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Gamma-jet tomography of high-energy nuclear collisions in NLO pQCD

Han-Zhong Zhang

Institute of Particle Physics, Huazhong Normal University, China

Collaborators: Enke Wang, J. Owens and X.-N. Wang

Weihai, 08-14 August, 2009

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H. Z. Zhang, J. Owens, E. Wang and X. –N. Wang, Phys. Rev. Lett, 103(2009)032302; 98(2007)212301

OUTLINE

Jet Quenching Gamma-jet in NLO pQCD Same (energy loss) formalism for

calculating gam-hadr, single/dihadron Gamma-jet tomography: volume vs

surface emission Conclusions

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Jet quenching:

The hard jet loses a significant amount of its energy

via radiating gluon induced by multiple scattering.

hadrons

q

q

hadrons

leadingparticle

leading particle

N-N collision

hadrons

q

q

hadrons

Leading particle suppressed

leading particle suppressed

A-A collision

X.-N.Wang and M.Gyulassy, Phys.Rev.Lett.68,1480(1992)

A powerful tool for the study of Quark Matter

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Three kinds of hard probes of Quark Matter

1) Single jet Single hadron spectra

2) Dijet Hadron-triggered away-side hadron spectra

3) Gamma-jet Photon-triggered away-side hadron spectra

Single hadron Dihadron Gamma-hadron

STAR Pre.

Experimental studies of jet quenching, suppression observed

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Single and di-jet tomography

Single HadronDihadronH. Z. Zhang, J. Owens, E. Wang and X. –N. Wang,

PRL98(2007)212301

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Gamma-jet probe

GamTp

1JetTp

2JetTp

GamTp

JetTp

LO (tree level)

GamT

JetT pp Gam

TJetT pp

NLO corrections: (e.g. 23)

PQCD parton model:

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H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)

Most accompanying hadrons arewithin a cone of angle radius coneR

Try to eliminate fragmentation photon

An “isolation” cut (IC) is often applied on the electromagnetic signal to separate the direct photons from other sources.

22 )()( coneR

Jet

Gamma

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With IC, the fragm. only 10%.

Data :

1.0/.,5.0 TT

cone pEradR

PHENIX, PRL 98 (2007) 012002

“the measured photon samples … are expected to be isolated from parton jet activity.”

Inclusive photons

we will focus mainly on photons with isolation cuts

Direct vs. fragmentation photons

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The effective fragmentation functions

The jet energy loss in a 1D expanding system:

),,(0

000

0

1

nrbd

dL

dEE g

L

dc

)/5.7/()6.1/( 02.1

001

EEdL

dE

d

Energy loss parameter

(a parameterization form of theory calculations) Enke Wang , X. -N. Wang , PRL87(2001)142301)

(X. -N. Wang , PRC70(2004)031901)

r

),(,11

11),,( 20

//20

//2

/ cch

Lcch

Lccch zDe

z

zD

zeEzD

Tcc pEz /

in medium in vacuum

0000

/1),,,(0

0

nrb

dLg

L

q̂0

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Jet energy loss is dependent on its transverse momentum and initial space origination, a function of the azimuthal angle, the passing distance, the medium particle density along the trajectory

Track any jet inside the medium

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dihadronsingle hadron

determined in a same energy loss formalism 0in most central Au+Au

fmGeV /1.25.10

Actually we will choose epsilon_0 1.68GeV/fm for gam-hadr

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Hard sphere versus Woods-Saxont(

r)

r/fm

dze

rt

RrR

Art

crzr

WSA

HSA

/)(

0

222

022

1)(

/12

3)(

A hard sphere overlap geometry differs at most about10% from a Wood-Saxon geometry.

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1) Same model: NLO pQCD parton model

2) Same geometry: hard sphere

3) Same PDFs in vacuum: CTEQ6M

4) Same FFs in vacuum: KKP

5) Same energy loss parameterization

and =1.68GeV/fm

Calculate gamma-hadron in the same formalism as for single/dihadron in HIC

0

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D_AA is a sum of FF’s of the away-side jets (quark and gluon) weighted with the fractional gamma-jet cross sections.

Sometimes we call D_AA as

the photon-triggered fragmentation functions.

ThTT

TABT

hhTT

hABThT

T

hAB

ABTAB

ppzwhere

dydpddydp

ddydydpdpdpddydydp

dz

dN

NzD

/

/

/1)(

The per-trigger photon-hadron spectra

Xba

Xbach

AAd

dDD

ˆ

ˆ~

~/

Gamma-triggered hadron spectra

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1) Fit data very well in p+p for different values of p_T^trig.

2) Fit data well in Au+Au. Agreement is not nontrivial. It reinforces the success of the parton energy loss picture.

3) Hadron-triggered FF’s > Photon-triggered FF’s.

H.Z. Zhang, J.F. Owens, E. Wang

and X.-N. Wang ,

Phys. Rev. Lett, 103(2009)032302

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Mainly because the fraction of hadron-triggered gluon jets is larger than the fraction of photon-triggered gluon jets at same pt^trig, And the hadron yield of gluon jets is larger than that of quarks.

50%

10%

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More stronger dependence of on

1) In LO energy balance limits for gam-hadr

2) NLO radiative correction permits for G-hadr. The two effects cause NLO very different from LO . NLO needed

3) Compared with h-h , Gam-hadr has more dependence of on .

)(/)()( TppTAATAA zDzDzI

1Tz

1Tz

TzhAAI

hhAAI

Quantify suppressions of hadron- and gamma-triggered FF’s in A+A relative to p+p collisions due to JQ

hAAI

hAAI

TzhAAI

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Large : more susceptible to Eloss --- Surface emission

1) Even a small amount energy loss can greatly suppress the large- gam-hadr yield.

2) Only those jets originating near and escaping through the surface will contribute without energy loss.

3) Large- is mainly determined by the thickness of the corona of the surface emission.

Tz

9.0Tz

Tz

hAAI

Tz

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1) For contributing to small- gam-hadr, high energy jet is “wealthy” enough to lose finite energy, and can therefore originate near the center region, --- volume emission.

2) Intermediate- gam-hadr are determined by the competition of the two emission mechanism.

Tz

Tz

Small : encounter more Eloss --- Volume emissionTz

3.0Tz

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Different centrality dependence of the suppression for different values of z_T

1) z_T < 0.6, volume emissions dominate, weaker dependence.

2) z_T > 1, surface emissions dominate, stronger dependence

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Conclusions

• Gamma-hadron correlation described well by NLO pQCD• Gamma-hadron suppression consistent with single hadron and dihadron suppression• Volume & surface emission in different region of kinematic (small and large z_T)• Gamma-jet study toward true tomography of dense matter

Thank for your attention!

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Gamma-jet by NLO pQCD parton model

LO pQCD

FFsdPDFsTd ABAA

NLO pQCD

J. F. Owens, Rev. Mod. Phys. 59, 465(1987)