Aneta Iordanova University of Illinois at Chicago For the collaboration

1. Collision Geometry Scaling of Mid-Rapidity Charged Particle Multiplicity in PHOBOS from √sNN = 19.6 to 200 GeV Aneta Iordanova University of Illinois at Chicago For the collaboration DNP04/Chicago

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 (October 2004) Burak Alver, Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Russell Betts, Abigail Bickley, Richard Bindel, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, Vasundhara Chetluru, Patrick Decowski, Edmundo García, Tomasz Gburek, Nigel George, Kristjan Gulbrandsen, Clive Halliwell, Joshua Hamblen, Ian Harnarine, Conor Henderson, David Hofman, Richard Hollis, Roman Hołyński, Burt Holzman, Aneta Iordanova, Jay Kane, Nazim Khan, Piotr Kulinich, Chia Ming Kuo, Wei Li, Willis Lin, Steven Manly, Alice Mignerey, Gerrit van Nieuwenhuizen, Rachid Nouicer, Andrzej Olszewski, Robert Pak, Heinz Pernegger, Corey Reed, 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, Sergei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger, Frank Wolfs, Barbara Wosiek, Krzysztof Woźniak, Alan Wuosmaa, Bolek Wysłouch ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS PAN, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

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

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

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

8. dh < 0.1 Tracklet Reconstruction First Pass • |df| = |fSearch–fSeed| < 0.3 • |dh| =|hSearch–hSeed| < 0.1 • smallestdh combination. fSearch ,hSearch Extrapolate fSeed ,hSeed hit Search Layer hit Seed Layer Top Vertex Reconstructed Vertex Second Pass • Tracklets with a common hit • in the “Search Layer” • smallest |dh| combination.

9. Combinatorial Background Acceptance + Efficiency Correction Factor Multiplicity Determination • formed by rotating Inner Vertex Detector layers 1800 about the beam pipe • corrects for: • azimuthal acceptance of the detector • tracklet reconstruction efficiency • secondary decays

10. Results

11. 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)

12. 200 GeV (measured UA5) 19.6 GeV (interpolated ISR) Measured pseudorapidity density per participant pair as a function of <Npart> • ‘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 90 % C.L.

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

14. 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-s.

15. Ratio of the two data sets –systematic and statistical errors • Final 1-s systematic and statistical error • Centrality dependent • central events 3% • mid-central events 7%

16. Ratio for the data sets • Data ratio • Au+Au1 (fixed fraction of cross-section) 1-s errors

17. 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-s errors

18. 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 <Npart>) • No centrality dependence 1-s errors

19. Ratio for the data sets • Models • Hijing • increase in mid-rapidity with centrality • Saturation Model (KLN) • flat centrality dependence as in data 1-s errors Phys. Rev. C70, 021902(R)(2004)

20. Other ‘Geometry Scaling’ observations in • Multiplicity • 200/130 GeV mid-rapidity ratio Phys.Rev.C65 061901(R) (2002) • 19.6-200GeV Nch/<Npart/2> • Plot from QM 2002 talks

21. Charged hadron pT spectra Ratio of yield for200 and 62.4 GeV is centrality independent for all measured pT bins Other ‘Geometry Scaling’ observations in

22. Conclusions • We measured charged-particle pseudorapidity density at mid-rapidity for Au-Au collisions at 200 and 19.6GeV • Centrality, derived from different h-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