Jet quenching and direct photon production

F.M. Liu 刘复明 Central China Normal University, China

T. Hirano University of Tokyo, Japan

K.Werner University of Nantes, France

Y. Zhu 朱燕 Central China Normal University, China

arXiv: hep-ph/0807.4771v2

SQM2008 Beijing Oct 6-10

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Outline

• Motivations• Calculation approach• Results• Conclusion

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Motivations• Heavy ion collisions at various centralities offer us various bulks of

hot dense matter.

• The interaction between jets ( hard partons) and the bulk has receiv

ed notable interest, i.e. jet quenching is one of the most exciting obs

ervables at RHIC.

• The interaction of partons inside the bulk and the properties of the b

ulk are of great interest, which may offer us some insight to quark c

onfinement.

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direct photons, jets and plasmaPRL94,232301(2005), PRL96,202301(2006)

1. Jet queching gives different effects to direct photons?

2. Direct photons (thermal, jet-photon conversion) are penetrating probes

for the interaction of partons inside the bulk and the interaction betwee

n jet and bulk. We can make cross check of the properties of the medi

um.

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Calculation approachA precise calculation requires careful treatments on

• The space-time evolution of the created hot dense matter

• The propagation of jets in plasma

( interaction between jet + plasma)

• All sources of direct photons

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Space-time evolution of Plasma

),,,,...(,,, zyxBsu

%) ,(d

dn

Described with ideal hydrodynamics in full 3D spaceConstrained with PHOBOS data

Tested with hadrons’ yields, spectra, v2 and particles correlation

For more details, read T. Hirano

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Jets (hard partons)

)ˆˆˆ()(ˆ

ˆ),(),( 2

/2

/AB2

jet

utscdabtd

dsMxGMxGdxdxTK

pdyd

dNbBb

abcdaAaba

t

AB

),()()()( /// AxRxGA

ZxG

A

NxG EKS

apaNaAa

MRST 2001 LO pDIS and EKS98 nuclear modification are employed

)(),2

(),2

(),(30 zy

bxTy

bxT

pd

dNrpf BA

Jet phase space distribution at τ=0:

)(),(),,(),( 00003 Evpptvrpfpdrpfxpf

at τ>0:

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Parton Energy Loss in a Plasma

• Energy loss of parton i=q, g, D: free parameter

• Energy loss per unit distance, by BDMPS

• Every factor depends on the location of jet in plasma , i.e.,

0

))(,())(,,( ),,( 00 rfridrpiE QGP

is EDri / ))(,,( *2

)/8ln()233(

6)(

cfs TTN

T

upE *

)(r

fQGP: fraction of QGP at a given point

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Fix D with pi0 suppression • From pp collisions:

• From AA collisions, parton energy loss is considered

via modified fragmentation function

),(1 20

/2t

2,t

2

0

QzDzpdyd

dNdz

pdyd

dNcc

c

cpp

gqcc

pp

Factorization scale and renormalization scale to be tpQM

functionion fragmentat KKP :),( 20/ QzD cc

),,( 2/ ccc EQzD X.N.Wang’s formula

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Raa(pi0, %) at high pt gives D=1.5

A common Dfor various Centralities!

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Sources of direct photons• Leading Order contr. from primordial NN scatterings

• Thermal contribution

upETExdpdyd

dN

t

**thermal

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thermal

),,(

qqg

gqq

)ˆˆˆ()(ˆ

ˆ),(),( 2

/2

/AB2

)LO(

utscdabtd

dsMxGMxGdxdxT

pdyd

dNbBb

abaAaba

t

AB

2004 al,et Rapp R.

1991 al,et Kapusta:),( *HG TE

AMY/

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*QGP

1

1

2

)(

9

6),( * C

eT

TTE

TEs

Interactions of thermal partonsare inside the rate!

...effect LPM

qqg

qq

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Sources of direct photons• Jet photon conversion

• Fragmentation contribution: similar to pi0 production

Ignored contributions: Medium induced radiation (mainly at low pt )

radiation from pre-equilibrium phase (short time)

qqg

gqq

C

Tg

TETxpfeTE q

qq

S22

*22

2*

JPC

24ln),(

4),(

),( *JPC

42

TExdpdyd

dN

t

JPC

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Results

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Centrality dependent pt-spectra(1)PHENIX data: PRL 98, 012002 (2007) & arXiv:0801.4020

Our predictions coincide with the precise measurement!

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Centrality dependent pt-spectra(2)PRL94,232302(2005)

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Pt spectrum from pp collisions

The PHENIX fit of pp spectrum is used for Raa of thermal photons.

PRL 98, 012002 (2007)

A good test for contributions from leading order +fragmentation without Elossin AA collisions.

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Raa: energy loss

• Data is reproduced within theory uncertainty.

• E loss makes about 40% decrease of total photon production

• Centrality independent ? central and peripheral results differ

by less than 5% with Eloss

by about 20% at intermediate pt w/o Eloss

PRL94,232302(2005);J.Phys.G34, S1015-1018,2007

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Jet quenching & bulk volume

• E loss does play a important role in fragmentation contribution and

jet photon-conversion contribution.

• This is centrality-dependent, similar to the suppression to pi0 production.

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Competition btw different sources

Thermal and LO dominate low and high pt region respectively.Raa is not sensitive to E loss, because of the centrality dependence of them.

When collisions move to perpherial, JPC becomes less importantwhile fragmentation becomes more important .

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Information from Thermal photonsEnergy density at plasma center

• High Temp. from fitting a pt spectrum A higher Temp. plasma

• More yields (shines) of thermal photons A bigger-size (longer-life) plasma.

Raa due to thermal source

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V2 of thermal photons

Contrary to hadronic v2 (ideal hydro predicted increase monotonically), the elliptic flow of thermal photons decrease at high pt!

( Information for the earlier evolution of the plasma?)

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Time evolution

• At initial time there is no transverse flow, so v2 vanishes.

• A big fraction of energetic thermal photons are emitted at early time: More than 50\% at pt=3GeV/c and more than 70\% at pt=4GeV/c within the first 0.3fm/c, though the whole evolution time is about 20fm/c.

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Discussion and Conclusion• Parton energy loss does make 40% decrease of Raa(γ)

• Raa(γ) is independent of centrality (within 5% accuracy) because of

1) the dominance of leading order contribution

2) strong suppression to JPC and frag. contributions due to E-loss

• Thermal photons can provide information of the temperature and size of

the plasma via the slope of pt spectrum and the yields.

• The elliptic flow of thermal photons is predicted to first increase and then

decrease with pt, contrary to hadronic v2, which does not carry the early i

nformation of the QGP.

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Thank you!

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purely leading order calculation:

Isosping mixture and nuclear shadowing make an evident decrease.

Why RAA decreases at high pt?

),()]()(

)([)( /// AxRxfA

zAxf

A

zxf EKS

aNapaAa

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Thermal fraction