Zbigniew Majka

1
Title
Review of early results from BRAHMS experiment
at RHIC Zbigniew Majka (for the BRAHMS
Collaboration) Department of Hot Matter Physics,
M. Smoluchowski Institute of Physics Jagiellonian
University, Kraków, Poland
2
Plan

• What was built and why?
• What has been accomplished?
• What is still to come?

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BRAHMS Experimental setup
Measurements p, K, ? identified over wide range
of rapidity 0
lt y lt 4 0.2 lt pt lt
3GeV/c central and fragmentation
region vs centrality
(with high precision).
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• Results address

Baryon
stopping. Reaction
mechanism and dynamics
Chemical equilibrium
Thermalization
Baryo-chemical potential
Particle production
Particle to antiparticle ratios
Mini-jet production
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Tracking and PID

• Forward Spectrometer
• Full Forward Spectrometer (2.3o lt ? lt 15o )
• High-momentum mode
• sweeping D1,D2
• tracking and momentum determination by T2-T5,
  D3,D4
• PID RICH (?/K/p) separation lt 25 GeV/c
• Tof-H2 (? /K lt 5, K/p lt 8.5 GeV/c with 4? cut)
• Low-momentum mode
• tracking and momentum determination by T1-T2, D2
• PID C1 (?/K) separation lt 9 GeV/c
• Tof-H1 (? /K lt 3.3, K/p lt 5.7 GeV/c with 4? cut
• Front Forward Spectrometer (15o lt ? lt 30o )
• Same as Low-momentum Mode
• Momentum resolution ?(dp/p) 1
• Mid-Rapidity Spectrometer
• Tracking and Momentum determination,
• MTP1,MTP2 and D5.
• PID TOFW (? /K lt 2.2, K/p lt 3.7 GeV/c with 4?
  cut).

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BRAHMS acceptance run 2
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Centrality determination vertex distribution

BeamBeam

SiMA (2.0lt ? lt2.0) PTMA (2.2lt ?
lt2.2) BBC (3 lt ? lt 4)
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MRS Hadron identification - TOF
m2p2( t2 / L2 -1)
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FFS Hadron identification
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FFS Hadron identification - C1
Cos? 1/n? gt ?th1/n
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BFS Hadron identification - RICH
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?sNN 130 GeV

• Rapidity dependence of antiparticle-to-particle
  ratio
• Pseudorapidity densities of charged particles
• Comparison with models

BRAHMS PRL 87 (2001) 112305
BRAHMS PL B523 (2001) 227
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?sNN 200 GeV Pseudorapidity densities
of charged particles
BRAHMS PRL 88 (2002) 202301
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N_part
?0
?3.0
p-p
?4.5
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Pseudorapidity densities - comparison
4630 charged particles produced for 0-5
central 14 increase over 130GeV 50 increase
over p ?p (UA5) gt significant medium effects
Bjorken energy density ?BJ
(1/?R2?0) ltp t gtdN ch/d? (where?? 0 1 fm/c, ltp
t gt 0.5 GeV/c) ? 5 GeV/fm3

(?QGPcritical ? 1 GeV/fm3)
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Saturation of excitation in fragmentation region
Data shifted to the beam frame of reference
BRAHMS PRL 88 (2002) 202301
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Antiparticle-to-particle ratio - vs rapidity
130 GeV
- centrality and pt dependence
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Correlation strange meson and baryon
antiparticle-to-particle ratios
BRAHMS submitted to PRL, nucl-ex/0207006
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Summary from BRAHMS

• The highest particle multiplicity observed in
  nuclear collisions
• ( Nch ?
  4600)
• Large(st) energy density
• (? ? 5
  ?crit )
• High reaction transparency
• (p-/p ?
  0.75 at y0)
• Decoupling between central and fragmentation
  regions
• Onset of boost invariant plateau
• Preliminary
• Partonic (color) description appears necessary
• Central region dominated by matter-antimatter
  balance
• 
  Qualitatively new physics regime