1

Direct Photon-Hadron Correlations

in Au+Au at 200GeV

Megan ConnorsFor the PHENIX Collaboration

Quark Matter 2009

2 M. Connors

Photon tagged jets as probes Photons do not interact

strongly

RAA≈1 implies no medium effect

γ-h correlations should be more sensitive to modified fragmentation function and energy loss than h-h γ energy ≈ jet energy

3 M. Connors

Photon Sources

q

qg

q

q g

γ

Compton Annihilation

γ

At high pT promptphotons dominate!

π0 suppression

Hadron decays Direct photons

Compton scattering 2 parton annihilation

Fragmentation Photons Final state partons

Jet-thermal annihilation Thermal radiation Bremsstrahlung

π0 γγ

Medium Induced

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γ-jet Correlations

q

qg

γ

?

5 M. Connors

Measurements with Photon Tagged Jets

Also can measure IAA, ratio of the direct photon-hadron yields

zp

phad ro n

je t

D zN

dN zdzq

evt

( )( )

1

γ energy ≈ jet energy

X.N. Wang, Z. Huang PRC55 (1997) 3047-3061

IAA = YAA/Ypp~DAA(z)/Dpp(z)

6 M. Connors

Statistical Method for Determining the Direct Photon Yield

0-h decayed via MC to get decay -h

YN

dNd

h

1

( )

pT [GeV]

R

For Singles: Nincl = Ndirect + Ndecay

Y Y Yinc lNN direct

NN deca y

d irect

in cl

decay

in cl

R NN

NN

incl

decay

d irec t

decay 1YR Y Y

Rd irect

incl decay

1

For Pairs:

π0 pT [GeV]

7 M. Connors

γ-h Correlation Inclusive

Decay

Direct γ-h

YR Y Y

Rd irect

in cl deca y

1

8 M. Connors

Away-side Yields Measurements have shown suppression of

awayside (PHENIX arXiv:0903.3399)

Run 7~4X statistics

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Run 7 Measurements Suppression and modified fragmentation

function

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How much suppression? Ratio to p+p yields Average from Run4: IAA=0.32±0.12±0.09

arXiv:0903.3399

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Modified Fragmentation Function

zppT

Th

T

D zN

dN zdzq T

evt

T

T( )

( )

1

γ pT ≈ jet pT

IAA ~DAA(zT)/Dpp(zT)

12 M. Connors

Surface emission? NLO, Longitudinal expansion Modification at low zT and suppression at high zT Zhang et al arXiv0902.4000v1

High zT

Low zT

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Effect of Fluctuations

Priv. comm.

LO, 3D hydro ASW Includes

probability function instead of an average energy loss

Result: Reduces the

geometric dependence

Flatter IAA vs zTT. R. and K. Eskola, Phys. Rev. C 75 (2007) 054910;T. R., Phys. Rev. C 78 (2008) 034904

14 M. Connors

Enhanced Low zT Multiplicity

ln

1z T

Analytic MLLA Radiative Energy

loss results in enhancement of soft particle multiplicity

Ratio of medium/vacuum curves is IAA

Borghini and Wiedemann arXiv:hep-ph/0506218

ln

1z T

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The Comparison Exhibits the

biggest zT dependence

Expanded awayside integration region

Borghini and Wiedemann arXiv:hep-ph/0506218

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Compare to 0-h Similar suppression in direct -h and 0-h!

See A. Adare’s Posterz

ppT

Th

Ttrig

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Fragmentation Function Measurement of modified FF in Au+Au compared

to p+p

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Slope comparisondNdz

N eT

bz T

Fit a universal curve for all jet energies

p+p slope of 6.89 ± 0.64 more consistent with quark fragmentation of b=8 than b=11 for gluons

Slope of Au+Au is 9.49 ±1.37

Au+Au slope is ~1.3σ higher than p+p

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Near Side Story Approximately zero direct γ–h yield in Au+Au No enhancement compared to p+p direct γ–h

20 M. Connors

Conclusions Moving toward precision measurements of

Energy loss Similar suppression as seen in 0-h Au+Au zT distribution appears steeper

than p+p No enhanced nearside yield observed

Not yet the final word on direct γ-h

21 M. Connors

How we measure γ–h correlations

Trigger on a high pT (>5GeV/c) inclusive photon Measure between photon and all hadrons

in the event within a certain pT range Event mixing corrects for acceptance and

combinatorial pairs

Δφ

Drift Chamber

Per-

Trig

ger Y

ield

Underlying Event =Combinatorial Bknd +Elliptic Flow

PbSc

Electromagnetic Calorimeters

PbGl