# Presentation: 885376

## Rapidity Dependence of Pion Elliptic Flow

- Hironori Ito1
- Erik Johnson2
- Steve Sanders2

- BRAHMS Collaboration

- 1Brookhaven National Laboratory
- 2University of Kansas

## BRAHMS Collaboration

**I.Arsene, I.G. Bearden****7****, D. Beavis****1****, S.Bekele****11****, C. Besliu****10****, B. Budick****6****, **
**H. Bøggild****7****, C. Chasman****1****, C. H. Christensen****7****, P. Christiansen****7****, R.Clarke****10****, **
**R. Debbe****1****,J. J. Gaardhøje****,**** K. Hagel****8****, ****H. Ito****1****, A. Jipa****10****, J. I. Jordre****10****, F. Jundt****2, **** ****E.B.Johnson****11****, J.I.Jordre****9****, C.Jørgensen****7****, R. Karabowicz****3****, E. J. Kim****11****,**
** ****T.M. Larsen****7****, J. H. Lee****1****, Y. K.Lee****5****,  S.Lindal****12****, G. Løvhøjden****2****, Z. Majka****3****,**
** ****M. Murray****11****, J. Natowitz****8****, B.S.Nielsen****7****,  D.Ouerdane****7****, R.Planeta****4****, F. Rami****2****, C.Ristea****7****, O.Ristea****10****, D. Röhrich****9****,  B. H. Samset****12****, ****S. J. Sanders****11****, R.A.Sheetz****1****, P. Staszel****3****, T.S. Tveter****12****, F.Videbæk****1****, R. Wada****8****, H.Yang****9****, Z. Yin****9****,**** ****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 **

## BRAHMS Detector Setup

**Notes:**
BRAHMS consists of 2 rotatable spectrometers: MRS and FS.  MRS rotates from 30 to 90 and FS rotates from 2.3 to 30 deg , covering large pseudorapidity ranges.  

## Event Plane Detectors

- Flow Ring 2

- Si Ring 1

- Tile Ring 1

- Flow Ring 3

- Flow Ring 2 (silicon strips):
- 42 segments per 2π
- -2 < η < 0.2
- Flow Ring 3 (silicon strips):
- 42 segments per 2π
- -2 < η < 1.2
- Si Ring 1 (silicon strips):
- 6 segments per 2π
- -2.4 < η < 0.8
- Tile Ring 1 (scintillator tile):
- 6 segments per 2π
- -2 < η < -0.8

**Notes:**
There are several event plane detectors.  Flow silicon ring detectors, Si rings, tile, Beam-beam.
Flow Ring 2 and 3 are reoriented silicon strip detectors, which provides 42 segments.  Tile and standard slicon have 6 sides and the Beam-beam counters have 8 segments.
By selecting vertex in wide range, their pseudorapidity coverage is different.

## Introduction to Flow

- Initial spatial anisotropy...

- ...final momentum anisotropy

- ...after
- rescattering
- leads to ...

- Elliptic Flow: n = 2

**Notes:**
Initial spatial anisotropy of collision.  There are large number of rescattering involved.  It translates to final momentum anisotropy.
In flow analysis, a cross section  are expanded by Fourier series.  In this expansion, coefficient of the series with n=2 corresponds to elliptic flow.

## Rapidity Dependence?Current Status

**Notes:**
At midrapidity, v2(pt) has been measured for several different particles species and centralities.  
How about data at forward rapdiity?  STAR and PHOBOS have measured integrated flow at wide range of pseudorapidities.  It has strong pseudorapidity dependence.
STAR has also measured v2(pt) at forward rapdiites.  It has large pt dependence.

## Method

- a,b and c are the Scintillator Tile, the Silicon Strip  and the Beam-Beam counters

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

- Nth order event plane

- Observed v2

- Event plane resolution correction

- Real v2

**Notes:**
We use the persedure described by Poskanzer and voloshin.  At first, it obtains event plane by using charged multiplicity.
2nd, using the obtained event plane, a particle track are reoriented against this angle.  Mean of cosine of this angle for many events are v2 observed.  
3rd,  using correlation, the correction due to event plane resolution is obtained.
4th, the final observed v2 are obtained.

## Particle Identification at Midrapidity

**Time-of-Flight  Identification**
-  Pions: 2.5 GeV/c
-  Protons:	4 GeV/c
-  Kaons: 	2 GeV/c

**Notes:**
At midrapiditiy, Time-of-flight is used for particle identification.   Pion, kaon and protons are clearly separated.

## Particle Identification at Forward Rapidity

**RICH identification**
-  Pions:28 GeV/c
-  Protons:  35 GeV/c
-  Kaons: 28 GeV/c

**Notes:**
At forward rapidity.  RICH detectors are used for particle identification.  

## Charged Hadron V2 at Midrapidity

- Charged Hadron V2 at Midrapidity

- Strong pT dependence is seen at midrapidity.

**Notes:**
As other experiments report, large pt dependence of charged hadron v2 is seen at midrapidity.
And, it is consistent with STAR.

## Pion V2 at Midrapidity

**Notes:**
For identified particle, pion v2 also shows the large v2 dependence.

## Pion V2 at Forward Rapidity

-  Pion
- Error is statistical only

- η = 3.2

- Pion V2 at Forward Rapidity

- 10-20%

**Notes:**
How about at the forward rapidity?  As you can see, the dependence of pt is still very large. 
Comparing with v2 at midrapidity, it is seen that both v2 are very similar.

## Rapidity Dependence

- η = 1

- η = 3.2

- η = 0

- Small η dependence found.

- Charged hadrons

**Notes:**
Going back to charged hadron v2, again, the pseudorapidity dependence is very similar.  
3D Hydro calculation also shows very small dependence on pseudorapidity.

## pT Integrated v2

- pT Integrated v2

- Integrated v2 are derived  using the measured spectra and v2(pT).   (See a poster by C. Ristea.)
- Integrated v2 are also relatively flat.
- <η>		<pT> GeV/c
-    0			0.55
-    1			0.53
-    2. h+	 	0.506
-        h-	 	0.423

**Notes:**
Using our preliminary v2, we can obtain the fit to this curve.  
Also, from the preliminary spectra, we fit with power low.  Keep in mind that our spectra does not extend back to 0.  It stops at around 300MeV/c.  
The systematic error consists of error in correction factor, which is common to all pseudorapidity, and systematic uncertainty in spectra.
It is interesting to note that v2(pt) of 3D hydro was equal to higher than our data at low pt.  However, its integrated v2(pt) has more pseudorapditiy dependence.
Two things are important: fit (or extrapolation) at low pt range as well as how much mean pt falls as you go larger pseudorapidity. 

## Centrality Dependence

- Large centrality dependence is seen for pT integrated v2 — consistant with PHOBOS results.

- Centrality dependence at midrapidity

**Notes:**
And, lastly, the centrality dependece of pt integrated v2 is shown.
It is consistent with phobos result.

## Conclusion

-  v2(pT) for charged hadrons and pions have been measured at midrapidity as well as at forward rapidity.
-  v2(pT) shows small pseudorapidity dependence—consistent with 3D Hydro model.
-  η dependence of pT integrated v2 is strongly sensitive to measured spectra.
-  Strong centrality dependence observed.
**Proton and Kaon results are forthcoming. **