Results from the BRAHMS Experiment at RHIC

1
Results from the BRAHMS Experiment at RHIC

• 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

2
Relativistic Heavy Ion Collider
2 Oclock IR
BRAHMS
PHOBOS

• June 2000 Startup of RHIC

PHENIX

• June - September 2000
• First Physics Run

STAR
AuAu _at_ two energies
?SNN 56 and 130 GeV

• July 2001- January 2002
• Second Physics Run

AuAu _at_ ?SNN 200 GeV (maximal design energy)
pp (reference data)
3
The BRAHMS Collaboration

• 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,
• 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 AM University, College Station. USA
  
• 9University of Bergen, Norway
  
  10University of Bucharest, Romania
• 11University of Kansas, Lawrence,USA
  
  12 University of Oslo Norway

55 physicists from 12 institutions
4
The BRAHMS Experiment at RHIC
Perspective view of BRAHMS
Forward Spectrometer 2.3o lt ? lt 30o

• Good Particle Identification
• over wide range of rapidities
• (0ltylt4) and transverse
• momenta (0.2ltptlt4GeV/c)

Mid-Rapidity Spectrometer 30o lt ? lt 95o

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

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Global Detectors in BRAHMS
BBC
SiMA
TPM1
TMA
BBC
SiMA Silicon strips TMA Scintillator tiles
BBC Cerenkov radiator
Charged Particle Multiplicity
? Primary Vertex
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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)112305 I.Bearden et
al, PLB523(2001)227 I.Bearden et al,
nucl-ex/0112001 submitted to
PRL
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EVENT CHARACTERIZATION COLLISION
CENTRALITY
AuAu _at_ ?SNN130GeV

• Measured with Multiplicity
• Detectors (TMA and SiMA)

Central b0
?
?
?
Central ?
? Peripheral
Peripheral b large
?

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

• For each Centrality Cut
• ? Evaluate the corresponding number of
  participants Npart
• (Glauber Model)

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dNch/d? measurements in BRAHMS
I.Bearden et al, Phys.Lett.B523(2001)227
AuAu _at_ ?SNN130GeV
0 - 5
5 -10
TPM1
? -ln (tan(?/2))
TMA

• Data from ? detectors
• ? Consistency
• By combining all results
• ? Cover wide ? range
• -4.7 ? ? ? 4.7

BBC
SiMA
dNch/d?
10-20
20-30
Complete distribution
30-40
40-50
Total Charged Particle Multiplicities
?
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dNch/d? distributions
AuAu
?SNN130GeV
?SNN200GeV
I.Bearden et al (BRAHMS) PLB523(2001)227
I.Bearden et al (BRAHMS) Submitted to PRL
Nch(-4.7lt?lt4.7)
3860 ? 300
4630 ? 370
? 0-5 ? 30-40?

• Forward ?s ? No Centrality
  Dependence
• Mid-rapidity (??0) ? Increase with centrality
• Centrality Dependence ? Relative
  contributions
• of Soft and
  Hard processes

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dNch/d? - Centrality Dependence

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

• Increase with Npart ?
• Onset of hard processes

dNch/d? A?Npart ? B?Ncoll
Superposition of Soft Hard
D.Kharzeev and M.Nardi, PLB 507(2001)121
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Comparison to Model Predictions

• AuAu data much larger than pp
• ? Not a simple superposition
• Medium effects ? important role
• in AA collisions

• Both models HIJING and EKRT
• reproduce the measured multiplicities

Central Collisions
For Central Collisions

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

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SUMMARY

• BRAHMS has measured dNch/d? distributions in
  AuAu
• collisions at two energies ?SNN130GeV 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?/(0.5ltNpartgt) 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 (SoftHard) and EKRT
  (Gluon Saturation)
• ? It would be interesting to investigate the
  Centrality Dependence in
• these models ? Stronger Constraints

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Limiting Fragmentation

• Fragmentation region

Central Collisions (5)
? ?SNN130GeV
Appropriate frame beam reference frame
? ?SNN200GeV
? PbPb at SPS
?SNN17.2GeV
Deines-Jones et al, PRC (2000) 4903
No Energy Dependence from SPS to
RHIC
? Observed in several reactions pp, ppbar,
p-emulsion, ?-emulsion
? Consistent with the Hypothesis of Limiting
Fragmentation
(Benecke et al, PRC 188(1969)2159)
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dNch/d? - Comparison to Model Predictions
?SNN130GeV
PLB523(2001)227
UrQMD Bass et al,Prog.Part. Nuc.Phys.41(98)255 HI
JING Wang and Gyulassy, PRD44(91)3501 AMPT Zhang
et al, PRC61(2001)067901 Lin et
al, PRC64(2001)011902
5
5-10
20-30
40-50

• Parton scattering models give a good description
  of the data

• AMPT ? wider distributions (includes hadronic
  rescattering)

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dNch/d? - Comparison to Model Predictions
AuAu _at_ ?SNN200GeV
AMPT Zhang et al, PRC61(2001)067901 Lin et
al, PRC64(2001)011902 High density QCD gluon
saturation Kharzeev and Levin, PLB523(2001)79
dNch/d?
Differences for Peripheral Collisions but Small
effect !
?
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Stronger Constraints on the models ...
?SNN130GeV
AuAu _at_
? Important to use different observables to
constrain models