Results from the BRAHMS Experiment at Results from the BRAHMS Experiment at RHICRHIC

F.Rami* for the BRAHMS Collaboration* Institut de Recherches Subatomiques

and Université Louis Pasteur, Strasbourg

Introduction The BRAHMS Experiment Main Physics Results Global features and event characterization Charged particle multiplicity distributions dNch/d vs. Centrality and SNN

Comparison to theoretical models Summary and Conclusion

Relativistic Heavy Ion Collider

2 O’clock IR

June 2000: Startup of RHIC

June - September 2000 First Physics Run

Au+Au @ two energies

SNN = 56 and 130 GeV

July 2001- January 2002 Second Physics Run

Au+Au @ SNN = 200 GeV (maximal design energy)

p+p (reference data)

PHOBOS

PHENIX

STAR

BRAHMS

I.G. Bearden7, D. Beavis1, C. Besliu10, Y. Blyakhman6,J. Bondorf7, J.Brzychczyk4, B. Budick6, H. Bøggild7, C. Chasman1, C. H.Christensen7, P. Christiansen7, J.Cibor4, R.Debbe1, J. J. Gaardhøje7, K. Grotowski4, K. Hagel8, O. Hansen7, H. Heiselberg7, A. Holm7, A.K. Holme12, H. Ito11, E.Jacobsen7,A. Jipa10, J. I. Jordre10, F. Jundt2, C. E. Jørgensen7, 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. Murray8, J.Natowitz8, B.S.Nielsen7, K. Olchanski1, D. Ouerdane7, R.Planeta4, F.Rami2, D.Roehrich9, B. H.

Samset12, S. J. Sanders11, I. S. Sgura10, R.A.Sheetz1, Z.Sosin3, P. Staszel7,T.S. Tveter12, F.Videbæk1

R.Wada8 and A.Wieloch3.

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, 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

The BRAHMS Collaboration

~55 physicists from 12 institutions

The BRAHMS Experiment at RHIC

Perspective view of BRAHMS

Good Particle Identification over wide range of rapidities (0<|y|<4) and transverse momenta (0.2<pt<4GeV/c)

Forward Spectrometer2.3o < < 30o

Mid-Rapidity Spectrometer30o < < 95o

Basic information on momentum spectra and yields of charged hadrons as a function of y and pt

SiMA Silicon strips

TMA Scintillator tiles

BBC Čerenkov radiator

SiMA

TPM1

BBCTMA

BBC

Global Detectors in BRAHMS

ChargedParticleMultiplicity

Primary Vertex

BRAHMS Physics Program

Probing Hot and Dense Nuclear Matter by studying:

Reaction Mechanisms and Dynamics Different Observables: dNch/d, pt spectra

Baryon Stopping (anti-particle/particle ratios)

Strangeness Production

Collective Flow

High pt hadron spectra (Jet Quenching effects)

First Results dNch/d and anti-particle/particle ratios

I.Bearden et al, PRL87(2001)112305I.Bearden et al, PLB523(2001)227I.Bearden et al, nucl-ex/0112001 submitted to PRL

EVENT CHARACTERIZATION COLLISION CENTRALITY

Au+Au @ SNN=130GeV

Measured with Multiplicity Detectors (TMA and SiMA)

Central Peripheral

Define Event Centrality Classes Slices corresponding to different fractions of the cross section

Central b=0

Peripheral b large

For each Centrality Cut Evaluate the corresponding number of participants

Npart

(Glauber Model)

dNch/d measurements in BRAHMS

0 - 5 % 5 -10%

10-20% 20-30%

30-40% 40-50%

TPM1

BBC SiMA

TMA

I.Bearden et al, Phys.Lett.B523(2001)227Au+Au @ SNN=130GeV

Data from detectors Consistency

By combining all results Cover wide range -4.7 4.7

= -ln (tan(/2))

“Complete” distribution

Total Charged Particle Multiplicities

dN

ch/d

dNch/d distributions

Au+AuSNN=130GeV SNN=200GeV

I.Bearden et al (BRAHMS)PLB523(2001)227

0-5% 30-40%

Forward ’s No Centrality Dependence Mid-rapidity (0) Increase with centrality

Centrality Dependence Relative contributions of Soft and Hard processes

Nch(-4.7<<4.7)

I.Bearden et al (BRAHMS)Submitted to PRL

3860 300 4630 370

=0 Steady increase =3 Flat dependence (dNch/d scales with Npart)

Increase with Npart Onset of hard processes

D.Kharzeev and M.Nardi, PLB 507(2001)121

dNch/d = ANpart BNcoll

% of hard

SNN=130GeV 20% 7%

SNN=200GeV 25% 7%

dNch/d - Centrality Dependence

Superposition of Soft + Hard

Comparison to Model Predictions

1

2

3

HIJING – Jet quenching

HIJING – No Jet quenching

EKRT (Gluon Saturation)

Wang & Gyulassy, PRL86(2001)3496

BRAHMS

1

2

3

|

|

|

|

Both models HIJING and EKRT reproduce the measured multiplicities

Au+Au data much larger than pp Not a simple superposition Medium effects important role in AA collisions

It would be interesting to explore the Centrality Dependence in these models Stronger constraints

Central CollisionsFor Central Collisions

SUMMARY

BRAHMS has measured dNch/d distributions in Au+Au collisions at two energies SNN=130GeV and 200GeV

Combining different sub-detectors in BRAHMS “Complete” dNch/d distributions

At Forward ’s No Centrality Dependence (dNch/d scales with Npart) No Energy Dependence ( Limiting Fragmentation)

At Mid-rapidity dNch/d/(0.5<Npart>) increases with Centrality Influence of hard scattering processes Two component analysis Significant contribution at RHIC

dNch/d measured in central collisions can be reproduced by two different models HIJING (“Soft+Hard”) and EKRT (“Gluon Saturation”) It would be interesting to investigate the Centrality Dependence in these models Stronger Constraints

Limiting Fragmentation

Central Collisions (5%)

SNN=130GeV

SNN=200GeV

Pb+Pb at SPS

Fragmentation region

Appropriate frame = beam reference frame

No Energy Dependence from SPS to RHIC

Consistent with the Hypothesis of Limiting Fragmentation

Observed in several reactions pp, ppbar, p-emulsion, -emulsion

Deines-Jones et al, PRC (2000) 4903

SNN=17.2GeV

(Benecke et al, PRC 188(1969)2159)

dNch/d - Comparison to Model Predictions

SNN=130GeV

5% 5-10%

20-30% 40-50%

UrQMDBass et al,Prog.Part.Nuc.Phys.41(98)255

HIJINGWang and Gyulassy,PRD44(91)3501

AMPTZhang et al,PRC61(2001)067901Lin et al,PRC64(2001)011902

Parton scattering models give a good description of the data AMPT wider distributions (includes hadronic rescattering)

PLB523(2001)227

dNch/d - Comparison to Model Predictions

Au+Au @ SNN=200GeV

AMPTZhang et al,PRC61(2001)067901Lin et al,PRC64(2001)011902

High density QCDgluon saturationKharzeev and Levin,PLB523(2001)79

Differences forPeripheral Collisions but Small effect !

dN

ch/d

Stronger Constraints on the models ...

Important to use different observables to constrain models

Au+Au @ SNN=130GeV