1. Rachid Nouicer1 The Latest Results from PHOBOS @ RHIC Rachid NOUICER University of Illinois at Chicago and Brookhaven National Laboratory for the Collaboration International Europhysics Conference on High Energy Physics July 17, 2003 Evidence of final-state suppression of high-p T hadrons in Au + Au collisions using d + Au measurements

2. Rachid Nouicer2 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 LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER 68 Participants; 8 Institutions; 3 Countries PHOBOS Collaboration (May 2003)

3. Rachid Nouicer3 Hard-scattered Partons are built-in QCD Probes within RHI Collisions Parton t =- few fm/c Partons within initial nuclei Parton t = 0 fm/c Hard scattering between partons t = + few more fm/c Hadronization “fragmentation” Hadrons “Leading” Hadron t = + few fm/c Scattered partons traverse created hot QCD medium A main goal of relativistic heavy ion physics is to investigate high-temperature, high-density QCD, by creating and then studying the highly-excited medium produced in high-energy nuclear collisions. Detector One method of diagnosing a QCD medium is to look for any modifications of the probes due to the medium. Nucleus We measure high-p T leading hadrons. The basic thing we look for: If scattered partons lose energy, the number of leading hadrons at high-p T will be depleted (suppressed). This is the main goal of this talk Central Collision

4. Rachid Nouicer4 The RHIC/AGS Accelerator Complex

5. Rachid Nouicer5 PHOBOS Detector for Au+Au at 200 GeV 1m Triggering ZDC Octagon 135000 Silicon Pad channels 4  Multiplicity Array Two Mid-rapidity Spectrometers TOF wall for High-Momentum PID Triggering: Scintillator Paddles, Zero Degree Calorimeter Spectrometer TOF Ring Counters

6. Rachid Nouicer6 Triggering on Collisions & Centrality Coincidence between Paddle counters at  t = 0 defines a valid collision Paddle + ZDC timing reject background PP Negative Paddles Positive Paddles Au x z PN Positive ZDC Negative ZDC Negative Cerenkov Positive Cerenkov Au CentralPeripheral HIJING +GEANT Glauber calculation Model of paddle trigger Data Data+MC

7. Rachid Nouicer7 Definition of N coll and N part Central Collision Peripheral Collision

8. Rachid Nouicer8 Bulk Tail Systematic Errors not shown p T Distribution of Charged Particles Au + Au at 200 GeV PHOBOS p T coverage p T = 0.030 – 5 GeV/c AuAu Can we use this tiny tail to probe the QCD medium ?

9. Rachid Nouicer9 780 0.2<y   <1.4 Data: PHOBOS, nucl-ex/0302015 Submitted to Phys. Lett. B Transverse Momentum Distributions vs Centrality Au+Au at 200 GeV 45-50% 35-45% 25-35% 15-25% 6-15% 0-6% Centrality Centrality range: from 3 to 10 fm from 6 to 3 N part 276 ± 9 N Coll 1050 300 500 200 ± 8 138 ± 6 175 93 ± 5 107 65 ± 4 syst. uncertainties for N coll : 10-15 %. 344 ± 12

10. Rachid Nouicer10 Relative to UA1 p+p Data: PHOBOS, nucl-ex/0302015 Submitted to Phys. Lett. B Ratio of Au+Au and p+p Spectra at 200 GeV High-p T suppression mid-peripheral N part = 65 ± 4 Is this a signature of the final state “jet quenching“ ?  ~ 3  ~ 6 central N part = 344 ± 12 We observe a significant change in the spectral shape between p+p 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.

11. Rachid Nouicer11 Is final state “jet quenching“ the only explanation ? Au + Au at 200 GeV PHOBOS, nucl-ex/0302015 Particle production scales approximately with Npart at high-p T Similar centrality dependence at p T = 0.5 and 4 GeV/c ! Rachid Nouicer Expectation for Ncoll-scaling Data: PHOBOS, nucl-ex/0302015 Submitted to Phys. Lett. B Saturation model : Kharzeev, Levin, McLerran hep -ph/0210332 Particle Yield/ Initial state parton saturation works qualitatively too … Expectation for Ncoll-scaling Saturation model prediction

12. Rachid Nouicer12 Conclusions of part I Strong final state suppression at high-p T ? –Indication of high density, strongly interacting matter! Strong initial state suppression persisting to high-p T ? –Indication of multipartonic effects in the nuclear wavefunction! We need a simpler system such as d + Au in order to understand a complex system Au + Au RHIC Accelerator response : no problem! The observed high-p T suppression suggest two possibilities:

13. Rachid Nouicer13 Predictions for d+Au Vitev, nucl-th/0302002, Phys.Lett.B in press Vitev and M.Gyulassy, Phys.Rev.Lett. 89 (2002) Central Peripheral Kharzeev, Levin, McLerran, hep-ph/021332 “~30% suppression of high-p T particles” (central vs peripheral) 16% increase central vs peripheral Nuclear Modification Factor R dAu Parton Saturation (initial state) pQCD (final state)

14. Rachid Nouicer14 This Year: PHOBOS Detector 2003 d+Au at 200 GeV  Moved TOF walls back ~ 5 m from interaction point  Installed new spectrometer trigger detector that selects on high p T tracks  Installed new “time-zero” (T0) Cerenkov detectors to provide triggering and on-line vertexing as well as a start time for our TOF walls.  Proton calorimeter on Gold and Deuteron “going” sides for dA run SPECTRIG T0 mini-pCal pCal  DAQ upgrade (x10)

15. Rachid Nouicer15 Centrality Determination in d+Au at 200 GeV Multiplicity distribution Pseudorapidity Glauber Calculation Hijing 1.383 Hulthen w.f. 41mb inelastic NN cross-section Full GEANT Simulation HIJING Simulation dN/d  Counts Centrality cuts (4 bins) 3.0<|  |<5.4 0.2<  <1.4

16. Rachid Nouicer16 Invariant Yields for Charged Hadrons vs p T d+Au at 200 GeV Systematic uncertainties : - tracking efficiency : 5-10 % - malfunctioning channels : 5 % Deuteron direction Data: PHOBOS, nucl-ex/0306025 Accepted in Phys. Rev. Lett. (2003)

17. Rachid Nouicer17 central Au+Au All syst. uncertainties: 90% C.L. Nuclear Modification factor (R dAu ) vs p T The observation and the absence of high-p T suppression in Au+Au and d+Au respectively, can be an indication of creation of highly interacting medium in Au+Au. Clear evidence of absence of high-p T suppression in d + Au Compare to p+p reference… From Glauber (HIJING 1.383) 41mb (same as for Glauber) From UA1, using Pythia to go from |  | < 2.5 to 0.2 <  < 1.4 Data: PHOBOS, nucl-ex/0306025 Accepted in Phys. Rev. Lett. (2003)

18. Rachid Nouicer18 Data disfavor initial state interpretation of Au+Au high-p T suppression syst. uncertainties: 15-20 %@ 90 C.L. Nuclear Modification Factor (R dAu ) vs Ncoll Parton saturation predicts : “ ~30% suppression of high-p T particles” (central vs peripheral) Data: PHOBOS, nucl-ex/0306025 Accepted in Phys. Rev. Lett. (2003)

19. Rachid Nouicer19 Summary Clear Evidence of high-p T suppression in more central collisions of Au + Au at 200 GeV Clear Evidence of absence of high-p T suppression in more central collisions of d + Au at 200 GeV Slight enhancement at high-p T (p T = 4 GeV/c) of nuclear modification factor of dAu vs Ncoll The latest news from PHOBOS: –We have compared central Au+Au to central d+Au. –This data strongly disfavors the “initial state” parton saturation interpretation of high-p T hadron suppression. More to come !