# Presentation: 285876

## Elliptic Flow in Au+Au Collisions at RHIC

- Carla M. Vale, MIT(*)
- for the Phobos Collaboration

*(*) now at Iowa State University*

## The Phobos 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 Garcia, 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,
- Siarhei Vaurynovich, Robin Verdier, Gábor Veres, Peter Walters, Edward Wenger,
- Frank Wolfs, Barbara Wosiek, Krzysztof Wožniak, Alan Wuosmaa, Bolek Wysłouch
- 	**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**

## Phobos Detector (Run 2001)

## Measuring Elliptic Flow in Phobos

** ****Large ******** coverage**
** ****Event-by-event**
** ****Uniform acceptance in ******
** ****Separated subevents**
** ****Does not require large event sample**

** ****Measures p****T ****dependence of v****2**
** ****Use of tracks reduces background effects**
** ****Minimal MC dependence**
** ****Subevents and tracks    widely separated in ******

**Hit-based Method(s)**

**Spectrometer Method**

## Hit-Based Method

- 

**Hit-Based Method**

- z

- 

- z

- 

- Ring

- Ring

- Octagon

- 1. Select events with vertices at negative z, to have symmetric detector in , 

- 2. Determine 2 using   subevents in -2 < < 2

- 3. Find v2 using hits with || < 5.3

## Track-Based Method

-  For each event, given its vertex position, select two equally sized regions, symmetric in  and 

-   coverage of the two subevents changes with vertex position; minimum interval between both is 4 units

## Tracking

-  of Spectrometer tracks is correlated with the reaction plane found in the Octagon to determine v2

-  Straight tracks (**1**): 0 < < 2 - used for v2(Npart) and v2() measurements
-  Curved tracks (**2**): 0 < < 1.5 - used for v2(pT) measurement

## -2 distributions vs. pT

- fitted with 2v2cos[2(-2)] function

******/2**

******/2**

- -2

## Results: v2(pT)

**Results: v****2****(p****T****)**

- 0 <  < 1.5

- nucl-ex/0407012

## Comparing to Other Results

- PHOBOS: 3-50% central, 0 <  < 1.5
- STAR: 5-53% central
- | | < 1.2

**PHOBOS 200GeV**
**STAR 130GeV RP**
**STAR 130GeV 2-cumulant**
**STAR 130GeV 4-cumulant**

## Both Methods: v2(NPart)

- nucl-ex/0407012

## Hit-Based v2()

**PRL 89, 222301 (2002)**

**nucl-ex/0407012**

## Track-based v2()

- Phobos
- 200 GeV Au+Au

## Both Methods: v2()

- Peripheral: 25-50%

## Both Methods: v2()

- Mid-central: 15-25%

## Both Methods: v2()

- Central: 3-15%

## Combined Result: v2()

-  dependence is seen already for 0 <  < 2, at least for peripheral events

- Phobos 200 GeV Au+Au

- nucl-ex/0407012

## Energy Dependence

- Phobos Au+Au
- 0-40% central

- ’=||-ybeam

- nucl-ex/0406021

## Comparison with Models I

- Phobos
- 200 GeV Au+Au

## Comparison to Models II

- Phobos 200 GeV Au+Au

## Conclusions

- v2(pT) is well described by hydrodynamic models up to
- 	pT ~ 1.5 GeV, for 0 <  < 1.5 and top 50% centrality; v2(NPart) is also well described by the same models, for the 20% most central events and 0 <  < 1.
- The  dependence of v2, sharp for high , starts already at mid-rapidity, at least for semi-peripheral collisions
- The shape of v2() appears to change only by a scale factor as a function of centrality, but a flattening of the shape for central events cannot be ruled out
- Non-flow effects can be reduced by using widely separated rapidity regions to determine the reaction plane and extract v2

## Outlook

- Latest 200 GeV Au+Au run at RHIC with higher statistics offers the possibility of expanding this analysis by adding particle identification
- New Au+Au data at 62.4 GeV available: new results already!
- Interesting issues to keep watching: theoretical modeling of v2(), improvement of methods to reduce non-flow effects - better quantification of these effects is needed