High p T Suppression at Forward Rapidities Catalin Ristea Niels Bohr Institute, Copenhagen for the BRAHMS Collaboration XXXV International Symposium on Multiparticle Dynamics Kroměříž, Czech Republic

The Relativistic Heavy Ion Collider BRAHMS p+p p+p, d+Au, Cu+Cu, Au+Au  s NN =200GeV Cu+Cu, Au+Au  s NN =62.4GeV Cu+Cu Au+Au d+Au The BRAHMS Experiment - global detectors - spectrometers & PID What we measure: - R CP and R AA Experimental results Comparing Au and Cu

The BRAHMS Collaboration I.Arsene 11, I.G. Bearden 6, D. Beavis 1, S. Bekele 6, C. Besliu 9, B. Budick 5, H. Bøggild 6, C. Chasman 1, C. H. Christensen 6, P. Christiansen 6, R. Clarke 9, R.Debbe 1, J. J. Gaardhøje 6, K. Hagel 7, H. Ito 1, A. Jipa 9, J. I. Jordre 9, F. Jundt 2, E.B. Johnson 10, C.E.Jørgensen 6, R. Karabowicz 3, E. J. Kim 4, T.M.Larsen 11, J. H. Lee 1, Y. K. Lee 4, S.Lindal 11, G. Løvhøjden 2, Z. Majka 3, M. Murray 10, J. Natowitz 7, B.S.Nielsen 6, D. Ouerdane 6, R.Planeta 3, F. Rami 2, C. Ristea 6, O. Ristea 9, D. Röhrich 8, B. H. Samset 11, D. Sandberg 6, S. J. Sanders 10, R.A.Sheetz 1, P. Staszel 3, T.S. Tveter 11, F.Videbæk 1, R. Wada 7, H. Yang 6, Z. Yin 8, and I. S. Zgura 9 1 Brookhaven National Laboratory, USA, 2 IReS and Université Louis Pasteur, Strasbourg, France 3 Jagiellonian University, Cracow, Poland, 4 Johns Hopkins University, Baltimore, USA, 5 New York University, USA 6 Niels Bohr Institute, University of Copenhagen, Denmark 7 Texas A&M University, College Station. USA, 8 University of Bergen, Norway 9 University of Bucharest, Romania, 10 University of Kansas, Lawrence,USA 11 University of Oslo Norway 48 physicists from 11 institutions

✔ excellent momentum resolution ✔ unique p T coverage at high y BRaHMS Broad Range Hadron Magnetic Spectrometers

q q hadrons leading particle leading particle Schematic view of jet production high p T particles are produced from the fragmentation of jets resulting in hard scatterings loose energy by gluon bremsstrahlung – sensitive to the properties of the traversed medium – jet quenching. aspects to consider: gluon saturation, gluon shadowing, parton recombination, jet-quenching. Cronin enhancement. High p T Suppression

BRAHMS acceptance leading particle ->high p T R CP – systematic errors cancel out * same data set, detector configuration pp reference not available R AA – more information different collision systems no collective effects in pp could emphasize different effects R AA & R CP ratios < 1 show medium effects wrt the ref. used higher η -> different medium density in longitudinal direction, reference pp softer Nucl-th/ Polleri and Yuan Beam Rapidity [H. Weber, UrQMD]

BRAHMS Acceptance small solid angle: ~6 msr for MRS and 0.5 msr for FS map out the particle phase-space by collecting data with many different spectrometer settings average spectrum, event normalization *, corrections

Global Detectors Beam-Beam counters: for vertex determination. TMA & SiMA: for centrality determination. more central collisions Silicon Strips Plastic Scintillator Tiles ZDC counters: at  18 meters. Measure spectator neutrons. 0-5% 5-10% 10-20% 20-30% 40-50% 30-40%

Corrections to the spectra Tracking efficiencies, geometrical acceptances

Spectra for inclusive charged hadrons in 200 GeV inclusive p T spectra for nonidentified h + and h - at η 0, 1, 2.6, 3.2 power-law shape, where the vast majority of particles are produced in the pt region below 2 GeV/c steeper at higher η

R CP in Au+Au at 200 GeV ● Not significant dependence on η ● η~3.2 h + /h - show the same behaviour ● Cronin like enhancement at 2 GeV/c

R AA in 200 GeV/c ● Suppression at all η ● Cronin like enhancement in central colls at all η (p T ~2GeV/c) ● R AA no dependent on η => pp changing the same as AuAu effects other than just medium ● Already for semi-peripheral events, R AA goes to 1.

Hydro + jet model 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 iv – place for more refined initial conditions Reasonable description of data at  =0 and  =2.2 and  =3.2

Strong energy absorption model from a static 2D matter source. ( Insprired by A.Dainese (Eur.Phys.J C33,495) and A.Dainese, C.Loizides and G.Paic (hep-ph/ ) ) Parton spectrum using pp reference spectrum Parton energy loss  E ~ q.L**2 q adjusted to give observed R AA at  ~1. The change in dN/d  will result in slowly rising R AA. The modification of reference pp spectrum causes the R AA to be approximately constant as function of . Can corona effect mask the lower parton density at  =3.2 ?

200 && 62.4 GeV 62.4 GeV pp reference is based on ISR collider data 200 GeV pp reference from BRAHMS data – PRL 93, (2004) 200 GeV 62.4 GeV

Au+Au, 62.4 GeV AuAu CuCu T.M. Larsen, QM2005 Preliminary Cu+Cu Au+Au

R 200, 62.4 GeV Little  dependence at 200 GeV… …but greater suppression approaching beam rapidity at 62.4 GeV. Preliminary kinematic limit CuCu 62.4 GeV... no suppression! Peripheral Au+Au is not pp!

PID - Time-of-flight TIME-OF-FLIGHT TOF2 TOF1 TOFW TOF1 TOF2  / K 2  cut K / p 2.0 GeV/c 3.0 GeV/c 4.5 GeV/c 3.5 GeV/c 5.5 GeV/c 7.5 GeV/c TOFW2 2.5 GeV/c 4.0 GeV/c MRS FS

PID - Cherenkov RICH C1 C4 RICH: Cherenkov light focused on spherical mirror  ring on image plane Ring radius vs momentum gives PID  / K separation 25 GeV/c Proton ID up to 35 GeV/c CHERENKOV (2 settings) C4 Threshold (MRS):  / K separation 9 GeV/c

R AA for identified particles No p suppression! Preliminary

R AA versus centrality for identified hadrons R. Karabowicz, QM2005

R CP for p, π, K at y ~ 3 Rcp for Identified particles at y~3 Suppression for all particles Rcp(p) > Rcp(K) > Rcp(  ) Cent bins using multiplicity in |  |<2 Glauber Model for 0-10% ~880 for 40-60% ~ 78 ±± K±K± p,pbar

R AA at y=0 and y=3.1 R AA for pions at forward rapidities agrees with R AA at midrapidity R. Karabowicz, QM2005

d+Au system... D. Kharzeev hep-ph/ initially used to emphasize the Au+Au suppression at η ~ 0 CGC - high enery, forward y, large nuclei, saturation CGC inspired much teoretical and experimental work

Charged hadron spectra In all p+p d+Au Au+Au systems, the distributions get softer at higher rapidities... dN/d η...

d+Au System Cronin like enhancement at  =0. Clear suppression as  changes up to 3.2 Same ratio made with dn/d  follows the low p T R dAu R dA   d 2 N d+Au /dp T d  d 2 N pp inel /dp T d  <> = 7.2 where = 7.2±0.3 At  =0 the central events have the ratio systematically above that of semi-central events. We see a reversal of behavior as we study events at  =3.2

D. Kharzeev, Y.V. Kovchegov, K. Tuchin, hep-ph/ (2004) CGC model describes R dAu and R CP Suppression comes in at y > 0.6 CGC saturation model

Recombination... important contributions from thermal/shower partons Parton recombination (up to moderate p T ) Variety of processes can result in suppression Quality of data is insufficient for ruling out models Hwa, Yang and Fries Phys.Rev.C71:024902,2005

Summary R AA show 200 GeV for h + /h - - no significant diff between h + /h - R AA moderate/no 63 GeV (AuAu, CuCu) R CP – not clear what's the dependency on different systems – few effects competting at intermediate pt (eg. v 2 ) centrality dependence of nucl. mod. factors && slow/NO dependency with η => consistency with jet surface emission (and hydro+jet model) different mechanism of suppression for baryons and mesons – recombination? R AA for dA consistent with CGC predictions, though quality of data is insufficient to rule out models