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Zbigniew MajkaM.Smoluchowski Institute of Physics, Jagiellonian University, Kraków, Poland

ufmajka@cyf-kr.edu.pl

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

•What was built and why?

•What has been accomplished?

•What is still to come?

BRAHMS Experimental setup

*Measurements:

p, K, identified over wide range of rapidity:

0 < |y| < 4

0.2 < pt < ~ 3GeV/c central and fragmentation region vs centrality (with high precision).

* Results address:

Baryon stopping.

Reaction mechanism and dynamics

Chemical equilibrium

Thermalization

Baryo-chemical potential

Particle production

Particle to antiparticle ratios

Mini-jet production

Forward Spectrometer

•Full Forward Spectrometer (2.3o < < 15o )High-momentum mode

–sweeping D1,D2

–tracking and momentum determination by T2-T5, D3,D4

–PID: RICH (/K/p) separation < 25 GeV/c

–Tof-H2 ( /K < 5, K/p < 8.5 GeV/c with 4 cut)

Low-momentum mode

–tracking and momentum determination by T1-T2, D2

–PID: C1 (/K) separation < 9 GeV/c

–Tof-H1 ( /K < 3.3, K/p < 5.7 GeV/c with 4 cut

•Front Forward Spectrometer (15o < < 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 < 2.2, K/p < 3.7 GeV/c with 4 cut).

Tracking and PID

BRAHMS acceptance – run # 2

SiMA (2.0< | |<2.0)

PTMA (2.2< || <2.2)

BBC (3 < | | < 4)

Centrality determination – vertex distribution

Silicon Strips

Plastic Scintillator Tiles

Beam–Beam

MRS Hadron identification - TOF

m2=p2( t2 / L2 -1)

FFS Hadron identification

FFS Hadron identification - C1

Cos = 1/n => th=1/n

BFS Hadron identification - RICH

Low field run

sNN = 130 GeV

BRAHMS: PRL 87 (2001) 112305 BRAHMS: PL B523 (2001) 227

•Rapidity dependence of antiparticle-to-particle ratio

•Pseudorapidity densities of charged particles

•Comparison with models

sNN = 200 GeV

Pseudorapidity densities of charged particles

BRAHMS: PRL 88 (2002) 202301

N_part

Centrality [%]

= 0 = 1.5

= 3.0 = 4.5 Nch Npart

0 – 5

553 36 625 55

554 37 627 54

372 37 470 44

107 15 181 22

3860 300 4630 370

352 346

5 – 10

447 29 501 44

454 31 515 48

312 36 397 37

94 13 156 18

3180 250 3810 300

299 293

10 - 20

345 23 377 33

348 25 386 35

243 27 309 28

79 10 125 14

2470 190 2920 230

235 228

20 – 30

237 16 257 23

239 16 267 23

172 18 216 17

59 8 90 10

1720 130 2020 160

165 164

30 – 40

156 11 174 16

159 11 182 16

117 13 149 14

43 6 64 7

1160 90 1380 110

114 114

40 – 50

98 7 110 10

104 7 115 11

77 9 95 9

30 4 43 5

750 60 890 70

* 4630 charged particles produced for 0-5% central

* 14% increase over 130GeV

* 50% increase over p +p (UA5)

=> significant medium effects

Bjorken energy density: BJ = (1/R20) <p t >dN ch/d

(where 0 =1 fm/c, <p t > = 0.5 GeV/c) 5 GeV/fm3

(QGPcritical 1 GeV/fm3)

Pseudorapidity densities - comparison

Saturation of excitation in fragmentation region

Data shifted to the beam frame of reference

BRAHMS: PRL 88 (2002) 202301

Antiparticle-to-particle ratio:

- vs rapidity

130 GeV

- centrality and pt dependence

Correlation –strange meson and baryon antiparticle-to-particle ratios

BRAHMS: submitted to PRL, nucl-ex/0207006

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 y=0) 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

Tranverse flow at y=0 not significantly different from SPS?

Moderate transverse source sizes

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