University of Illinois at Chicago and Brookhaven National Laboratory

1. The Latest Results from PHOBOS @ RHIC Pseudorapidity Distribution of Charged Particlesin d + Au Collisions at 200 GeV University of Illinois at Chicago and Brookhaven National Laboratory for the Collaboration Seminar at BNL November 14, 2003 Rachid NOUICER

2. PHOBOSCollaboration (October 2003) 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, 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, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Michael Ricci, Christof Roland, Gunther Roland, Joe Sagerer, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Siarhei Vaurynovich, Robin Verdier, Gábor Veres, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch, Jinlong Zhang ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER 68 Participants; 8 Institutions; 3 Countries

3. Outline • PHOBOS multiplicity detector • Centrality determination and cross checks • Minimum-bias pseudorapidity distribution • Systematic errors • Comparison of d + Au to Au + Au and p + p systems • Comparison to the predictions of parton saturation model and microscopic models (HIJING, RQMD, AMPT) • Summary

4. PHOBOS Multiplicity Detector • 4p Multiplicity Array: • - Central Octagon Barrel : • - 6 Rings at higher Pseudorapidity : • Triggering: Scintillator counter arrays ZDC 1m Triggering “Scintillatorcounter arrays” RingCounters Octagon ZDC Sample Silicon pad sizes Octagon Detector: 2.7 x 8.8 mm2 Ring Counter: 20 –105 mm2

5. PHOBOS Capability in Charged Particle Multiplicity Analysis Event display of One collision event f Octagon region Rings Rings • Two analysis methods : • 1- Hit-Counting Analysis based on ratio of hit pads to empty pads using Poisson statistics • 2- Analog Analysis based on particle energy deposited in each pad

6. Extensive systematic Au + Au data Phys. Rev. Lett., 91, 052303 (2003) 200 GeV 19.6 GeV 130 GeV PHOBOS PHOBOS PHOBOS dN/dh Typical systematic band (90%C.L.) h h h • Phys. Rev. Lett. 85, 3100 (2000) • Phys. Rev. Lett. 87, 102303 (2001) • Phys. Rev. C 65 , 31901R (2002) • Phys. Rev. Lett. 88 , 22302 (2002) • Phys. Rev. C 65 , 061901R (2002) • Phys. Rev. Lett. 91, 052303 (2003) • nucl-ex/0301017, subm. to PRL • nucl-ex/0311009, subm. to PRL PHOBOS Multiplicity papers :

7. Parton Saturation Describes Au + Au Kharzeev & Levin, Phys. Lett. B523 (2001) 79 Au + Au at 130 GeV • We need a simpler system such as d + Au in order to understand a complex system Au + Au • The results of d+Au are crucial for testing the saturation approach

8. PHOBOS Capability in Charged Particle Multiplicity Analysis • The goal is to measure minimum-bias dNch/dh • Challenge is to correct for trigger and event selection bias • Measure the dNch/dh in narrow bins of centrality and integrate over centrality to produce a minimum-bias result • Cross check influence of auto-correlations

9. Centrality Determination ETot signal distributions Data and MC unbiased Efficiency distribution for ETot signal Overall trigger and vertex-finding efficiency is ~ 83 %

10. Centrality Determination Comparison of the signal distributions from Data and MC (HIJING) DATAmeasured cross section Normalize MC distribution with trigger and vertex bias Scale • Data and MC (biased) distributions match well • - Data cut = MC cut X scale factor Scaling factor =1.046 Details of centrality determination were presented in DNP talks: A. Iordanova and R. Hollis at UIC

11. Centrality Determination - Unbiased ETot signal distribution represents the full geometrical cross section - Slice this distribution into percentile bins - For each slice we extract dN/dh HIJING • Number of participants for minimum-bias is

12. Five Distinct Silicon Centrality Methods for Cross Checks 2) EOct method | h | < 3 3) EAuDir method h < -3 1) ETot method | h | < 5.4 EOct ETot EAuDir Centrality methods 5) ERing method 3 <|h | < 5.4 4) EdDir method h > 3 EdDir ERing

13. Minimum-Bias dN/dh Obtained from the Five Distinct Silicon Centrality Methods The distributions agree to within 5% PHOBOS DATA PHOBOS DATA

14. Final Minimum-Bias distribution obtained from Silicon Centrality Methods PHOBOS DATA • In the following, we will discuss the systematic errors

15. Systematic Errors These systematic errors are h-dependent and the major contributions are : • Analysis methods : Digital and Analog methods : 1-5% • Silicon centrality methods : 1-5% • HIJING/ feed-down corrections: 5-18% • Efficiency from a different Monte-Carlo simulation : 5% At h=0 the total systematic error is ~ 7%

16. Final Minimum-Bias Pseudorapidity Distribution in d + Au at 200 GeV nucl-ex/0311009 and Submitted to PRL • Charged particle pseudorapidity density near midrapidity is PHOBOS DATA • Integrated primary charged particle multiplicity in the measured region is

17. Missing charged particle multiplicity is Estimates of the Total Charged Particle Production Using AMPT Model Using Triple Gaussian fit • Upper limit including systematic errors : • Estimated total charged particle multiplicity is

18. Comparison of d + Au to Au + Au and p + p Systems at the Same Energy • Normalized to the number of participants / 2 • Compared to p + p collisions: • increase in particle production in the gold direction • reduction of particle production in the deuteron direction • The total integrated charged particle multiplicity normalized to the number of participant in d + Au and p + p is approximately the same: nucl-ex/0311009 and Submitted to PRL PHOBOS DATA d + Au and Au + Au

19. Comparison to Parton Saturation and RQMD Models nucl-ex/0311009 and Submitted to PRL • Parton saturation (KLN) and RQMD models are inconsistent with the data • KLN model overestimates the height of the gold side peak, underestimates its width, and predicts the peak at h ~ -3 rather than h= -1.9 as in data. Parton saturation model predictions for d + Au: D. Kharzeev et al., arXiv:hep-ph/0212316

20. Comparison to AMPT and HIJING Models nucl-ex/0311009 and submitted to PRL • The HIJING calculation • reproduces the deuteron side and the peak of the gold-side • fails to reproduce the tail in the gold direction (h < -2.5). • AMPT predictions • With & without final-state interactions fall close to the data. • FSI appear to broaden the gold-side peak, leading to moderate increase of the particle multiplicity in the region h < -3.5. AMPT predictions for d + Au : Zi-Wei Lin et al., arXiv:nucl-ph/0301025

21. Ratio of the Model predictions to data • Quantitative evaluation of the model predictions, expressed as the ratio of the model prediction to the data nucl-ex/0311009 and submitted to PRL

22. Summary • The dN/dh in d + Au collisions at 200 GeV has been measured • The distribution is broader than pp and peaked in the gold direction • The average pseudorapidity density is • The measured integrated charged particle multiplicity is • The total integrated charged particle multiplicity normalized to the number of participant in d + Au and p + p is approximately the same • Comparison to the predictions of microscopic models is made • The data disfavors the predictions of parton saturation model • The latest news from PHOBOS: More to come !