# Presentation: 686735

## Elliptic Flow Analysis at BRAHMS 

- Hironori Ito
- BNL

- Charged-particle anisotropy over pseudorapidity and transverse momentum ranges of 0<η<3 and 0<pt<3 in AuAu Collisions at √s_NN = 200 GeV

- Current status of BRAHMS elliptic flow analysis

## BRAHMS Collaboration

**I.G. Bearden****7****, D. Beavis****1****, C. Besliu****10****, Y. Blyakhman****6****, J.Brzychczyk****4****, B. Budick****6****,**
**H. Bøggild****7 ****, C. Chasman****1****, C. H. Christensen****7****, P. Christiansen****7****, J.Cibor****4****, R.Debbe****1****,**
**E. Enger****12****, J. J. Gaardhøje****7****, M. Germinario****7**** , K. Grotowski****4**** , K. Hagel****8****, O. Hansen****7****, **
**A.K. Holme****12****, ****H. Ito****1****, E. Jacobsen****7****, A. Jipa****10****, J. I. Jordre****10****, F. Jundt****2****,  ****E. Johnson****11****, C.E.Jørgensen****7****, R. Karabowicz****4**** , T. Keutgen****9****, E. J. Kim****5****, T. Kozik****3****, T.M.Larsen****12****, **
**J. H. Lee****1****, Y. K.Lee****5****,  G. Løvhøjden****2****, Z. Majka****3****, A. Makeev****8****, B. McBreen****1****,**
** ****M. Mikkelsen****12****, M. Murray****11****, J. Natowitz****8****, B.S.Nielsen****7****, K. Olchanski****1****, D. Ouerdane****7****,**
** ****R.Planeta****4****, F. Rami****2****, D. Roehrich****9****,  B. H. Samset****12****, D. Sandberg****7****, ****S. J. Sanders****11****,**
** ****R.A.Sheetz****1****, Z.Sosin****3****, P. Staszel****7****, T.S. Tveter****12****, F.Videbæk****1****, R. Wada****8****,**
** ****A.Wieloch****3 ****and I. S. Zgura****10**** **
**1****Brookhaven National Laboratory, USA, ****2****IReS and  Université Louis Pasteur, Strasbourg, France**
**3****Jagiellonian University, Cracow, Poland, ****4****Institute of Nuclear Physics, Cracow, Poland**
**5****Johns Hopkins University, Baltimore, USA,**** 6****New York University, USA**
**7****Niels Bohr Institute,  University of Copenhagen, Denmark**
**8****Texas A&M University, College Station. USA, ****9****University of Bergen, Norway **
**10****University of Bucharest, Romania, ****11****University of Kansas, Lawrence,USA   **
**12**** University of Oslo Norway **

## Size of created medium?

- y=0

- y=1

- y=2

- y=3

- Vast majority of data as well as models about elliptic flow concentrates on describing phenomena at midrapidity.
- That is great, but how about  longitudinal information?

## Elliptic flow at Midrapidity

- Phys. Rev. Lett. 86 (2001) 402

- STAR

- STAR

- At midrapidity, several measurements of elliptic flow have been reported at RHIC.
- Hydrodynamic models describe the data well up to ~ 2GeV/c

- PHENIX

- Phys. Rev. Lett. 89 (2002) 212301-1

## Not  at midrapidity

- PRC 65 (2002) 011901

- nucl-ex/0403050

- Phys. Rev. Lett. 88 202301 (2002)

- BRAHMS

- BRAHMS

- At forward and backward rapidities, understanding of v2 is not quite sufficient.
- The model seems to follow the pseudorapidity distribution.
- The data exhibits sharp triangular shape.

## BRAHMS Detector System

- Two rotatable spectrometers: MRS and FS
- Small solid angles

## Modification to the SMA detector

- Silicon has two rings

- Before

- After

- 6 segments per 2π

- 42 segments per 2π

- SMA is rotated 90 deg
- normal to the beam axis.

- Up to RUN03, BRAHMS had not been able to measure event planes.

## Idea to measure event plane and flow

- Measure event plane by global detectors: SMA and BBC
- Look at the correlation between the angle between observed event angle and particle angle measured in the tracking detectors (MRS and FS).

- Complication?
- They are small solid angle spectrometers.
- --- require large number of events.
- --- may require corrections due to the missing
- solid angles.

## Equations

- Reaction plane angle:

- Currently, the preliminary data suggests the event plane resolutions of ~ 27 deg with n = 2.

- Methods describe by
- A. M. Poskanzer and S. A. Voloshin
- Phys. Rev. C58 (1998) 1671

## Conclusion

- We can not show any results of elliptic flow at this time.
- Can we measure v2 at η=0, 1, 2 and 3?
- 	Do we have enough events?   Yes (Hopefully)
- 	Can we measure event plane?  Yes.
- Recently, it was pointed out that it is not necessary for flow measurement to know event planes.