Photon-hadron Correlations Measured in Au-Au Collisions

at √sNN=200 GeV with the

PHENIX Detector

Megan Connors Stony Brook University

For the Phenix CollaborationAPS Meeting April 13, 2008

M. Connors APS Meeting 4/13/082

Photon tagged jets as probes

Photons are excellent probes because they do not interact with the strongly coupled medium

RAA≈1 implies no medium effect

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

M. Connors APS Meeting 4/13/083

Photon Sources

q

qg

q

q g

γ

Compton Annihilation

γ

At high pT direct

photons dominate! π0 suppression

Hadron decays

Prompt photons Compton scattering Quark anti-quark annihilation

Fragmentation Photons Final state partons

Jet conversions Thermal radiation

π0 γγ

Medium Induced

M. Connors APS Meeting 4/13/084

γ-jets

q

qg

γ

?

M. Connors APS Meeting 4/13/085

What are 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 γ h

hγ

hγ

PbSc

Electromagnetic Calorimeters

PbGl

Event Mixing

M. Connors APS Meeting 4/13/086

Determining the Direct Photon Yield

0-h decayed via MC to get decay -h

YN

dN

dh

1

( )

pT [GeV]R

For Singles: Nincl = Ndirect + Ndecay

Y Y Yincl

NN direct

N

N decayd irect

incl

decay

incl

RN

NNN

incl

decay

d irect

decay 1 YR Y Y

Rd irect

incl decay

1

For Pairs:

π0 pT [GeV]

M. Connors APS Meeting 4/13/087

γ-h Correlation for p+p

Inclusive

Decay

Direct γ-h

Run 5 p+p 200 GeV

YR Y Y

Rd irect

incl decay

1

M. Connors APS Meeting 4/13/088

Subtraction technique works over wide range of pT!

Direct γ-h Correlations in Run 6 p+p

[rad]

1/N

trig

dN

/d

(A

.U.)

pT, hadron

pT,

ph

oto

n

Little or no near-side

production associated with direct

photon triggersObserve

opposing jet contribution in

away-side yields

M. Connors APS Meeting 4/13/089

Additional challenges and sources of uncertainty in Au+Au analysis: underlying event subtraction (ZYAM) and elliptic flow subtraction

Working to reduce uncertainties and finish analyzing remaining statistics

Little or no near-side

production associated with direct

photon triggersAway-side

yields indicate large jet

suppression in +jet channel

0

7-9 X 2-3 GeV7-9 X 1-2 GeV 7-9 X 3-5 GeV

Direct γ-h Correlations in Run 7 Au+Au

M. Connors APS Meeting 4/13/0810

Higher pT

Although the production rate is lower at higher pT, the direct photon signal to background ratio (Rγ) improves.

The accuracy of pQCD calculations also improves at higher pT.

pT, γ

Run7 Au+Au 200 GeV

Near/Away-Side > 1

Run7 Au+Au 200 GeV

Near/Away-Side ≈ 1

12-15 X 3-5 GeV7-9 X 3-5 GeV

Inclusive-h

M. Connors APS Meeting 4/13/0811

Summary

Photon tagged jets are important probes of the medium Expect photon-hadron correlations to be more

sensitive to modified fragmentation function Run 6 provides baseline measurements for

comparisons Observed suppression of away side jet in

early Run 7 AuAu γ-h analysis Measurements extend over large range of pT

M. Connors APS Meeting 4/13/0812

Plans

Integrate away-side yields Run 6 provides baseline measurements for determining

fragmentation function Observe modifications to the fragmentation function

from Run 7 AuAu γ-h Polished results coming soon!

zp

phadron

jet

D zN

dN z

dzqevt

( )( )

1

γ energy ≈ jet energy

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

Back up Slides

M. Connors APS Meeting 4/13/0814

Theoretical Predictions

Pt spectrum of charged particles from fragmentation in the opposite direction of a photon tagged jet

Ratio of fragmentation functions, IAA, with and without energy loss

Wang et. al. Phys. Rev. Lett. 77 (1996) 231-234

2/8/2008Quark Matter 200815 2/8/2008Quark Matter 200815

Constructing the Decay Background To start: Construct (0) - h

from 0-h Problem is that tagging the

0’s introduces a bias

<pT> tagged = <pT> true

Developed a pair-by-pair correction procedure

Per-Trigger Yield from 0):

07

5

( , )T T TP p p dp

Correction Procedure:

Decay kinematics reproduced by Monte Carlo include energy resolution effects and acceptance (P)This is just for 0’s, What about contributions from other mesons? This approach implicitly assumes they have the same associated yield

00 0

00

0

( )d, , ,T

hhTT T TT TT p YPY pp p p ppp

000( ) (( )TrueTagged

)

TT T

d dNdN

dp

N

d dpp

– Analytic Form – w/ Detector Response

Sample MC weight function: