Why does high-pt suppression persist at forward rapidity?

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

1

HEHI BRAHMS

Why does high-pt suppression persist at forward rapidity?

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

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

2

HEHI BRAHMS

Outline

Introduction– Experiment

– RAA

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

Model Comparisons Summary

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

3

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!

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

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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

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

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HEHI BRAHMS

Charged hadron pT spectra 200GeV Au+Au

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

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

6

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

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

HEHI BRAHMS

RAA @ 62.4 GeV ~3.1~0

Au+Au

Cu+Cu

BRAHMS Preliminary

BRAHMS Preliminary

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

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HEHI BRAHMS

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

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

BRAHMS Preliminary

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

HEHI BRAHMSRAuCu0- 10% & 40-60%

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

HEHI BRAHMS

RAuCu with same Ncoll at Midrapidity

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

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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)

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

HEHI BRAHMS

RAuCu with same ‘radius’

BRAHMS PRELIMINARY

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

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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

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

HEHI BRAHMS

RAA at 62.4 GeV, y=0BRAHMS PreliminaryBRAHMS Preliminary

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

HEHI BRAHMS

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

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

16

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

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

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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

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

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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

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

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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?

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

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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