BRAHMS

Zhongbao Yin

for the for the BRAHMSBRAHMS Collaboration Collaboration

Key Results from the Key Results from the BRAHMS ExperimentBRAHMS Experiment

BRAHMS

● The BRAHMS experiment● Baryon stopping● High pT hadron production● Summary

Outline

BRAHMS

The BRAHMS ExperimentThe BRAHMS Experiment Broad RAnge Hadron Magnetic Spectrometers

BRAHMS

StoppingStopping

● Energy released for particle production● Information on the initial condition for

the evolution of relativistic heavy-ion collisions– Landau model: full stopping+isentropic

expansion– Bjorken scenario: transparency => net baryon

poor region at mid-rapidity

● Information on the baryon transport mechanism

BRAHMS

Stopping: pbar/pStopping: pbar/p At Mid-rapidity

= 1.01 ±0.04

K-/K+ = 0.95 ±0.05

pbar/p = 0.75 ±0.04

● Pair production is predominant ● The ratios, within errors, are constant in the interval y=0-1 =>Considerable transparency

Moving away from the central region, the influence of the participant baryons becomes more and more important

PRL 90, 102301(2003)PRL 90, 102301(2003)

Au+Au @ 200 GeVAu+Au @ 200 GeV

BRAHMS

Stopping: Net ProtonsStopping: Net Protons

PRL 93, 102301(2004)

Au+Au @ 200 GeV

HIJING without Baryon Junction is the least un-favoured.

BRAHMS

Rapidity and Energy Rapidity and Energy LossLoss

y2.06 0.16y = 2.04 0.10

E=26+-5GeV/nE= 30+-2GeV/n

py BB

partp dy

dy

ydNy

Nyy

0

)()(2

p

p

y

y

BBT

part

dydy

dNym

NE cosh

1

Rapidity loss saturates while around 75% of initial energy is available for particle production.

BRAHMS

Simplified Landau model predicts: dN/dy (y) of Gaussian with width ln(2mp)

(Caruthers & Duong-van,

PRD8(1973)859)

Landau or Landau or Bjorken ?Bjorken ?

PRL 94, 162301(2005)

Au+Au @ 200 GeV

BRAHMS

Hard ScatteringHard Scattering

Study modifications modifications of high pT production in AA, dA with respect to pp via Ncoll scaling at different rapidities to disentangle different effects

recombination

BRAHMS

Nuclear Modification Factors: Nuclear Modification Factors: RRAAAA

RAA = (yieldAA/<Ncoll>)/yieldpp

Rcp = (yieldcentral/<Ncoll>cent)/(yieldperipheral/<Ncoll>peripheral)

BRAHMS PRL91(2003)072305

RdAu : enhancement

Large high pt suppression aslo at forward rapidity.

BRAHMS

RRdAdA in d+Au in d+Au

CollisionsCollisions

Enhancement at mid-rapidity, while high pt suppression at = 3.2

BRAHMS PRL 93, 242303 (2004)

BRAHMS

RRcp cp in d+Au in d+Au

collisionscollisions

● At mid-rapidity, stronger enhancement for more central collision, but reversed centrality dependence at forward rapidity.

● Consistent with CGC prediction: PLB599(2004)23

BRAHMS

But… Recombination also But… Recombination also worksworks

with only the recombination of soft and shower partonsno multiple scattering, or gluon saturation put in explicitly

Hwa,Yang & Fries: PRC(2005)024902Hwa,Yang & Fries: PRC(2005)024902

BRAHMS

RRAAAA for identified hadrons for identified hadrons

● Strong suppression for pions but no suppression for (anti-)protons at both mid-rapidity and forward rapidity

BRAHMS Preliminary

BRAHMS

p/p/ ratios in Au+Au ratios in Au+Au collisionscollisions

● Ratios are large compared with 0.2 from fragmentation of energetic partons

● Ratios are smaller at forward rapidity while pion suppression persists.

● Qualitative agreement with parton recombination with collective flow effect

BRAHMS Preliminary

BRAHMS

SummarySummary● RHIC reaches a net-baryon poor region

at mid-rapidity● In Au+Au collisions at 200 GeV:

– High pt suppression of charged hadrons and pions at both mid- and forward rapidities

– No high pt (anti-)proton suppression at both mid- and forward rapidities

● In d+Au collisions at 200 GeV:– significant reduction of the nuclear

modification factor at forward rapidity and this suppression increases with rapidity and centrality

BRAHMS

The BRAHMS CollaborationI. Arsene10, I. G. Bearden7, D. Beavis1, C. Besliu10, B. Budick6, H. Bøggild7, C. Chasman1,

C. H. Christensen7, P. Christiansen7, J. Cibor4, R. Debbe1, E. Enger12, J. J. Gaardhøje7, M. Germinario7,

K. Hagel8, O. Hansen7, H. Ito1, 11, A. Jipa10, F. Jundt2, J. I. Jørdre9, C. E. Jørgensen7,

R. Karabowicz3, E. J. Kim5, T. Kozik3, T. M. Larsen12, J. H. Lee1, Y. K. Lee5, S. Lindal12, R. Lystad9,

G. Løvhøiden2, Z. Majka3, A. Makeev8, B. McBreen1, M. Mikelsen12, M. Murray8, 11, J. Natowitz8,

B. Neumann11, B. S. Nielsen7, J. S. Norris11, D. Ouerdane7, R. Planeta4, F. Rami2, C. Ristea10, O. Ristea10,

D. Röhrich9, B. H. Samset12, D. Sandberg7, S. J. Sanders11, R. A. Scheetz1, P. Staszel7, T. S. Tveter12,

F. Videbæk1, R. Wada8, Z. Yin9, I. S. Zgura10

1Brookhaven National Laboratory, USA

2IReS and Université Louis Pasteur, Strasbourg, France

3Jagiellonian University, Krakow, Poland

4Institute of Nuclear Physics, Cracow, Poland

5Johns Hopkins University, Baltimore, USA

6New York University, USA

7Niels Bohr Institute, 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