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

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

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

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

Charged Particle Multiplicity 0-5% central Au+Au: Total charged particle multiplicity: 4630370 (PRL 88, 202301(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)

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%)

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

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 ???

Chemical freeze-out Kinetic freeze-out BRAHMS preliminary Increasing y PRL90,102301 (2003) Chemical freeze-out Kinetic freeze-out BRAHMS preliminary At y=0: -/+ = 1.0, K-/K+ = 0.95 ±0.05 pbar/p = 0.75 ±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, 102301(2003)

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

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”)

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

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

Data versus Hydro-Jet Model Hirano & Nara (nucl-th/0307087) 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

Evolution of RdAu with rapidity nucl-ex/0403005 Cronin like enhancement at =0 Clear suppression at =3.2 Low pt consistent with measured dNch/d

pQCD versus data @  = 3.2 A. Accardi, M. Gyulassy, nucl-th/0402101 Geometrical shadowing with opacity from fit to PHENIX (y~0, 0)

Color Glass Condensate explanation =0 =1 =2.2 =3.2 D. Kharzeev at al. hep-ph/0405045 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

Rapidity dependence for d+Au Submitted to PRL nucl-ex/0401025 Curves: Saturation Model from Kharzeev, Levin, Nardi NPA730 (2004) 448

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