1. Measurement of the Centrality Dependence of Charged Particle Pseudorapidity Density with the PHOBOS Detector Michael Reuter University of Illinois at Chicago for the Collaboration APS Spring Meeting: Washington, DC April 29, 2001

2. April 29, 2001APS Meeting: Washington, DCPage 2 The PHOBOS Detector Ring Counters Paddle Counter Spectrometer TOF Octagon+Vertex : ANL, BNL, IFJ, MIT, NCU, UIC, UMD, UR Au+Au  s NN =130 GeV

3. April 29, 2001APS Meeting: Washington, DCPage 3 The BIG Picture Q: Entropy Production Thermal Equilibration Hadro-Chemistry Charged Particle Density C. Vale J12.004 Event Anisotropy - Flow Particle Ratios C. Henderson Q12.003 Energy Dependence (vs. AGS/SPS Data) System Size (p+p, N part Dependence) Context:

4. April 29, 2001APS Meeting: Washington, DCPage 4 Selecting Collisions Negative Paddles Positive Paddles ZDC NZDC P Au x z PP PN Coincidence between Paddle counters Paddle + ZDC timing reject background Sensitive to 97% of inelastic cross-section for Au+Au at  s NN = 130 GeV

5. April 29, 2001APS Meeting: Washington, DCPage 5 Determining Centrality N part HIJING + GEANT Glauber Calculation Model of Paddle Response Paddle signal (a.u.) Counts

6. April 29, 2001APS Meeting: Washington, DCPage 6 Estimating 97% when really 94% overestimates N part Uncertainty on N part % Error on N part N part Measurement sensitive to trigger bias –“Minimum-bias” still has bias –Affects most peripheral events Paddle signal (a.u.) Counts

7. April 29, 2001APS Meeting: Washington, DCPage 7 Detector Geometry

8. April 29, 2001APS Meeting: Washington, DCPage 8 Vertex Finding cm Vertex @ Z = -0.12 cm +z cm Vertex Resolution:  x ~ 450  m  y ~  z ~ 200  m Counts

9. April 29, 2001APS Meeting: Washington, DCPage 9 Tracklet Analysis Tracklets are two point tracks that are constrained by the event vertex. |  | < 0.04 |  | < 0.3 Vertex Spectrometer D =  2 +  2 D < 0.015  =  1 –  2  =  1 –  2 D

10. April 29, 2001APS Meeting: Washington, DCPage 10 Tracklet Systematic Error Reconstruction: Vertex selection, Tracklet algorithm etc. VTX: 1.8% SPEC: 3% Weak Decays: Mostly Ks and  VTX: 2% SPEC: <1% Background: Combinatorial,  -electrons VTX: 1.5% SPEC: 1% MC Generators: Different particle production, background etc. VTX: 5% SPEC: <<1% Total: VTX: 7.5% SPEC: 4.5%

11. April 29, 2001APS Meeting: Washington, DCPage 11 Measured dN ch /d  PHOBOS Preliminary CentralPeripheral

12. April 29, 2001APS Meeting: Washington, DCPage 12 Centrality Dependence of dN ch /d  dN ch /d  /(0.5 N part ) N part Yellow Band: Systematic Error

13. April 29, 2001APS Meeting: Washington, DCPage 13 Comparison to SPS General features (rapid rise/flat top) similar Note that WA98 dN ch /d  measured in lab frame Au+Au Pb+Pb

14. April 29, 2001APS Meeting: Washington, DCPage 14 Summary Measured centrality dependence of charge particle pseudorapidity density at |  | < 1 for Au+Au at  s NN = 130 GeV Charged particle density 40% larger than pp and 70% larger than at SPS Similar centrality dependent features as SPS Eagerly awaiting 200 GeV data! Species scan helps understanding of N part