Charged Particle FLOW measurement for | h |<5.3 with the PHOBOS detector

1. Charged Particle FLOW measurement for |h|<5.3 with the PHOBOS detector Inkyu Park (Univ. of Rochester) for the PHOBOS Collaboration

2. PHOBOS Collaboration ARGONNE NATIONAL LABORATORY Birger Back, Nigel George, Alan Wuosmaa BROOKHAVEN NATIONAL LABORATORY Mark Baker, Donald Barton, Alan Carroll, Stephen Gushue, George Heintzelman, Robert Pak, Louis Remsberg, Peter Steinberg, Andrei Sukhanov INSTITUTE OF NUCLEAR PHYSICS, KRAKOWAndrzej Budzanowski, Roman Holynski, Jerzy Michalowski, Andrzej Olszewski, Pawel Sawicki, Marek Stodulski, Adam Trzupek, Barbara Wosiek, Krzysztof Wozniak MASSACHUSETTS INSTITUTE OF TECHNOLOGYWit Busza*, Patrick Decowski, Kristjan Gulbrandsen, Conor Henderson, Jay Kane, Judith Katzy, Piotr Kulinich, Johannes Muelmenstaedt, Heinz Pernegger, Corey Reed, Christof Roland, Gunther Roland, Leslie Rosenberg, Pradeep Sarin, Stephen Steadman, George Stephans, Gerrit van Nieuwenhuizen, Carla Vale, Robin Verdier, Bernard Wadsworth, Bolek Wyslouch NATIONAL CENTRAL UNIVERSITY, TAIWANWillis Lin, Jawluen Tang UNIVERSITY OF ROCHESTERJoshua Hamblen,Erik Johnson, Nazim Khan, Steven Manly, Inkyu Park, Wojtek Skulski, Ray Teng, Frank Wolfs UNIVERSITY OF ILLINOIS AT CHICAGORussell Betts, Clive Halliwell, David Hofman, Burt Holzman, Wojtek Kucewicz, Don McLeod, Rachid Nouicer, Michael Reuter UNIVERSITY OF MARYLANDRichard Bindel, Edmundo Garcia-Solis, Alice Mignerey *spokesperson Flow@PHOBOS - Inkyu Park

3. Physics Goal of RHIC & Flow b (reaction plane) Initial state anisotropy Flow Equation of state Flow strength Degree of thermalization dN/d(f -YR ) = N0 (1 + 2V1cos (f-YR) + 2V2cos (2(f-YR) + ... ) Directed flow Elliptic flow In-plane OR Out-of-plane affect other physics: HBT, Spectra, etc Flow@PHOBOS - Inkyu Park

4. PHOBOS Detector setup 2000 Paddle Trigger Counter TOF Spectrometer Ring Counters Octagon+Vertex See Robert Pak’s talk Flow@PHOBOS - Inkyu Park

5. x z Paddle counters : Trigger & Centrality Positive Paddles Negative Paddles ZDC N ZDC P Au Au PN PP Npart Data MC Paddle Signal Paddle Signal See Judith Katzy’s talk Flow@PHOBOS - Inkyu Park

6. Spectrometer : Vertex Reconstruction form 3D vertex z Flow@PHOBOS - Inkyu Park

7. Octagon and Ring detectors • |h| < 5.3 (Dh = 0.05-0.1), 0f2p (Df = 2p/32 -2p/64) 5.0m 1.1m 2.3m octagon Ring counter Interaction Point -1.1m -5.0m -2.3m Flow@PHOBOS - Inkyu Park

8. Hit Definition Charged particle deposit energy in pad hit event vertex 1 hit = pad with energy > 60 keV Particle direction Octagon Ring 60keV 60keV Energy deposit (keV) Energy deposit (keV) Flow@PHOBOS - Inkyu Park

9. Event Selection Rings N vertex available Rings P f -56cm -14cm z Octagon • To cover pseudo-rapidity -2.0 to 2.0, only events with vertex -38cm to -30 cm are used • Rings will cover 3.0 < |h| < 5.3 • 13K events are used finally for the analysis Flow@PHOBOS - Inkyu Park

10. Centrality Bins Triggered Accepted Normalized Paddle Signal Acceptance affected by various strict vertex cuts Flow@PHOBOS - Inkyu Park

11. Event Plane Reconstruction Yn = tan-1 ( Xn / Yn ) / n (Xn,Yn)=( S w cos(nf), S w sin(nf) ) Yn Yn Xn f(pixel number) Z (pixel number) w is weight to compensate for detector related azimuthal asymmetries (inverse of hit density) Flow@PHOBOS - Inkyu Park

12. Particle distribution w.r.t. Event Plane DN / Nµ V 2 Flow@PHOBOS - Inkyu Park

13. Flow Analysis* (Subevent correlation) • If we know the reaction plane perfectly: Vn = < cos (n(f-YR)) > -2.0 < h < -0.1 0.1 < h < 2.0 Yna Ynb RingN RingP SubE (a) SubE (b) • In real experiment, YR is unknown: use Yn • Vnobs = < cos (n(f-Yn)) > • <cos(n(Yna,b-YR))> = ( <cos(n(Yna- Ynb))> )1/2 • Finally, correct for event plane resolution • Vn= Vnobs / < cos (n(Yn -YR)) > * Phys. Rev. C 58, 1671 A. M. Poskanzer, S. A. Voloshin Flow@PHOBOS - Inkyu Park

14. Subevent Plane Correlation Normalized Paddle Signal Flow@PHOBOS - Inkyu Park

15. Occupancy Correction V2raw = < cos (2(fhit-Y2)) > With our hit counting method, high occupancy reduces flow signal Occupancy = fraction of hit pads MC V2corr =V2raw / (1 -Occ) Independent of the magnitude of flow Centrality bin V2 = V2corr / ( < cos (2(Y2a- Y2b)) > )1/2 Flow@PHOBOS - Inkyu Park

16. Centrality Dependence V2 PHOBOS Preliminary Centrality bin midrapidity : |h| < 1.0 Flow@PHOBOS - Inkyu Park

17. Centrality Dependence midrapidity : |h| < 1.0 V2 Hydrodynamic model Preliminary SPS AGS Normalized Paddle Signal Errors are statistical only (systematic errors ~ 0.007) (STAR : Normalized Nch ) Flow@PHOBOS - Inkyu Park

18. Pseudorapidity dependence of V2 V2 Averaged over centrality PHOBOS Preliminary h Errors are statistical only (systematic errors ~ 0.007) Flow@PHOBOS - Inkyu Park

19. Pseudorapidity dependence of V2 V2 PHOBOS Preliminary STAR (PRL) SPS NA49 (QM99) Pion (b<11fm) h rapidity PHOBOS Errors are statistical only (systematic errors ~ 0.007) STAR : averaged over their centrality Flow@PHOBOS - Inkyu Park

20. Summary • Elliptic Flow at midrapidity reaches 6-7% in peripheral collisions, and drops in central collisions • Elliptic Flow is a strong function of pseudorapidity • Indication of sensitivity to V1 (we are studying…) Thank you! Flow@PHOBOS - Inkyu Park