Eun-Joo Kim ( Chonbuk Nat. Eun-Joo Kim ( Chonbuk Nat. Univ.)Univ.)Hongyan Yang ( Univ. of Bergen Hongyan Yang ( Univ. of Bergen ))

For the CollaborationFor the Collaboration

NNuclear M Modification FFactors at BRAHMS

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OutlineOutline

• BRAHMS experiment Nuclear Modification Factor - rapidity, system, energy

dependence - dAu, AuAu, CuCu at 200, 62.4 GeV• Summary

90o

30o

2.3o

15o

30o

It’s the BBroad RARAnge HHadron MMagnetic SSpectrometers!

AuAu AuAu

AuAudd

CuCu CuCu

pp pp

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pp collisions: we need this as reference

dAu collisions

AuAu & CuCu collisions

At the RHIC energies, hard scattering processes at high-pT become important. Partons are expected to loose energy in the dense matter Different rapidities provide different densities of the medium : study nuclear modification in AA, dA with respect to pp via Ncoll scaling to learn about QCD dynamics “Dialing” initial condition channel Rapidity dependent high pT suppression factors : provide information on dynamical medium effect

BRAHMS studies the properties of the produced medium as a function of rapidity…

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NNuclearuclear M Modification odification FFactoractor

yield in AA/number of equivalent pp collisions

yield in ppRAA =

- quantify the nuclear effects- quantify the nuclear effects

yield in central events/<Ncoll>cent

Rcp =yield in peripheral events/<Ncoll>peripheral

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RRdAudAu, R, RAuAuAuAu at at =0, =0, ~2.2~2.2

BRAHMS, PRL91, 072305(2003)

suppression

enhancement

▪ R(~0) ~ R(~2) ▪ Absence of suppression in dAu at =0▪ Suppression in central AuAu is a final-state effect

Nuclear Modification at higher rapidity ▪ Longitudinally extended source? ▪ And/or Initial state effect enhancing?

RCP(~2.2) RCP(~0)

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dAu results as a function of dAu results as a function of & & centralitycentrality

increasing suppression

Cronin like enhancement centrality dependence

reversed at forward rapidities

Increasing centrality

BRAHMS PRL93(2004)242303

The data are described by CGC models [PLB599(2004) 23], and have given rise to

other interpretations and additional measurements. [Phys. Rev. C71 (2005) 024902]

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Rapidity dep. RRapidity dep. RAuAuAuAu : h : h±± at 200 at 200 GeVGeV

High pT suppressions persists over 3 units in

0-10%

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Centrality dependence of RCentrality dependence of RAuAu AuAu & & RRCPCP

more peripheral, less suppression !

no s

ign

ifican

t rap

idity

dep

en

den

ce

no significant rapidity dependence

▪ high pT suppression (pT > 2 GeV/c) observed at higher is similar to that at ~0

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RRAuAu AuAu of hadron at 62.4 of hadron at 62.4 GeV GeV

▪ RAuAu(62.4GeV) > RAuAu(200 GeV) at pT > 2GeV/c & no rapidity dependence▪ more peripheral, less suppression : depends on geometry of system?

▪ consistent with weak effects of a dense medium and a return of Cronin type enhancements

BRAHMS Preliminary

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RRCPCP, R, RCuCu CuCu of hadron at 62.4 of hadron at 62.4 GeV GeV

▪ no pT dependence▪ no dependence on system size ▪ no rapidity dependence

BRAHMS

Preliminary

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RR from R from RCP CP at 200, 62.4 at 200, 62.4 GeVGeV

BRAHMS Preliminary

AuAu 200 GeV

R = RCP() / RCP(~0)

▪ R(~3) < R(~0.9) at low energy

62.4 GeV

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RRCPCP at CuCu 200 GeV at CuCu 200 GeV

▪ more suppression as the collisions become more central▪ little dependence on ▪ similar effect as seen AuAu

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RRCPCP : dependency on system size at 200 : dependency on system size at 200

GeVGeV

= 3.2

Forward central CuCu looks like forward peripheral AuAu▪ RCP depends on Npart

(0-20%)/(60-80%)

centrality Npart

------------------------------------ CuCu 0-10% 96.8 40-60% 17.2 AuAu 33-43% 96.3 59-79% 15.7 dAu 0-20% 14.6 60-80% 4.8

pT [GeV/c]

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RRAuAu, AuAu, RRCPCP((, p), p) at 200GeV at 200GeV

0-10%

y~1y=0 y~3.1

BR

AH

MS

P

relim

inary

RAuAu: < p

Suppression for pions

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RRdAudAu at Forward rapidity at Forward rapidity

PHENIX PRC74, 024904(2006)

BRAHMS Preliminary

more suppressed at forward

baryons : enhancement

RdAu(p) suppression up to 2.5 GeV/c

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Forward Tomography: Dynamics + Forward Tomography: Dynamics +

GeometryGeometry

G. Barnaföldi, Lévai, Papp, Fai: nucl-ph/0609023

▪ Shadowing + MS + Energy Loss

▪ “Extracted“ opacity indicates longitudinally traveling partons see less colored field

▪ Not a prediction : Assuming rapidity independent suppression factor

L/: mean value of jet scattering

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SummarySummary

Nuclear Modification Factors at BRAHMS show

▪ suppression at forward rapidity, too : RAA, RdAu

▪ no significant rapidity dependence : RAA

Non-hadronic energy loss through the medium in |y|<3: ▪ centrality dependence ( the geometry of the system ) : RAA, RdAu

▪ energy dependence, quantitatively : RAA

▪ more suppression for mesons than baryons : RAuAu, RdAu

Forward suppression indicates that there most likely is some interplay between initial state (e.g. shadowing) and final state (suppression in the thin slice of deconfined matter, energy loss) effects.

needs more theoretical/experimental understanding on dynamics of particle suppression/production at RHIC

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The CollaborationThe CollaborationI. Arsene9,11, I.G. Bearden6, D. Beavis1, S.Bekele10, C. Besliu9, B. Budick5, H. Bøggild6 ,

C. Chasman1, C. H. Christensen6, P. Christiansen6, R.Clarke9, R.Debbe1,

J. J. Gaardhøje6, K. Hagel7, H. Ito1, A. Jipa9, J. I. Jordre8, F. Jundt2, E.B.Johnson10,

J.I.Jordre8, C.Jørgensen6, R. Karabowicz3, E. J. Kim10, T.M.Larsen6, J. H. Lee1,

Y. K.Lee4, S.Lindal11, G. Løvhøjden2, Z. Majka3 , M. Murray10, J. Natowitz7,

B.S.Nielsen6, D.Ouerdane6, R.Planeta3, F. Rami2, C.Ristea6, O.Ristea9,

D. Röhrich8, B. H. Samset11, S. J. Sanders10, R.A.Sheetz1, P. Staszel3, T.S. Tveter11,

F.Videbæk1, R. Wada7, H.Yang8, Z. Yin8, and I. S. Zgura9

1Brookhaven National Laboratory, USA2IReS and Université Louis Pasteur, Strasbourg, France

3Jagiellonian University, Krakow, Poland4Johns Hopkins University, Baltimore, USA

5New York University, USA6Niels Bohr Institute, University of Copenhagen, Denmark

7Texas A&M University, College Station. USA,8University of Bergen, Norway

9University of Bucharest, Romania10University of Kansas, Lawrence,USA

11University of Oslo, Norway