Why does high-pt suppression persist at forward rapidity?

5. Februar 2008 Quark Matter '08 Jaipur IndiaI.G.Bearden, Niels Bohr Institute

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HEHI BRAHMS

Why does high-pt suppression persist at forward rapidity?

I.G. Bearden ( for the BRAHMS collaboration)Niels Bohr Institute, Københavns Universitet


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HEHI BRAHMS

Outline

Introduction– Experiment

– RAA

Results– 200GeV Au+Au– 62GeV Cu+Cu & Au+Au

Model Comparisons Summary


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HEHI BRAHMS

Two small solid anglespectrometers (FS and MRS)

2.3°30°

that can rotate from 2.3° to 30°

90°

30°

and 30° to 90° (MRS)

provide excellent PID over broad range in y-pT

The BRAHMS ExperimentIt’s Broad RAnge Hadron Magnetic Spectrometers!


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HEHI BRAHMS

Quantified by:

ddpNdN

ddpNdR

TNN

bin

TAB

AB /

/2

2

• Cronin Enhancement• Shadowing/Saturation• Jet-quenching

- yield relative to that fromN+N collisions, scaled forthe nuclear geometry (Nbin)

Nuclear Modification


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HEHI BRAHMS

Charged hadron pT spectra 200GeV Au+Au

Nb: filled are +, open -, drawn lines are BRAHMS p+p data


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HEHI BRAHMS

RAA for charged hadrons

at high pT, the charged hadron yields produced in

most central 0-10% Au+Au collisions are

suppressed compared to binary scaled p+p yields

Little change in RAA as function of eta

BRAHMS Preliminary


HEHI BRAHMS

RAA @ 62.4 GeV ~3.1~0

Au+Au

Cu+Cu

BRAHMS Preliminary

BRAHMS Preliminary


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HEHI BRAHMS

Caveat emptor!How to make pT=2GeV/c

p=2.8GeV/c p=28GeV/c

BRAHMS Preliminary


HEHI BRAHMSRAuCu0- 10% & 40-60%


HEHI BRAHMS

RAuCu with same Ncoll at Midrapidity


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HEHI BRAHMS

RAuCu mid vs. forward 62.4GeV (10%)

0

0,2

0,4

0,6

0,8

1

1,2

0 1 2 3

forwardmid

pT

RA

uC

u

MidrapidityForward (3.1)


HEHI BRAHMS

RAuCu with same ‘radius’

BRAHMS PRELIMINARY


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HEHI BRAHMSRAA for identified particles

(200GeV)

the charged pion yields are suppressed by a factor of ~ 2-3 as

compared with binary scaled p+p pion yields.

RAA for pions is independent of rapidity

the proton and antiproton yields in central Au+Au at 200 GeV

do not show suppression, baryon meson difference remains

protons

BR

AH

MS P

relim

inarypions


HEHI BRAHMS

RAA at 62.4 GeV, y=0BRAHMS PreliminaryBRAHMS Preliminary


HEHI BRAHMS

RAA, y>3, 62GeV, top 10% central


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HEHI BRAHMS

Interesting regionIntermediate pT

(2<pT<3.5 GeV/c)Pion hard, Proton soft

PT, cross~ 1.8 GeV/c for 1.8 GeV/c for

2.7 GeV/c for K2.7 GeV/c for K3.7 GeV/c for p3.7 GeV/c for p

Hydro+Jet: Interplay between Soft and Hard

T.H

irano &

Y.N

ara

(’0

3),

PR

C6

9(2

00

4)0

34

90

8

Au

+A

u a

t b

=2 f

m

Calculation based on two models with

CGC as initial conditions:

3D hydrodynamic model (Hydro)

dynamics of high pT partons (Jet)

pT,cross is the point for Yieldsoft=Yieldhard

pT,cross moves toward high pT with mass

of particles because of the effects of

radial flow.

y=0


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HEHI BRAHMS

Model comparison I

pT spectra for charged hadrons in 200 GeV Au+Au reasonably

well described by the hydro+jet model calculation at η=3.2


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HEHI BRAHMS

Model comparison II

opacity n = L/l

L – effective length of matter

l – mean free path

less jet quenching +

stronger initial effects

at higher rapidities

maintain rapidity independent NMF

Barnafoldi et al. hep-ph 0609023


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HEHI BRAHMS

Is RAA an appropriate measure?

~15 GeV

Re

nk,

Esk

ola

, P

RC

75

, 0

54

91

0 (

20

07

)

Wicks,H

oro

witz,D

jord

jevic,G

yula

ssyN

PA

78

4, 4

26

(20

07

)

Two rather differentModels both reproduceRAA. RAA only tells us that we only ‘see’ particles from near Hot&dense surface?


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HEHI BRAHMS

Summary

At 200GeV, intermediate pT constant behavior vs. y

For 62.4GeV, pronounced changes from mid to forward rapidity.

+ suppressed for 62.4 y=3.4, p “suppressed” but not partonically

RAA flat for same mean radius (<R>). Why?

Hydro+jet looks good, but... RAA not a good model descriminator

Documents

Why does high-pt suppression persist at forward rapidity?