Update on flow studies with PHOBOS

1. Update on flow studies with PHOBOS Flow Workshop BNL, November 2003 S. Manly University of Rochester Representing the PHOBOS collaboration

2. The Phobos Collaboration Birger Back,Mark Baker, Maarten Ballintijn, Donald Barton, Bruce Becker, Russell Betts, Abigail Bickley, Richard Bindel, Andrzej Budzanowski, Wit Busza (Spokesperson), Alan Carroll, Zhengwei Chai, 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 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 Wožniak, Alan Wuosmaa, Bolek Wysłouch, Jinlong Zhang ARGONNE NATIONAL LABORATORY BROOKHAVEN NATIONAL LABORATORY INSTITUTE OF NUCLEAR PHYSICS, KRAKOW MASSACHUSETTS INSTITUTE OF TECHNOLOGY NATIONAL CENTRAL UNIVERSITY, TAIWAN UNIVERSITY OF ILLINOIS AT CHICAGO UNIVERSITY OF MARYLAND UNIVERSITY OF ROCHESTER

3. Flow in PHOBOS

4. 5m 2m 5 4 3 2 1 0 1 2 3 4 5 1m h coverage for vtx at z=0 •  coverage • Data at 19.6, 130 and 200 GeV

5. Pixelized detector Hit saturation, grows with occupancy Sensitivity to flow reduced Can correct using analogue energy deposition –or- measure of occupied and unoccupied pads in local region assuming Poisson statistics Poisson occupancy correction

6. Poisson occupancy weighting

7. Octagonal detector Require circular symmetry for equal phase space per pixel Pixel’sazimuthalphase space coverage depends on location Acceptance (phase space) weighting Relative phase space weight in annular rings = <Nocc>-1

8. f z + Non-flow background • Non-flow Backgrounds flow signal Dilutes the flow signal • Remove Background • Estimate from MC and correct

9. Detector Beampipe dE (keV) cosh h Background! h Background suppression Demand energy deposition be consistent with angle Works well in Octagon Technique does not work in rings because angle of incidence is ~90

10. Vtx holes RingsN Octagon RingsP Spec holes f h

11. High Resolution Low Resolution Determining the collision point octagon hit density peaks at vertex z position extrapolate spectrometer tracks

12. Vtx holes Spec holes RingsN Octagon RingsP f h Detector symmetry issues where SPEC vertex efficiency highest Most data taken with trigger in place to enhance tracking efficiency

13. PHOBOS flow analyses based on subevent technique Poskanzer and Voloshin, Phys. Rev. C58 (1998) 1671. Azimuthal symmetry is critical Strategies: Hit-based analyses Track-based analysis: Avoids holes for reaction plane determination Uses tracks passing into spectrometer • Avoid the holes – Offset vtx method • Use the holes – Full acceptance method • Use a different type of analysis, such as cumulants

14. RingsN Octagon RingsP f h Limited vertex range along z Offset vtx method Subevents for reaction plane evaluation Technique used for published 130 GeV data • Good azimuthal symmetry • Fewer events, no 19.6 GeV data • Gap between subevents relatively small

15. RingsN Octagon RingsP Vertex range -10<z<10 f h Full acceptance method Subevents for reaction plane evaluation vary with analysis • Good statistics, 19.6 GeV data in hand • Gap between subevents large • Requires “hole filling”

16. Inner layer of vertex detector fills holes in top and bottom. Must map hits from Si with different pad pattern and radius onto a “virtual” octagon Si layer RingsN Octagon RingsP f h Dealing with the holes

17. RingsN Octagon RingsP f h Dealing with the holes Fill spectrometer holes by extrapolating hit density from adjoining detectors onto a virtual Si layer. (Actual spec layer 1 is much smaller than the hole in the octagon.)

18. RingsN Octagon RingsP Vertex range -8<z<10 f h Track-based method Subevents for reaction plane • Momentum analysis • 200 GeV data • Gap between tracks and subevents large • Little/no background

19. Reaction plane determined by hits in widely separated subevent regions, symmetric in ,  Vertex measurement Track-based method – detector space

20. Correlate tracks in spectrometer to reaction plane to determine v2 Track-based method – detector space

21. A question to this workshop: Are there non-flow correlations that stretch across 3-6 units of ? Full acceptance v1: sep=6 Full acceptance v2: sep=5.2 Offset vertex v2: sep=0.2-1.0 Track-based analysis h vz(cm)

22. Preliminary v2200 Final v2130 v2 vs. centrality and energy |h|<1 v2 200 130 PHOBOS Au-Au <Npart> 130 GeV result: PRL 89:222301, 2002

23. v2 vs.centrality, method comparison v2 |h|<1 v2200 (hit) v2200 (track) track hit PHOBOS Preliminary 200 GeV Au-Au <Npart>

24. v2 vs.pT 0<h<1.5 v2 PHOBOS preliminary h+ + h- 200 GeV Au-Au track-weighted centrality averaging (top 55%) 17% scale error

25. v2 vs. and energy <Npart>~190 v2 Preliminaryv2200 PHOBOS Au-Au Final v2130 200 130 Hit-based result v2200 & v2130 similar h 130 GeV result: PRL 89:222301, 2002

26. A. Poskanser showed in his talk that STAR agrees with the PHOBOS v2(). It will be interesting to see if it is possible to deconvolute the STAR and BRAHMS results in the forward region to determine what fraction of the drop in v2() comes from dN/dpT and what fraction comes from v2(pT).

27. Input flow Directed flow: MC analysis, resolution and background corrected, used event plane from 1st harmonic A little Quark Matter preview

28. Preliminary directed flow sensitivity PHOBOS preliminary h+ + h-Au-Au data A little Quark Matter preview

29. Flow at PHOBOS: What’s new? • 200 GeV analyses • Finalizing systematics • Plan to release soon final results in 3 bins of centrality • Directed flow (v1) • Still optimizing analysis and working to understand fine points of data analysis using full acceptance technique • Goal is to release preliminary v1() at 19.6, 130 and 200 GeV for Quark Matter