Rachid Nouicer1 University of Illinois at Chicago and Brookhaven National Laboratory for the Collaboration Seminar at BNL November 14, 2003 The Latest Results from RHIC Pseudorapidity Distribution of Charged Particles in d + Au Collisions at 200 GeV Rachid NOUICER

Rachid Nouicer2 68 Participants; 8 Institutions; 3 Countries PHOBOS Collaboration (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 LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

Rachid Nouicer3 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

Rachid Nouicer4 PHOBOS Multiplicity Detector 1m Triggering “Scintillator counter arrays” ZDC Ring Counters Octagon 4  Multiplicity Array: - Central Octagon Barrel : - 6 Rings at higher Pseudorapidity : Triggering: Scintillator counter arrays Sample Silicon pad sizes Octagon Detector: 2.7 x 8.8 mm 2 Ring Counter: 20 –105 mm 2

Rachid Nouicer5 PHOBOS Capability in Charged Particle Multiplicity Analysis Event display of One collision event Octagon region 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

Rachid Nouicer6 Extensive systematic Au + Au data dN/d    19.6 GeV130 GeV 200 GeV PHOBOS Typical systematic band (90%C.L.) Phys. Rev. Lett., 91, (2003) Phys. Rev. Lett. 85, 3100 (2000) Phys. Rev. Lett. 87, (2001) Phys. Rev. C 65, 31901R (2002) Phys. Rev. Lett. 88, (2002) Phys. Rev. C 65, R (2002) Phys. Rev. Lett. 91, (2003) nucl-ex/ , subm. to PRL nucl-ex/ , subm. to PRL PHOBOS Multiplicity papers :

Rachid Nouicer7 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

Rachid Nouicer8 The goal is to measure minimum-bias dN ch /d  Challenge is to correct for trigger and event selection bias Measure the dN ch /d  in narrow bins of centrality and integrate over centrality to produce a minimum-bias result Cross check influence of auto-correlations PHOBOS Capability in Charged Particle Multiplicity Analysis

Rachid Nouicer9 Centrality Determination ETot signal distributions Data and MC unbiased Efficiency distribution for ETot signal Overall trigger and vertex-finding efficiency is ~ 83 %

Rachid Nouicer10 Centrality Determination DATA measured cross section MC distribution with trigger and vertex bias - Data and MC (biased) distributions match well - Data cut = MC cut X scale factor Normalize Scale Comparison of the signal distributions from Data and MC (HIJING) Scaling factor =1.046 Details of centrality determination were presented in DNP talks: A. Iordanova and R. Hollis at UIC

Rachid Nouicer11 Centrality Determination - Unbiased ETot signal distribution represents the full geometrical cross section - Slice this distribution into percentile bins - For each slice we extract dN/d  HIJING Number of participants for minimum-bias is

Rachid Nouicer12 Five Distinct Silicon Centrality Methods for Cross Checks 1) ETot method |  | < 5.4 2) EOct method |  | < 3 3) EAuDir method  < -3 4) EdDir method  > 3 5) ERing method 3 <|  < 5.4 ETot EOct EAuDir EdDir ERing Centrality methods

Rachid Nouicer13 Minimum-Bias dN/d  Obtained from the Five Distinct Silicon Centrality Methods The distributions agree to within 5% PHOBOS DATA

Rachid Nouicer14 Final Minimum-Bias distribution obtained from Silicon Centrality Methods PHOBOS DATA In the following, we will discuss the systematic errors

Rachid Nouicer15 Systematic Errors 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% These systematic errors are  dependent and the major contributions are : At  the total systematic error is ~ 7%

Rachid Nouicer16 Final Minimum-Bias Pseudorapidity Distribution in d + Au at 200 GeV Charged particle pseudorapidity density near midrapidity is Integrated primary charged particle multiplicity in the measured region is nucl-ex/ and Submitted to PRL PHOBOS DATA

Rachid Nouicer17 Estimates of the Total Charged Particle Production Using Triple Gaussian fit Missing charged particle multiplicity is Using AMPT Model Upper limit including systematic errors : Estimated total charged particle multiplicity is

Rachid Nouicer18 Comparison of d + Au to Au + Au and p + p Systems at the Same Energy nucl-ex/ and Submitted to PRL 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: PHOBOS DATA d + Au and Au + Au

Rachid Nouicer19 Comparison to Parton Saturation and RQMD Models 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  ~ -3 rather than  = -1.9 as in data. nucl-ex/ and Submitted to PRL Parton saturation model predictions for d + Au: D. Kharzeev et al., arXiv:hep-ph/

Rachid Nouicer20 nucl-ex/ and submitted to PRL Comparison to AMPT and HIJING Models The HIJING calculation reproduces the deuteron side and the peak of the gold-side fails to reproduce the tail in the gold direction (  < -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  < AMPT predictions for d + Au : Zi-Wei Lin et al., arXiv:nucl-ph/

Rachid Nouicer21 Ratio of the Model predictions to data nucl-ex/ and submitted to PRL Quantitative evaluation of the model predictions, expressed as the ratio of the model prediction to the data

Rachid Nouicer22 Summary The dN/d  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 !