1. Recent Results from the BRAHMS
Experiment at RHIC
Paweł Staszel, Jagellonian University
for the BRAHMS Collaboration
Eighth Workshop on Non-Perturbative QCD
Paris, 7 – 11 June, 2004

2. The Relativistic Heavy Ion Collider
BRAHMS
Au+Au
Top energy:
sNN=200GeV
d+Au
p+p

3. The BRAHMS Collaboration
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, A.K. Holme12, H. Ito11,
A. Jipa10, J. I. Jordre10, F. Jundt2, C.E.Jørgensen7, R. Karabowicz3, E. J. Kim5,
T. Kozik3, T.M.Larsen12, J. H. Lee1, Y. K.Lee5, G. Løvhøjden2, Z. Majka3,
A. Makeev8, B. McBreen1, M. Mikkelsen12, M. Murray8, J. Natowitz8, B.S.Nielsen7,
K. Olchanski1, D. Ouerdane7, R.Planeta4, F. Rami2, D. Röhrich9, B. H. Samset12,
D. Sandberg7, S. J. Sanders11, R.A.Sheetz1, P. Staszel3,7, T.S. Tveter12,
F.Videbæk1, R. Wada8, Z. Yin9, and I. S. Zgura10
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, 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
50 physicists from 12 institutions

4. Agenda of this talk General Characteristics of the Au+Au sNN=200GeV
- particle production
- nuclear stopping
- statistical model description (particle ratios)
- transvers dynamics (particle pt spectra)
Nuclear modification of spectra Au+Au (QGP)
Rapidity evolution of nuclear modification for
d+Au (CGC)
Summary

5. Charged Particle Multiplicity
0-5% central Au+Au:
Total charged particle
multiplicity: 4630370
(PRL 88, (2002))
50% increase over p+pbar (UA5)
0-5%
5-10%
10-20%
20-30%
30-40%
40-50%
p+p
Energy density: Bjorken 1983
eBJ = 3/2 (<Et>/ pR2t0) dNch/dh
 4.0 GeV/fm3
(<Et>=0.5GeV, t0=1fm/c)

6. Limiting Fragmentation
Shift the dNch/d distribution by the beam rapidity, and scale by Npart. Lines up with lower energy  limiting fragmentation
Au+Au sNN=200GeV (0-5% and 30-40%)
Au+Au sNN=130GeV (0-5%)
Pb+Pb sNN=17GeV (9.4%)

7. Baryon stopping y = yb - y y = 2.03  0.16 y = 2.00  0.1
Gaussians in pz
6 order polynomial
Total E=25.72.1TeV
72GeV per participant

8. Baryon stopping II ? LHC SNN=63 GeV ??? 8.9 y =0.58yp
scaling broken
empirical scaling
8.9
LHC
y = 2.2, E/A=2800GeV
(Ebeam/A=3500GeV, yp=8.9) ?
SNN=63 GeV
???

9. Chemical freeze-out Kinetic freeze-out BRAHMS preliminary
Increasing y
PRL90, (2003)
Chemical freeze-out
Kinetic freeze-out
BRAHMS preliminary
At y=0: -/+ = 1.0, K-/K+ = 0.95 ±0.05
pbar/p = ±0.04
Good statistical model description with
B= B(y),
At |y|<1 materanti-matter
T115 Mev, T0.7c at y=0
Flow velocity decreases with rapidity.
Lower density  lower pressure  less flow
Temperature increases with rapidity.
Lower density  faster freeze out 
higher temperature
Phys. Rev. Lett. 90, (2003)

10. High pt Suppression  Jet Quenching
Particles with high pt’s (above ~2GeV/c)
are primarly produced in hard scattering
processes early in the collision
 Probe of the dense and hot stage q
hadrons
leading
particle
leading particle
Schematic view of jet production
p+p experiments  hard scattered
partons fragment into jets of hadrons
In A-A, partons traverse the medium
If QGP  partons will lose a large
part of their energy
(induced gluon radiation)
 Suppression of jet production
 Jet Quenching
Experimentally  depletion of the high pt region in hadron spectra

11. Charged hadron invariant spectra
RAA =
Yield(AA)
NCOLL(AA)  Yield(NN)
Scaled N+N reference
Nuclear Modification Factor
RAA<1  Suppression relative to
scaled NN reference
BRAHMS, PRL91(2003)072305
h=0
h=2.2
Reference spectrum
p+pbar spectra (UA1)
SPS:
data do not show suppression
enhancent (RAA>1) due to initial state
multiple scatering (“Cronin Effect”)

12. High pt suppression in Au+Au @ SNN=200 GeV
BRAHMS, PRL91(2003)072305
mid-rapidity (=0)
At central collisions clear suppression
At peripheral no suppression (as expected)
forward rapidity (=2.2)
the same trend
no p+p reference large sys. errors
Yield(0-10%)/NCOLL(0-10%)
Yield(40-60%)/NCOLL(40-60%)
RCP=
RCP shows suppression at both
=0 and =2.2

13. Control measurement: d+Au @ SNN=200
Suppression in AuAu due
to Jet Quenching or due to
Initial State Parton
Saturation (CGC)?
What about d+Au?
- Jet Quenching – No
- CGC Yes/No?
Excludes alternative interpretation
in terms of Initial State Effects
 Supports the Jet Quenching for
central Au+Au collisions
+ back-to-back azimuthal correlation
by STAR

14. Data versus Hydro-Jet Model
Hirano & Nara (nucl-th/ )
i Hydro  description of the soft
part of the produced matter
ii Hard part  use a pQDC model
(PYTHIA)
i+ii – generation of jets is evolving
medium
Reasonable description
of data at both =0 and =2.2

15. Evolution of RdAu with rapidity
nucl-ex/
Cronin like enhancement at =0
Clear suppression at =3.2
Low pt consistent with measured dNch/d

16. pQCD versus  = 3.2
A. Accardi, M. Gyulassy,
nucl-th/
Geometrical shadowing with opacity from fit to PHENIX (y~0, 0)

17. Color Glass Condensate explanation
=0
=1
=2.2
=3.2
D. Kharzeev at al.
hep-ph/
quark dipole-nucleus scattering amplitude
Two free parameters fitted to data:
y0 – onset of saturation
c - onset of quantum regime
Overal good description
of RdAu
With general trend of
RdAu  1/Npart, this model
accounts also for resonable
description of RCP

18. Rapidity dependence for d+Au
Submitted to PRL nucl-ex/
Curves: Saturation Model from Kharzeev, Levin, Nardi NPA730 (2004) 448

19. Summary Large hadron multiplicies
 Almost a factor of 2 higher than at SPS ( higher )
 Much higher than in pp ( medium effects)
Identified hadron spectra
 Broken lower energy scaling of rapidity loss
 Good description by statistical model
 large transvers flow
Suppression of high pt particles in central Au+Au collisions
observed at =0 and 2.2
 Consistent with a Jet Quenching scenario
Evolution of nuclear modification in d+Au data
 absence of the suppression in d+Au data at =0 supports
Jet Quenching scenario
 forward data consistent with onset of suppression in the
Color Glass Condensate