1. Results from PHOBOS at RHIC David Hofman University of Illinois at Chicago For the Collaboration European Physical Society HEP2005 International Europhysics Conference on High Energy Physics EPS (July 21st-27th 2005) in Lisbon, Portugal

2. HEP2005 2 What PHOBOS has studied so far… Relativistic Heavy Ion Collisions of – Au+Au √s NN = 19.6, 55.9, 62.4, 130.4 200.0 GeV – Coming soon: Cu+Cu (√s NN = 22.5, 62.4 and 200.0 GeV) “Benchmark” Collisions of – d+Au √s NN = 200.7 GeV –Coming soon: p+p (√s NN = 200, 410 GeV)

3. HEP2005 3 What PHOBOS has measured so far… Centrality dependence of primary charged particle production vs. –  (dN/d  out to |  <5.4) –  –  R (flow) – p T (0.2 – ~5 GeV/c) Identified charged particles –antiparticle/particle ratios –low p T yields of ,K,p (0.03 – 0.2 GeV/c) –Spectra for p T = 0.3 – ~4 GeV/c

4. HEP2005 4 What PHOBOS has observed so far… We have created a state of matter at RHIC with high energy-density, that is nearly net-baryon free and interacting very strongly. Transition to this high-energy state of matter does not create abrupt changes in observables at RHIC energies. The data exhibit many “simple” scaling behaviors. The data exhibit a remarkable factorization of collision energy and geometry. “White Paper”: nucl-ex/0410022, Nucl. Phys. A 757, 28

5. HEP2005 5 The Collaboration (June 2005) Burak Alver, Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Richard Bindel, Wit Busza (Spokesperson), Zhengwei Chai, Vasundhara Chetluru, Edmundo García, Tomasz Gburek, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Corey Reed, Eric Richardson, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Artur Szostak, Marguerite Belt Tonjes, Adam Trzupek, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Donald Willhelm, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Shaun Wyngaardt, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

6. HEP2005 6 The PHOBOS Detector (2005) NIM A 499 (2003) 603 TOF T0 Trigger Counter Triggering & Centrality Determination 4-  Multiplicity Detector Magnetic Spectrometer + ToF Spectrometer pCAL Paddle Trigger Counter SpecTrig ZDC Vertex Octagon Rings SpecCal

7. Properties of this “State of Matter”

8. HEP2005 8 High Energy Density & Approaching a Baryon-Free Environment PHOBOS nucl-ex/0405003 Energy Density Estimate: Largest Uncertainty is the Time to Equilibration. Conservative estimates put  eq ~ 2 fm/c &  RHIC > 3 GeV/fm 3  Energy Density ~6x energy density inside nucleons (20x for nuclei)   B ~ 27 MeV at full RHIC energy, a factor ~10x smaller than at SPS. Particle Ratios: Approaching Unity PHOBOS PRC 67, 021901R

9. HEP2005 9 Strongly Interacting Matter Elliptic flow reaches hydro limit Suppressed high-pT production No enhanced low-pT production PHOBOS nucl-ex/0407012 FLOW PHOBOS PRC 70, 051901(R) low-p T 0 0 0.5 1.0 1.5 R AA, R dAu Au+Au 0-6% d+Au 0-20% high-p T PHOBOS PRL 91, 072302 200 GeV Au+Au, 0-50% hydro 200 GeV Au+Au p T (GeV/c)

10. Gradual Transition to this “State of Matter”

11. HEP2005 11 Smooth Evolution of Mid-rapidity dN ch /d  EnergyCentrality 200 GeV (measured UA5) 19.6 GeV (interpolated ISR) 90 % C.L. PHOBOS nucl-ex/0410022 PHOBOS PRC 70, 021902(R) Au+Au data Au+Au & Pb+Pb data

12. HEP2005 12 Au+Au 0-40% central Smooth Evolution of Bulk Elliptic Flow I ENERGY  v2v2 v2v2 PHOBOS PRL 94, 122303

13. HEP2005 13 Smooth Evolution of Bulk Elliptic Flow II Centrality PHOBOS nucl-ex/0407012 Au+Au @ 200 GeV

14. HEP2005 14 Smooth R AA Evolution I PHOBOS PRL 94, 082304 ENERGY R AA 0 1 2 pTpT 62.4 GeV 200 GeV

15. HEP2005 15 Smooth R AA Evolution II CENTRALITY Red = 62.4 GeV Blue = 200 GeV PHOBOS PRL 94, 082304 & PLB 578, 297 line for illustration only

16. “Simple” Scaling Behaviors 1)Extended Longitudinal Scaling

17. HEP2005 17 p + p inel. dN/d  Reminder: Limiting Fragmentation in p+p Data: UA5 (Alner et al.), Z.Phys.C33, 1 (1986)  Ansatz: At high collision energy, d 2 N/dy’dp T and particle mix, reach a limiting value and become independent of energy around beam rapidity (i.e. in the fragmentation region). Ansatz: Benecke, Chou, Yang, Yen, Phys. Rev. 188, 2159 (1969) Shift to Rest Frame of Beam

18. HEP2005 18 dN/d  : Extended Longitudinal Scaling Au+Au PHOBOS PRL 91, 052303 & preliminary 62.4

19. HEP2005 19 v 2 flow: Extended Longitudinal Scaling Au+Au PHOBOS PRL 94, 122303

20. HEP2005 20 dN/d  : Extended Longitudinal Scaling d+Au dAu & pEmulsion per incident nucleon and approx. same N part PHOBOS nucl-ex/0409021 (accepted PRC-RC)

21. “Simple” Scaling Behaviors 1)Extended Longitudinal Scaling 2)The Importance of N part

22. HEP2005 22 N part Participants N coll Binary Collisions N part /2 ~ A L~A 1/3 N coll ~ A 4/3 Participants (N part ) and Collisions (N coll ) b

23. HEP2005 23 Collisions per Participant Pair Glauber Monte Carlo Au+Au

24. HEP2005 24 Extracting in Au+Au and d+Au PHOBOS arXiv:nucl-ex/0409021 (accepted PRC-RC) PHOBOS PRL 91, 052303  Utilize PHOBOS broad  coverage (and the observed extended longitudinal scaling) to extract

25. HEP2005 25 PHOBOS PRL 91, 052303 N part scaling for N ch in Au+Au  Total charged particle yield per participant pair is flat with centrality  Reduction at mid-rapidity balanced by increase at larger rapidity PHOBOS arXiv:nucl-ex/0301017

26. HEP2005 26 N part scaling for N ch in d+Au arXiv:nucl-ex/0409021 (accepted PRC-RC)

27. HEP2005 27 N part R AA at 62.4 and 200 GeV PHOBOS 62.4 GeV 200 GeV PHOBOS PRL 94, 082304 N coll (N coll scaled)  Yields normalized by N part are less centrality-dependent Au+Au N coll

28. “Simple” Scaling Behaviors 1)Extended Longitudinal Scaling 2)The Importance of N part 3)Universality of Total Charged Particle Production

29. HEP2005 29 N ch production in pp, dA, AA and ee PHOBOS nucl-ex/0301017 & nucl-ex/0410022  “Match” for d+Au & p+p e+e-e+e- 200 GeV 130 GeV 20 GeV  “Match” for Au+Au & e + e -

30. HEP2005 30 Universal Curve for N ch vs  (s) at RHIC Energies PHOBOS nucl-ex/0301017 & nucl-ex/0410022 When pp and dA “corrected” for energy loss to leading baryon Au+Au PHOBOS d+Au (@ √s/2)

31. Factorization of Collision Energy and Geometry

32. HEP2005 32 Factorization of Energy and Centrality Dependence for Mid-rapidity dN ch /d  PHOBOS PRC 70, 021902R PHOBOS PRC 65, 061901R Preliminary: 62.4 GeV Au+Au Collisions

33. HEP2005 33 Factorization: p T Dependence Centrality factorization appears to be p T independent Au+Au: Ratio 200/62.4 GeV lines for illustration only PHOBOS Data PRL 94, 082304 & PLB 578, 297

34. HEP2005 34 R AA Evolution with Centrality… Universal? PHOBOS 62.4 GeV 200 GeV N part N coll PHOBOS PRL 94, 082304

35. HEP2005 35 R AA Evolution with Centrality… Universal? PHOBOS 62.4 GeV 200 GeV N part Energy-independent Weak function of N part, p T Energy-dependent Strong function of √s, p T N coll PHOBOS PRL 94, 082304

36. HEP2005 36 What has PHOBOS learned so far? We have created a state of matter at RHIC with high energy-density, that is nearly net-baryon free, and that is strongly interacting. Transition to this high-energy state of matter does not create abrupt changes in observables at RHIC energies. The data exhibits “simple” scaling behaviors and factorizations that unite the data. → suggests strong global constraints and illustrates the importance of the collision geometry in the initial state and the very early evolution of the colliding system. Waiting for Cu+Cu results! → test these features in a smaller system.