1

Surface （表层） versus volume （深层） emission

in photon-hadron correlations

Han-Zhong Zhang

Institute of Particle Physics, Huazhong Normal University, China

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

The international workshop for QCD/HIC July 10-12 , 2008

I. IntroductionII. Analysis on photon-hadron correlationsIII. Conclusion

2

I. Introduction

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)

What happens for a jet propagating inside QGP?

3

Three kinds of hard probes of QGP

1) Single jet Single hadron spectra

2) Dijet Hadron-triggered away-side hadron spectra

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

Single jet Dijet Gamma-jet

?H.Z. Zhang, J.F. Owens, E. Wang and X.-N. Wang , PRL 98(2007)212301

4

Gamma-jets were suggested for studying jet energy loss in dense matter. X. -N. Wang, Z. Huang,

and I. Sarcevic, PRL 77(1996) 231-234.

• The NLO study of the photon-triggered away-side hadron spectra will help to obtain the detailed picture of jet quenching in the whole $z_T$ region.

• The sensitivity of Gamma-jets to probe the dense matter.

)(

)()(./

/

/1)(

Tpp

TAATAAT

hTT

TAAT

hhTT

hAAThT

T

hAA

AATAA

zD

zDzIppzwhere

dydpddydp

ddydydpdpdpddydydp

dz

dN

NzD

Motivation

5

Gamma-jet by NLO pQCD parton model

LO: NLO corrections: (e.g. 23)

FFsdPDFsTd ABAA

GamT

JetT pp

GamTp

JetTp

GamT

JetT

GamT

JetT

GamT

JetT

pp

pp

pp

,

,GamTp

1JetTp

2JetTp

),( 21 JetT

JetT

JetT ppofOnep

therefore leading to hadrons with transverse

momentum larger than that of the photons

6

The fragmentation of the jets off the dense matter

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

),()],(/),()[1(),,( 20/

/2'0/

'2'0

/

'/2

/ cch

Lggh

c

gcch

c

cLccch zDezD

z

zLzD

z

zeEzD

,//),/( ''cTgcTcTc EpLzEppz

medium vacuum

0000

/1),,,(0

0

nrb

dLg

L

7

II. High p_T photon-triggered away-side hadron spectra within a NLO pQCD parton model in heavy ion collisions

High p_T photon sources in p+p:

1) Direct photon from hard scattering

Annihilation Compton 23 one-loop

LO NLO

J. F. Owens, Rev. Mod. Phys. 59, 465(1987);H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)

8

2) Fragmentation (bremsstrahlung) contributions(accompanied by nearly collinear hadrons on the same side)

J. F. Owens, Rev. Mod. Phys. 59, 465(1987);H. Baer, J. Ohnemus, and J. F. Owens, Phys. Rev. D. 42, 61(1990)

Most accompanying hadrons arewithin a cone of half-angle coneR

9

isolation cuts (IC):

Isolated photons in p+p at RHIC

Because of IC selected at

RHIC, most fragmentation

contributions from parton

jets are taken out.

1.0/.,5.0 TT

cone pEradR

The left are mainly from

annihilation and Compton

processes, direct photon.

PRL 98 (2007) 012002

10

If we only consider the events where the photon has no nearly collinear hadrons accompanying on the same side, high p_T photon/photon-hadron will be dominated by annihilation and Compton processes.

Only consider on annihilation and Compton photons !!!

Focus on isolated photons now

No consideration for energy loss of jets fragmentated into photons in AA.

The left direct photons don’t encounter energy loss. Quenching picture is simply and clearly exhibited by

the correlated parton jets.

11

Turbide, Gale, Jeon, Moore, Phys. Rev. C. 72 (2005) 014906

High p_T direct photon dominatesin central Au+Au at RHIC

Annihilation and Compton processes dominate for high p_T photons in AA.

12

Per-trigger yield for photon-hadron production in p+p

Data from “Matthew Nguyen for PHENIX, talk at QM2008”

dydp

dNdy

dydpdydp

dNdydpdy

T

pp

hhTT

hpp

hhT

Per-trigger yield as a

function of the p_T of

the triggered photon:

NLO pQCD results describe

the behavior of the data for

photon-hadron produced in

p+p at 200GeV

13

Qualitatively, Iaa in small z_Tregion is slightly more sensitiveto epsilon_0 than Iaa in large z_Tregion.

LO

Per-trigger yield for photon-hadron in central Au+Au

NLO

Why?

)(

)()(./

/

/

1)(

Tpp

TAATAAT

hTT

TAAT

hhTT

hAAThT

T

hAA

AATAA

zD

zDzIppzwhere

dydpddydp

ddydydpdpdpddydydp

dz

dN

NzD

14

T

hTT ppz

NLO Nh > 0 at z_T>1: surface emissionvacuumch

Lmediumch

LAAch DeDeD /

//

// )1(

At large z_T:medium contributions vanishdue to jet quenching, dominated by vacuum contributions.

9.0Tz

0

For LO, the jet’s energy can’t exceed the gamma’s energy, no contributions for z_T>1 region.

For NLO, because of contributions from 2->3 processes, the jet’s energy can exceed the gamma’s energy, have z_T>1 contributions.

15

T

hTT ppz

For small z_T: Volume emission

vacuumch

Lmediumch

LAAch DeDeD /

//

// )1(

At small z_T: both contribute

3.0Tz

16

The averaged distance <L> for the gamma-triggered parton jets passing through the quark matter.

Surface versus Volume emission

Small zt probes the matter deeper than large zt, so more sensitive.

Surface emission

Volume emission

3.0Tz

9.0Tz

17

Data from “A. Hamed for

STAR, talk at QM2008”

18

Single hadron Dihadron Photon-hadron

More sensitive probe?

NLO

19

Small-zt gamma-jets vs single jets

Gamma-jet Single jetsmall zT

Gam-jets for small zt probes the matter deeper than single jets.

3.0Tz 9.0Tz

20

Small-zt gamma-jets vs dijets

Gamma-jet Dijetsmall zT

Because of punch-through jets for dihadrons, it is not sure that small-zt gam-jets are more sensitive than dijets.

3.0Tz

9.0Tz3.0Tz 9.0Tz

21

Comparisons between gamma-h and dihadron in pp/AuAu

T

hh

hTrigT

h

Trig dz

dN

Nvs

dz

dN

N

11

)()(

)()(

ThAuAuT

hhAuAu

ThppT

hhpp

zDzD

zDzD

e.g. Trig=8GeV, zt=0.5

hadr:8 jet:12 jet:12 assoc: 4

gamm:8 jet:8 assoc: 4

p+p: Per trigger

22

Data from “A. Hamed for

STAR, talk at HP2008”

T

hh

hTrigT

h

Trig dz

dN

Nvs

dz

dN

N

11

)()(

)()(

ThAuAuT

hhAuAu

ThppT

hhpp

zDzD

zDzD

e.g. Trig=8GeV, zt=0.5

hadr:8 12 12 assoc: 4

gamm:8 8 6 assoc: 4

Au+Au: Per trigger

Volume emission

Tangential ~pp

Comparisons between gamma-h and dihadron in pp/AuAu

23

III. Conclusions

1) The suppression factor for hadrons with large z_T is controlled mainly by the surface emission of the gamma-jet events, while small z_T region will be volume emission bias.

2) Gamma-jets for small z_T region probe the dense matter deeper than those for large z_T region, so the gamma-jets for small z_T region are slightly more sensitive to the dense matter properties.

Thank for your attention!

24

Thank for your attention!

谢谢！

25

Dominated by jets close and perpendicular to the surface

Dominated by dijets close and tangential to the surface and the

punch-through dijets

dihadron

Color strength: single/dihadron yield from the jets originating from the square

Thickness of the outer corona

single hadron

Spatial transverse distribution of the initial parton production points that contribute to the single and dihadron along a given direction at RHIC

25% contribution

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1) No parton jet energy loss

2) Isospin effects

3) Shadowing effects

NLO direct photon in central Au+Au at RHIC

),(1),(),,(),,( 2

/2

/2

/2

/ xfA

Zxf

A

ZrxSrxf napaAaAa

Data from “PRL. 98 (2007) 012002”

27

For large z_T,

vacuumch

mediumchLL

vacuumch

vacuumch

Lmediumch

L

ppch

AAch

AA

vacuumch

Lmediumch

LAAch

D

Dee

D

DeDe

D

DI

DeDeD

/

///

/

//

//

/

/

//

//

/

)1(

)1(~

)1(

/

/

///

//

)1(

0)(,0)(

LLOAA

vacuumch

mediumchLLNLO

AA

NLOmediumch

LOmediumch

eI

D

DeeI

DD

“NLO I_AA ” > “LO I_AA” for large z_T

28

Parton jet energy loss per unit length:

),(5.0

),/5.7/()6.1/( 02.1

01

partpartd

NsdN

dNEE

dL

dE

E. Wang , X. -N. Wang , PRL87(2001)142301)

B. B. Back for PHOBOS, nucl-ex/0604017v1

),(

),(

0

0

part

part

Nsf

Nsf

Initial gluon density coefficient

Energy loss parameter

),(5.0

),(

00

partpart

part NsdN

dNNsf

(a parameterization form of theory calculations)