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BRAHMSBRAHMSProgress & PerspectiveProgress & Perspective

F.VidebækF.VidebækForFor

The BRAHMSThe BRAHMS collaborationcollaboration

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The BRAHMS CollaborationThe BRAHMS Collaboration- 53 people from 12 institutions-

I.Arsene10, I.G. Bearden7, D. Beavis1, C. Besliu10, B. Budick6,H. Bøggild7 ,

C. Chasman1, C. H. Christensen7, P. Christiansen7,J.Cibor4, R.Debbe1,E.Enger,J. J. Gaardhøje7,M. Germinario7 , K. Hagel8, O. Hansen7, H. Ito1, A. Jipa10, J. I. Jordre10, F. Jundt2,

C.E.Jørgensen7, R.Karabowisz3, E. J. Kim11,, T. Kozik3, T.M.Larsen12, J. H. Lee1, Y. K.Lee5, S.Lindal, G.Lystad,G. Løvhøjden2, Z. Majka3, A. Makeev8, M.Mikelsen, M. Murray11, J. Natowitz8,

B. Neuman11,B.S.Nielsen7, D. Ouerdane7, R.Planeta3, F. Rami2, C.Ristea10, O.Ristea10,D. Roehrich9, B. H. Samset12, D.Sandberg, S. J. Sanders11,, R.A.Sheetz1,

P. Staszel3, T.S. Tveter12, F.Videbæk1, R. Wada8, Z. Yin9 and I. S. Zgura10

1Brookhaven National Laboratory, USA, 2IReS and Université Louis Pasteur, Strasbourg, France3Jagiellonian University, Cracow, Poland, 4Institute of Nuclear Physics, Cracow, Poland

5Johns Hopkins University, Baltimore, USA, 6New York University, USA7Niels Bohr Institute, Blegdamsvej 17, 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

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Brahms Physics GoalsBrahms Physics GoalsProbing and characterizing Hot and Dense Nuclear Matter by

studying:Particle ProductionReaction Mechanisms and DynamicsBaryon StoppingHard Processes (high pt spectra)------------------------------------------------------------------------

Through High Precision Measurementsof Identified Hadrons over wide range of

Rapidity: 0 < y < 4(Central and Fragmentation regions)

Transverse momentum: 0.2 < pt < 4 GeV/c

BRAHMS measurement capabilities (PID and momentum ) at large y are unique in the RHIC Program. The PID capability at y~0,1 is at par or better than other exp.

Significant progress on the base program is achieved with the Au-Au data from RUN-2, in particular RUN-4 Au-Au, the d-Au, and pp data from Run-3

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Multiplicity Multiplicity measurementsmeasurements

0-5%

5-10%

10-20

At mid-rapidity dN/dh in Au-Au is significantly enhanced compared to pp. The increase with beam energy is modest.In the Bjorken scenario

with t~ 1 fm/c => ε>5 GeV/fm3

The present charged-particlepseudorapidity density data can be reproduced by the gluon saturation model.

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Gaussians in pz:

∑±

±− 2

2

2))sinh((

exppz

zN pymσ

=yδ 2.03 ± 0.16

Rapidity loss: ∫ −−=−= py BB

partpp dy

dydN

yN

yyyy0

)(2δ

Total ∆E=25.7±2.1TeV

-HI collisions are transparent at RHIC-The finite baryon number at y~0 is important for QCD description of baryon number transport

Nuclear StoppingNuclear Stopping

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Meson rapidity distributionsMeson rapidity distributions

No wide No wide ““plateauplateau”” observed in rapidity observed in rapidity for identified mesons. Close to afor identified mesons. Close to aGaussianGaussian shape (shape (σσ((ππ+) =2.35 ~ +) =2.35 ~ σσ(k+) (k+) =2.39) for all produced particles=2.39) for all produced particles

The RMS of The RMS of ππ distributions from low distributions from low energy to RHIC is close to prediction of energy to RHIC is close to prediction of Landau Hydro model (Landau Hydro model (CarruthersCarruthers)

Data from AGS, SPS, RHICData from AGS, SPS, RHIC

The agreement with this The agreement with this Landau hydro picture vs. Landau hydro picture vs. energy is excellent and energy is excellent and striking.striking.)

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Y < 1 : consistent with Hadron Gas Stat. ModelK+/π+ : 15.6 ± 0.1 % (stat)K−/π− : 14.7 ± 0.1 % (stat)[Phys. Lett. B 518 (2001) 41]

Divergence at higher y :Associated K+ productionNo single source with unique T and µB

RAPIDITY DEPENDENCE

BRAHMS, PRL90 (2003) 102301T~constant, µB varies with y

Increasing y

Strangeness with Strangeness with KaonsKaons

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Bulk propertiesBulk properties

The estimated energy density from particle The estimated energy density from particle production and reaction times is large enough to production and reaction times is large enough to create energy densities at least ~ 5 GeV/fm**3.create energy densities at least ~ 5 GeV/fm**3.The longitudinal expansion is, surprisingly, The longitudinal expansion is, surprisingly, consistent with the Landau picture that also consistent with the Landau picture that also relies on short formation time and a relies on short formation time and a hydrodynamic expansion of the matter formed.hydrodynamic expansion of the matter formed.Composition of particle production (Composition of particle production (ππ,K,p) is ,K,p) is determined from essentially the determined from essentially the µµBB , with an , with an overall rapidity independent freezeoverall rapidity independent freeze--out out temperature. Such analysis does not necessarily temperature. Such analysis does not necessarily prove that equilibration has been reached.prove that equilibration has been reached.

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d+Au Nuclear Modification d+Au Nuclear Modification ηη =0=0

High pT enhancement observed in d+Au collisionsat √sNN=200 GeV consistent with Cronin effect.

Comparing Au+Au to d+Au⇒ strong effect of

dense nuclear medium

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Suppression at Lower EnergiesSuppression at Lower Energies(preliminary data from 63 GeV)(preliminary data from 63 GeV)

Rcp Yield (0-10)% vs (40-60%) scaled by mean number of binary collisions.

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Au+Au d+Au p+p

pbar/π- ratio probes extent of dense

medium in y

P/π lower at higher rapidities in AA while suppression persists.

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HighHigh--ptpt

The highThe high--pt suppression has been firmly pt suppression has been firmly established at RHIC AA collisionsestablished at RHIC AA collisions–– Suppression of charged particle production at Suppression of charged particle production at

y~0 and 2.y~0 and 2.–– Pions are suppressed at large rapidity where Pions are suppressed at large rapidity where

the the dNdN//dydy is ~ 2/3 of that at midis ~ 2/3 of that at mid--rapidity.rapidity.–– The onset of suppression is smooth with The onset of suppression is smooth with

energy; present at 63, 130 and 200 GeV.energy; present at 63, 130 and 200 GeV.–– Interplay between preInterplay between pre--hadronic (hadronic (gluonic gluonic

degrees of freedom) and hadronic degrees of freedom) and hadronic absorption/reabsorption/re--scattering in hot system has to scattering in hot system has to be quantified.be quantified.

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RRdAudAuCroninCronin like like enhancement enhancement at at ηη=0.=0.

Clear Clear suppression suppression as as ηη changes changes up to 3.2up to 3.2

Same ratio Same ratio made with made with dn/ddn/dηηfollows the follows the low plow pTT RRdAudAu

Centrality dependence reversed at large η.

RdA = 1 d2Nd+Au/dpTdη<Ncoll> d2Npp

inel/dpTdη

where << Ncoll> = 7.2> = 7.2±0.3

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Initial StateInitial Statethe color glass condensate ?the color glass condensate ?

Initial StateInitial State

–– The particle production is slowly growing The particle production is slowly growing consistently with both the parton saturation consistently with both the parton saturation models, and the slow logarithmic growth in models, and the slow logarithmic growth in the pp multiplicities.the pp multiplicities.

–– The forward suppression The forward suppression RRcpcp, , RRdada in din d--Au Au collisions shows a reaction picture consistent collisions shows a reaction picture consistent with the parton saturation in the Auwith the parton saturation in the Au--wawe wawe function.function.

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SummarySummary

The BRAHMS results on rapidity dependence of The BRAHMS results on rapidity dependence of multiplicities, particle production have shed multiplicities, particle production have shed important light on:important light on:Energy densitiesEnergy densitiesHighHigh--ppt t suppression (2suppression (2--4 GeV/c) persist to large 4 GeV/c) persist to large y.y.Initial state in AA and Initial state in AA and dA dA can be described by can be described by parton saturation (CGC), albeit not uniquelyparton saturation (CGC), albeit not uniquely

The analysis of the large data set from RunThe analysis of the large data set from Run--4 and 4 and the nearthe near--term run with lighter specie(s) will give term run with lighter specie(s) will give much more detailed information of the properties much more detailed information of the properties of the hot and dense matter created in HI of the hot and dense matter created in HI collisions.collisions.