, 22302 (2002) PRL 91, 052303 (2003)

6. In Au+Au Collisions at sNN = 200 GeV • Maximum released energy is at mid-rapidity • In a system at rest with the center of mass • Energy/volume > 5 GeV/fm3 • It makes no sense to describe it in terms of simplehadronic degrees of freedom

7. …The high energy system is strongly interacting…

8. Evidence from flow: 200 GeV Au+Au PHOBOS preliminary Hit-based method Track-based method 200 GeV Au+Au PHOBOS preliminary 0 < h < 1.5 0-55% central, h+ + h- Nucl. Phys. A715 (2003) 611-614

9. PHOBOS Au+Au sNN=200 GeV 15% central -0.1< y <0.4 Evidence from the small number of particles produced with very low pT: In a large volume weakly interacting system you could expect the development of particles with long wavelength arXiv:nucl-ex/0401006

10. Evidence from the suppression of high-pT particles: PHOBOS dAu 200 GeV AuAu 200GeV PRL 91, 072302 (2003)

11. RAA as a function of sNN arXiv:nucl-ex/0405003

12. Discovery of simple scaling rules……in other words, discovery of global constraints

13. Energy dependence of particle production Universality of total multiplicity Limiting fragmentation

14. Note: In pp collisions, on average, approximately half the energy goes into the leading baryon Brenner et al arXiv:nucl-ex/0301017 A.Brenner et a., Phys.Rev.D26 (1982) 1497l

15. Energy dependence of particle production Universality of total multiplicity Limiting fragmentation

16. () PHOBOS Preliminaryv2200 Minimum Bias () PHOBOSv2130 h± () Nucl. Phys. A715 (2003) 611-614 () PRL 89, 222301 (2002) No boost invariant central plateau seen in v2 or dN/dy PRL 91, 052303 (2003)

17. E895 E895 E895 3.1 GeV Au+Au NA49 BRAHMS prel. NA49 3.6 GeVAu+Au 4.1 GeVAu+Au 200 GeVAu+Au 8.8 GeVPb+Pb 17.3 GeVPb+Pb No boost invariant central plateau seen in dN/dy PHOBOS 19.6 GeV 130 GeV 200 GeV AuAu  PRL 91, 052303 (2003) Prel. arXiv:nucl-ex/0403033 G.Roland, QM2004

18. 6% central dNch/dh ¢/<Npart>/2 Au+Au Universal limiting fragmentation In rest frame of one of the incident particles: arXiv:nucl-ex/0403033 PRL 91, 052303 (2003)

19. Elliptic flow: Phobos preliminary NA49 To see the limiting behavior, imagine that RHIC collided beams with asymmetric energy, withh´ = -2 corresponding to y = 0. PRL 91, 052303 (2003)

20. Centrality dependence

21. Npart scaling for: pA, KA, pA, dA, AA 10 GeV to 200 GeV Npart from 2 to 344 PHOBOS 200 GeV Phobos and E178 data arXiv:nucl-ex/0301017 E178: J.E.Elias et al., Phys.Rev.D22(1980) 13 arXiv:nucl-ex/0403033

22. 200 GeV 130 GeV 19.6 GeV Centrality Dependence at |h| < 1 Npart scaling for symmetric collisions: Phobos Phobos PRL 91, 052303 (2003) Nucl.Phys. A715 (2003) 65-74

23. Npart scaling for asymmetric collisions: arXiv:nucl-ex/0403033

24. 62.4 GeV 200 GeV arXiv:nucl-ex/0405003 PHOBOS

25. To date, in Heavy Ion Collisions, there is no evidencefor the weakly interactingQGP, as naively imagined by a large segment of the community before RHIC turn-on,… • …however, • We have discovered a strongly interacting medium with extremely high energy density whose description in terms of simple hadronic degrees of freedom makes no sense; • Furthermore, we have discovered that much of the data can be expressed in terms of simple scaling rules which suggest the existence of strong global constraints.