1. Measuring Mid-Rapidity Multiplicity in PHOBOS Aneta Iordanova University of Illinois at Chicago For the collaboration

2. Outline Multiplicity Analysis Technique –Vertex Tracklet reconstruction method Results –Mid-rapidity charged-particle multiplicity and its centrality dependence for 19.6 and 200GeV –Compare the results with model predictions Conclusions

3. Collaboration 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, Clive Halliwell, Joshua Hamblen, Adam Harrington, Michael Hauer, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, 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, Christof Roland, Gunther Roland, Joe Sagerer, Helen Seals, Iouri Sedykh, Wojtek Skulski, Chadd Smith, Maciej Stankiewicz, Peter Steinberg, George Stephans, Andrei Sukhanov, Marguerite Belt Tonjes, Adam Trzupek, Carla Vale, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch ARGONNE NATIONAL LABORATORYBROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOWMASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWANUNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLANDUNIVERSITY OF ROCHESTER

5. Multiplicity measurement at mid-rapidity (|  |<1)

6. Vertex Detector Top Bottom 62.1mm 50.4mm Z,  Beam pipe  1 channel Y X 8192 silicon channels Outer Layer: 2 × 2048 channels, 0.47mm × 24.1mm Inner Layer: 2 × 2048 channels, 0.47mm × 12.0mm

7. Tracklet Reconstruction Tracklet Two-hit combinations from Outer and Inner Vertex (Top or Bottom), pointing to the reconstructed vertex. Reconstructed vertex –from Spectrometer Detector 19.6GeV  x,y,z =0.3,0.3,0.4 mm (central)  x,y,z =0.6,0.5,0.8 mm (mid-central)

8. First Pass Second Pass  Seed Layer Search Layer Reconstructed Vertex hit  Search,  Search Extrapolate  Seed,  Seed |  | = |  Search –  Seed | < 0.3 |  | =|  Search –  Seed | < 0.1 smallest  combination. Tracklets with a common hit in the “Search Layer” smallest  combination. Top Vertex Tracklet Reconstruction

9. Acceptance + Efficiency Correction Factor Combinatorial Background Multiplicity Determination

10. Acceptance and Efficiency Correction Factor   depends on: –Z-vertex position –multiplicity in detector (hits)  Hits in Outer Vertex Layer / 20 19.6 GeV 200 GeV  corrects for: –azimuthal acceptance of the detector –tracklet reconstruction efficiency –secondary decays

11. Combinatorial Background Correction Factor  Combinatorial background: –formed by rotating Inner Vertex Detector layers 180 0 about the beam pipe Z,  Beam pipe 

12. Combinatorial Background Correction Factor  Tracklets/Background for 80 to 100 Hits in Outer Vertex Layer, 19.6 GeV  = N bg_tracklets /N reconstructed –  =0.76 Data MC Counts 

13. Results

14. Centrality Determination Select the “same” regions at 200 and 19.6 GeV Have two centrality methods at each energy –One at mid-rapidity –One away from mid-rapidity Mechanism for comparing ‘like’ regions to see systematic effects Results presented here –for a) and c) Regions are ‘matched’ according to the ratio of beam rapidities (a) with (c) (b) with (d)

15. Measured pseudorapidity density per participant pair as a function of ‘Geometry-normalized’ multiplicity in Au-Au collisions higher than corresponding values for inelastic Percentile cross-section –0-50% for 200 GeV –0-40% for 19.6 GeV 200 GeV (measured UA5) 19.6 GeV (interpolated ISR) 90 % C.L.

16. Models predictions –Hijing does not follow data trend –Saturation Model (KLN) Phys.Lett.B523 79 (2001) arXiv:hep-ph/0111315 better agreement 90 % C.L. Measured pseudorapidity density per participant pair as a function of

17. Ratio of the two data sets – systematic errors Most of the systematic errors on the individual measurements at the two energies will cancel in the ratio –Analyses performed with the same method –Detector –Centrality determination Percentile cross-section used in ratio – top 40% Errors are estimated as 1- .

18. Ratio of the two data sets – systematic errors R  –Most of geometry/efficiency effects cancel in the ratio –Contribution from secondary decays R  –  is found to be the same for Data/MC for the two data sets –Uncertainty from measured y- beam position R Npart –Nucleon-nucleon inelastic cross-section –MC simulations of the detector response –Glauber model calculations RR R  19.6 R  200 R Npart 2%0.4%

19. Ratio of the two data sets – systematic and statistical errors R Nrec –Counting statistics –Uncertainty in trigger efficiency (centrality bin position) central events 0% mid-central events 6% Final 1-  systematic and statistical error –Centrality dependent central events 3% mid-central events 7% RR R  19.6 R  200 R Npart R Nrec R central 2%0.4% 2.2%3.0%

20. Ratio for the data sets Data ratio –Au+Au1 (fixed fraction of cross-section) 1-  errors

21. Ratio for the data sets Data ratio –Au+Au1 (fixed fraction of cross-section) No centrality (geometry) dependence R = 2.03 ± 0.02 ± 0.05 (simple scale-factor between 19.6 and 200GeV) 1-  errors

22. Ratio for the data sets Data ratio –Au+Au1 (fixed fraction of cross-section) No centrality dependence R = 2.03 ± 0.02 ± 0.05 –Au+Au2 (fixed ) No centrality dependence 1-  errors

23. Ratio for the data sets Models –Hijing increase in mid-rapidity with centrality –Saturation Model (KLN) flat centrality dependence as in data 1-  errors http://xxx.lanl.gov/nucl-ex/0405027

24. Other ‘Geometry Scaling’ observations in Multiplicity –200/130 GeV mid-rapidity ratio Phys.Rev.C65 061901(R) (2002) –19.6-200GeV N ch / Plot from QM 2002 talks

25. Charged hadron p T spectra –Ratio of yield for 200 and 62.4 GeV is centrality independent for all measured p T bins Other ‘Geometry Scaling’ observations in

26. Conclusions We measured charged-particle pseudorapidity density at mid-rapidity for Au-Au collisions at 200 and 19.6GeV –Centrality, derived from different  -regions for each of the two Au-Au collision energies, yield consistent results –An increase in particle production per participant pair for Au-Au compared to the corresponding values for collisions –The ratio of the measured yields for the top 40% in the cross section gives a simple scaling factor between the two energies