# Presentation: 895549

## Rapidity Dependence of Charged Hadron Yields in Central Au+Au Collisions at 200 GeV

**Djamel Ouerdane**
**Niels Bohr Institute**
- for the **BRAHMS Collaboration**

**Quark Matter 2004**
**Oakland, January 11-18**

## The BRAHMS Collaboration

- I.G. Bearden7, D. Beavis1, C. Besliu10, Y. Blyakhman6, J.Brzychczyk4, B. Budick6,
- H. Bøggild7 , C. Chasman1, C. H. Christensen7, P. Christiansen7, J.Cibor4, R.Debbe1,
- E. Enger12, J. J. Gaardhøje7, M. Germinario7 , K. Grotowski4 , K. Hagel8, O. Hansen7,
- A.K. Holme12, H. Ito11, E. Jacobsen7, A. Jipa10, J. I. Jordre10, F. Jundt2, C.E.Jørgensen7,
- R. Karabowicz4 , T. Keutgen9, E. J. Kim5, T. Kozik3, T.M.Larsen12, J. H. Lee1, Y. K.Lee5,
- G. Løvhøjden2, Z. Majka3, A. Makeev8, B. McBreen1, M. Mikkelsen12, M. Murray8,
- J. Natowitz8, B.S.Nielsen7, K. Olchanski1, D. Ouerdane7, R.Planeta4, F. Rami2,
- D. Roehrich9,  B. H. Samset12, D. Sandberg7, S. J. Sanders11, R.A.Sheetz1, Z.Sosin3,
- P. Staszel7, T.S. Tveter12, F.Videbæk1, R. Wada8, A.Wieloch3 and I. S. Zgura10
- 1Brookhaven National Laboratory, USA, 2IReS and  Université Louis Pasteur, Strasbourg, France
- 3Jagiellonian University, Cracow, Poland, 4Institute of Nuclear Physics, Cracow, Poland
- 5Johns Hopkins University, Baltimore, USA, 6New York University, USA
- 7Niels Bohr Institute,  University of Copenhagen, Denmark
- 8Texas A&M University, College Station. USA, 9University of Bergen, Norway
- 10University of Bucharest, Romania, 11University of Kansas, Lawrence,USA
- 12 University of Oslo Norway

## Outline of the Talk

- The BRAHMS experiment
- Rapidity Dependence of Spectra and Yields
- From Stopping to Transparency
- Pions  : Landau or Bjorken ?
- Kaons : Is Strangeness Equilibrated ?
- Summary & Conclusions

## The BRAHMS Detector

- MRS

- FFS

- BFS

## Particle Identification

*Particle Identification*

*TIME-OF-FLIGHT*

- Particle Separation: pmax (2 cut)=

- TOFW

- TOF1

-  TOF2

-  / K

- 2 cut

- K / p

- 2 GeV/c

- 3 GeV/c

- 4.5 GeV/c

- 3.5GeV/c

- 5.5GeV/c

- 7.5GeV/c

## Particle Spectra 

- differential yields

**Kaon** invariant
- differential yields

- By combining all data sets and averaging over the number of collisions,
- we get the final invariant yields over a broad range of phase-space

*Particle Spectra *

**Proton** invariant
- differential yields

## Particle Spectra 

- Top 5% central collisions

## Rapidity Densities

- N()    ~ 1780   N(+)  ~1760
- N(K+)    ~   290   N(K) ~  240
- N(pbar) ~     85

*Rapidity Densities*

- At y ~ 0, dN/dy is
-        ~ 300 (300) for + (-)
-        ~   47   (44) for K+ (K)
-        ~   27   (20) for p (pbar)

- N() >> N(K) > N(p)
- N(+) = N()

- N(K+) > N(K)  and
- N(p) > N(pbar)  systematically

- Little decrease of transverse flow

## Nuclear Stopping

- BRAHMS, submitted
- to PRL, nucl-ex/0312023
- P. Christiansen
- Ph.D. Thesis

- Net-proton rapidity densities (top 5% central collisions)

## Nuclear Stopping

*Nuclear Stopping*

- Net-baryon after
- feed-down & neutron corrections

- Rapidity loss:
- (Npart= 357  10)

## Pion Production: Landau or Bjorken?

*Pion Production: Landau or Bjorken?*

- BRAHMS

- NA49

- AGS

- Entropy ~ Pion production
- Expansion ~ isentropic
- Pion gas = 3D relativistic fluid
- [L.D. Landau, Izv. Akad. Nauk SSSR 17 (1953)  52
-  P.Carruthers, M.Duong-van, PRD 8  (1973)  859]

## Strangeness with Kaons 

*Strangeness with Kaons *

*RAPIDITY DEPENDENCE*

## Strangeness with Kaons 

- At y = 0, ratios converge
- to ~ 15 %

*Strangeness with Kaons *

## Summary & Conclusions

- Transverse momentum spectra of ’s, K’s and p’s measured in rapidity range  |y| < 3.6, |yK| < 3.4 and |yp| < 3.1
- Stopping: dN/dy (p-pbar) ~ 7 at y = 0 => near transparency
-                 increases with increasing rapidity
-                 rapidity loss ~ 2 units,  scaling broken at 200 GeV
- Pions:      N(+) = N(-) over the rapidity range covered
-                 Landau picture close to data as a 1st approximation only
- Kaons:     N(+) ~ N(-) at mid-rapidity (47 and 44) but
-                 N(+) > N(-) at y > 2 due to associated K+ production
- K / :       converge to ~ 15% at y ~ 0 (plateau y < 1)
-                 same within systematic errors for full phase-space ratios
-                   possible indication of strangeness equilibration