1. Centrality Dependence of Charged Antiparticle to Particle Ratios Near Mid-Rapidity in d+Au Collisions at √s NN = 200 GeV Abigail Bickley Univ. of Maryland, Chemistry Dept. for the Collaboration September 22, 2003

2. Collaboration Birger Back, Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts, Abigail Bickley, Richard Bindel, 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, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Inkyu Park, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Joe Sagerer, Pradeep Sarin, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Robin Verdier, Gábor Veres, Frank Wolfs, Barbara Wosiek, Krzysztof Wozniak, Alan Wuosmaa, Bolek Wyslouch, 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

3. Outline I.Motivation II.Specifics of the detector Changes implemented for Run 2003 Spectrometer acceptance III.Analysis details Particle identification Corrections & systematic errors Centrality determination IV.Final ratios vs Centrality d+Au and Au+Au ratios comparison V.Model comparison VI.Implications of results

4. Motivation In d+Au collisions little reinteraction is expected thus the ratios should reflect the initially produced yields Recent results suggest conditions in Au+Au collisions are different than those observed in d+Au collisions Do these different conditions influence the measured particle ratios? Initial StateFinal State Interactions

5. Centrality Dependence The more collisions a participating nucleon suffers the greater the baryon number transport to mid-rapidity PeripheralCentral d+Au:Au+Au: N coll = # collisions, N part = # participants, N d part = # deuteron participants

6.  p  /p Ratio vs Centrality If the total proton yield is equal to the sum of the transported and produced components  Expect the  p/p  ratio to decrease with increasing number of collisions; i.e with centrality

7. PHOBOS Detector 2003 SPECTRIG mini-pCal pCal i.New T0 Cerenkov detectors ii.DAQ upgrade (x10) iii.Forward proton calorimeters iv.Moved TOF walls back v.New on-line high p T Spectrometer Trigger TOF Walls Rings T0 Paddle Spectrometer

8. Proximity to interaction point reduces secondaries and feed- down products Particle tracking expanded to include outer spectrometer wing  results in extended acceptance z -x 10 cm y 70 cm Reversible 2T magnetic field Two symmetric spectrometer arms  Two independent measurements Acceptance and efficiency cancel

9. Magnetic Field Reversals Bending toward beampipe: h - B -, h + B + near mid-rapidity Z Field Polarity: B -  - -  + +  - -  + + near mid-rapidity Z Field Polarity: B +  + +  + +  - -  - - Bending away from beampipe: h - B +, h + B -

10. Particle Identification Cut bands lie 3 RMS deviations from the expected mean Cutoffs minimize contamination from other particle species

11. Spectrometer Acceptance Au+Au Bend away from beampipeBend towards beampipe Contours represent where the acceptance has fallen to 10% of the maximal value d+Au Bend towards beampipeBend away from beampipe Acceptance extended to higher p T and lower rapidity range

12. Raw Ratios: Acceptance Corrected Assumptions: the following must be the same for antiparticles and particles for each bending direction and centrality bin –Acceptance and tracking efficiency Field strength (B+/B-) : agree within 0.2% Centrality Fractions (E Ring ) : agree within 1% –Kinematic distributions  p T ,  p T 2  and  y  : agree within 2%

13. Corrections Protons: –Absorption  3.5% ± 1.4% (syst.) –Secondary  1.6% ± 0.3% (stat.) ± 0.2% (syst.) –Feed-down  -0.5% ± 1% (syst.) Kaons: total correction <1% Pions: total correction <0.5%

14. Corrections Counts dca of track to beam orbit (cm) cut position Primaries Secondaries All Particles HIJING:

15. Systematic Uncertainties Kaon and Proton Ratios: –Centrality Measure: 2% –Kinematic Acceptance (p T and y): 1% Proton Ratios: –Dead & hot spectrometer channels: 0.5% –Spectrometer arm asymmetries: 1% –Polarity-dependent vertex correction: 1% Pion Ratios: –Electron Contamination: < 0.1%

16. Trigger Elements Normal Trigger: (dAVertex) –30M events collected –required T0 coincidence and |vz|<50cm Peripheral Trigger: (dAPeriph) –20M events collected –required dAVertex and Paddle occupancy <50% Negative Paddles Positive Paddles dAu x z PP PNT0NT0P Positive T0s Negative T0s

17. Energy (arb. units) Pseudorapidity Glauber Calculation Hijing 1.383 Hulthen w.f.  inelastic = 41mb Full GEANT Simulation HIJING Simulation dN/d  Trigger% Centrality  N coll  dAVertex60-1002.9(1.7)2.2(1.3)0.20 dAVertex30-607.0(3.0)4.0(1.8)0.61 dAVertex10-3012(3.6)6.1(1.8)0.78 dAVertex0-1016(4.0)8.1(2.0)0.84 dAPeriph60-1002.8(1.7)2.2(1.3)0.18 dAPeriph30-606.2(2.7)3.7(1.6)0.24 Measuring Centrality in d+Au

18. Particle Ratios vs Centrality - Any final state effects in Au+Au collisions do not modify the produced meson yields Au+Au proton ratio is significantly lower than d+Au ratios All d+Au particle ratios appear to be independent of centrality  -  /  + ,  K -  /  K +  d+Au ~  -  /  + ,  K -  /  K +  Au+Au

19. Baryon Transport d+Au vs Au+Au comparison: – central d+Au > central Au+Au BUT !! –(p/p) central d+Au > (p/p) central Au+Au Relative fraction of transported protons in central d+Au collisions is half that in central Au+Au collisions! 

20. Model Comparison Models agree with the expectation that baryon transport increases with increasing thus resulting in a decreased  p/p  ratio Data does not exhibit this behavior d+Au

21. Conclusions d+Au and Au+Au  and K ratios are consistent  any final state interactions in Au+Au collisions do not modify the ratio of initially produced meson yields Relative fraction of transported protons in central d+Au half that in central Au+Au collisions  may be evidence of collective behavior in Au+Au collisions that affects baryons d+Au particle-antiparticle ratios show a surprising lack of centrality dependence Results are in clear disagreement with AMPT, HIJING and RQMD predictions nucl-ex/0309013

22. Backup Slides

23. Raw Particle Yields TriggerCentrality % Evts (M) -- ++ K-K- K+K+ ppEvt (M) -- ++ K-K- K+K+ pp dAVertex60-1001.02787138791635373105561.1153203087524157543421 dAVertex30-602.914963749418242732160434943.181368161642930101129982282 dAVertex10-302.4196639575511133660219845582.6103027206413780123738242903 dAVertex0-101.313628643578032497154930771.46966414303267586426491958 dAPeriph60-1003.0825540530455141686618413.2429318797150946115641085 dAPeriph30-603.013666657997372353146131823.37154214435244982127072009 dAVertex: 30M evts; required T0 coincidence and |vz|<50cm dAPeriph: 20M evts; required dAVertex and Paddle occupancy <50%

24. Final Particle Ratios TriggerCentrality %  -  /  +  K -  /  K + p/pp/p dAVertex60-1002.2 (1.3)0.995  0.015  0.017 0.97  0.07  0.03 0.84  0.04  0.04 dAVertex30-604.0 (1.8)1.004  0.007  0.017 0.95  0.03  0.03 0.80  0.02  0.03 dAVertex10-306.1 (1.8)1.008  0.006  0.017 0.97  0.02  0.03 0.83  0.02  0.03 dAVertex0-108.1 (2.0)1.016  0.007  0.017 0.97  0.03  0.03 0.86  0.02  0.03 dAPeriph60-1002.2 (1.3)0.996  0.008  0.017 1.02  0.04  0.04 0.86  0.03  0.03 dAPeriph30-603.7 (1.6)1.014  0.007  0.017 0.97  0.03  0.04 0.82  0.02  0.03 Au+Au0-12~5.21.025  0.006  0.018 0.95  0.03  0.03 0.73  0.02  0.03 Superscript = statistical uncertainty; Subscript = systematic uncertainty