1 V Latin American Symposium on Nuclear Physics Brasil, Setembro 2003 Edmundo García University of Illinois at Chicago for the PHOBOS collaboration Recent Results From the Phobos Experiment at RHIC

E. García UIC2 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 García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Stephen Gushue, Clive Halliwell, Joshua Hamblen, Adam Harrington, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, 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 Woźniak, Alan Wuosmaa, Bolek Wysłouch, 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

E. García UIC3 AGS:  s NN = 4.8 GeV SPS:  s NN = 17 GeV RHIC: pp, dA, AA  s NN : 20 to 200 GeV Relativistic Heavy Ion Collider LHC  s NN = 5.6 TeV

E. García UIC4 RHIC in nuclear matter phase diagram p/p K – /K + A+A central collisions Au + Au 200 GeV:   >5 GeV/fm 3 (0.5 GeV/fm 3 inside protons)    27 MeV  with  ~170 MeV dN ch /d  ||  |

E. García UIC5 SPECTRIG T0 mini-pCal pCal 4π Multiplicity Detector 2 Arm Multiparticle Spectrometer Trigger Detectors Zero Degree Calorimeters (PCAL ZDC) TOF AuAu Run dAu Run

E. García UIC6  dN/dEta dN ch /d h 19.6 GeV*130 GeV 200 GeV PHOBOS: Phys. Rev. Lett. 91,052303(2003) centrality Broad range of collision energy measured Large pseudorapidity coverage Comprehensive coverage in centrality Typical systematic errors AuAu Centrality 0 - 6% % – 25% – 35% – 45% – 55%

E. García UIC7 Centrality determination Multiplicity distribution Counts Glauber Calculation Hijing Full GEANT Simulation centrality bins AuAu dAu

E. García UIC8 Total charged multiplicity scaling with N part Shaded band is uncertainty on extrapolation procedure Errors include contributions from Nch and Npart scaling Open symbols are UA5 data at 200 GeV and results from an interpolation at lower energies PHOBOS: nucl-ex/

E. García UIC9 “Participant” Scaling N coll = # of NN collisions: ~N 4/3 L~N 1/3 N part /2 = # of participating nucleons: N “Collision” Scaling N part /2 = # of participating pairs of nucleons: = 1 N coll = # of NN collisions: = 1 Wang,Gulassy Phys. Rev. Lett 86(2001) Kharzeev,Nardi Phys. Lett. B 501(2001)

E. García UIC10 p T spectra for 200 GeV 0.2<y   <1.4 PHOBOS, nucl-ex/ accepted to Phys Lett B

E. García UIC11 PHOBOS, nucl-ex/ Similar centrality dependence at p T = 0.45 and 4.25 GeV/c Centrality dependence vs. p T Yield per participant pair divided by a fit to most peripheral bin as a function of the number of participants

E. García UIC12 Yield/ /p+p fit Yield per participant pair divided by a fit to the invariant cross-section for proton antiproton collisions (200 GeV UA1) p T (GeV/c) No increase in particle production above p T ~ 2 GeV/c for all centralities : Yield suppression Yield Relative to ppbar Initial State Coherence? Interaction in Dense Medium? PHOBOS, nucl-ex/

E. García UIC13 Is suppression an initial or final state effect? Initial state Final state How to discriminate? Turn off final state  d+Au collisions From STAR: BNL colloquium June 18 strong modification of Au wave function (gluon saturation) partonic energy loss in dense medium generated in collision

E. García UIC14 d+Au p T spectra PHOBOS d+Au: nucl-ex/ All syst. uncertainties: 90% C.L. Centrality Selection 0 – 20% – 40 % – 70 % – 100 %

E. García UIC15 PHOBOS d+Au: nucl-ex/ central Au+Au From Glauber (HIJING 1.383) 41mb (same as for Glauber) From UA1, using Pythia to go from |  | < 2.5 to 0.2 <  < 1.4 PHOBOS data Nuclear modification factor

E. García UIC16 Centrality dependence of R dAu For p T = 4 GeV/C: Data disfavor initial state interpretation of high-p T suppression Initial state parton saturation model predicts (hep-ph/ ) decrease of ~25 % from > 6 to central collisions PHOBOS d+Au: nucl-ex/ All syst. uncertainties: 15 – 20 at 90% C.L.

E. García UIC17 Conclusions Data disfavors the “initial state” parton saturation interpretation of high-p T hadron suppression Yield quenching of high p T in AuAu can not be accounted by initial state interactions that should be also present in dAu. The suppression of the inclusive yield observed in central Au+Au collisions is consistent with final-state interactions with a dense medium generated in such collisions. Similar evidence was corroborated by the other 3 experiments at RHIC BRAHMS: Strong suppression of high p T yield in central collisions PHENIX: Jet Suppression STAR: strong suppression of the inclusive yield and back-to-back correlations at high p T

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E. García UIC20 Backup transparencies from here

E. García UIC21 dN/d  Background subtraction on tracklets < ±5% Uncertainty on  due to model differences < 5%  Total contribution due to feed-down correction < 4% (typically 1%)  Total fraction lost due to stopping particles < 5%  Both are corrected via MC normalization Total uncertainty on dN/ d  is ±8%  N part  Loss of trigger efficiency at low-multiplicity <10%  Uncertainty on  N part  <1% Uncertainty in modeling paddle fluctuations  Uncertainty on  N part  <6% ( dN/d  /  N part  ) 130 / ( dN/d  /  N part  ) 56 Many uncertainties cancel in the ratio Systematic Uncertainties

E. García UIC22 Initially released energy density >5GeV/fm 3 Note: energy density inside proton ≈ 0. 5GeV/fm 3 Total energy released ~2000GeV Max. initial overlap volume

E. García UIC23 ZDC N ZDC P Au x z PP PN CN CP Trigge r Event Selection (average number of participants ) tt

E. García UIC24 Preliminary Transverse Momentum and Centrality dependence of particle ratios PHOBOS 200 Au+Au Errors bars are systematic uncertainties 10% most central

E. García UIC25 Particle Spectra – low p T 200GeV Au+Au PHOBOS y=-0.1 to 0.4, 15% most central  K p ± ± ± p T (GeV/c) (2  p T ) -1 d 2 N/dydp T Current systematic errors ~20%(  ) ~40%(K) ~50%(p) (Ullrich QM2002) Centralities: Brahms 10 % Phenix 5 % Star 5% Phobos 15% nucl-ex/ July 2003

E. García UIC26 atoms particles nucleus qgp energy/density Big quark-gluon p + n low mass nuclei neutral atom star dispersion of TODAY Bang plasma formation formation formation formation heavy elements time s s 3 min 400,000 yr 10 9 yr >10 9 yr 15x10 9 yr Copyright 1998 Contemporary Physics Education Project (CPEP) time Physics

E. García UIC27 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 …for each centrality bin