Photon-Hadron Correlations at Photon-Hadron Correlations at RHICRHIC

Saskia Mioduszewski

Texas A&M University

E-M Workshop of RHIC/AGS Users’ Meeting

27 May, 2008

Outline

• Review of High pT Probes

• Motivation for -jet Measurement

• Experimental Methods

• Current Status of Measurement

Single-Particle Spectra

Hadrons are suppressed, photons are not – photons serve as the “control” experiment

High-pT suppression comparison to theory: GLV dNg/dy ~ 1000 I. Vitev and M. Gyulassy, PRL 89 (2002) 252301

Initial energy density: 0 ~ 15 GeV/fm3

PHENIX, Phys. Rev. Lett. 96, 202301 (2006)

From Single-Particle Spectra to 2-Particle Correlations

Single inclusive hadrons Di-jets (hadron-hadron correlations)

Suppression quantified by RAA =

YieldAA/(Yieldpp *<Nbinary>pp)Single particle is leading

hadron of jet

Probe of density of medium, but

Strong surface bias for “trigger” particle

Suppression quantified by IAA =

Jet-assoc. YieldAA/Jet-assoc. Yieldpp

Hadrons associated with high-pT trigger particle

More differential probe of energy loss Surface bias reduced, but not

removed

Trigger

“Reappearance of away-side jet”

With increasing trigger pT, away-side jet correlation reappears

4 < pT,trig< 6 GeV/c, 2< pT,assoc< pT,trig

STAR, Phys. Rev. Lett. 97 (2006) 162301

Surface Bias of Di-Jets?

Renk and Eskola, hep-ph/0610059

8 < pT,trig< 15 , 4< pT,assoc< 6 GeV/c

8 < pT,trig< 15 GeV/c

STAR, Phys. Rev. Lett. 97 (2006) 162301

Higher pT Jet“Near Side”

“Punch-through” Jet?

Tangential emission?

7

-Jet: “Golden Probe” of Energy Loss

• emerges unscathed from the medium- This probe is valuable for comparison with di-hadron correlations - Modification as a function of “true” parton energy real fragmentation

function D(z)

h

q

g

q

QCD analog ofCompton Scattering

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

Challenging Measurement

• Trigger photon measured with electro-magnetic calorimeter

• Large background from hadronic decays (0, )

• Background sources of photons (hadronic decays) suppressed in central Au+Au collisions

PHENIX, Phys. Rev. Lett. 94, 232301 (2005)

-jet Analysis in Au+Au collisions

Thomas Dietel, Quark Matter 2005

Challenging Measurement

• Trigger photon measured with electro-magnetic calorimeter

• Large background from hadronic decays (0, )

• Background signal is di-jet (0-jet)

• Subtract true signal

• Need large-statistics data set for -jet measurement

Statistical measurement of -jet yields

• Measure inclusive -jet correlation function

• Measure 0-jet correlation function

• Construct resulting decay-jet correlation

taking into account decay kinematics

• Subtract decay-jet from inclusive-jet

Result is direct-jet

PHENIX -jet in p+p

PHENIX Preliminary PHENIX Preliminary

J.Jin, QM 2006

Comparison to PYTHIA – verification of method

hdecYhincRYRhdir

Y

11

# inclusive # inclusive ##decay decay

R =

PHENIX -jet in Au+Au collisions

J.Jin, QM 2006 Black: Au+Au Red: p+p

1/N

trigd

N/d

(rad)

Away-side integrated yields

• trigger pT dependence• assoc. h pT dependence• centrality dependence + pp

• p+p shows a systematic trend of having higher yields in the away side than Au+Au!

J. Jin, QM 2006

Cu+Cudirect-h+/- vs. -h+/- jet yields

per-trigger jet pair yields: systematic from R

systematic from subtraction method

A. Adare, WWND 2007

h- Correlations

10-

6

X10-6

X10-3

X10-3

h-inclusive

h-decay

h-direct

- Idea: By triggering on a hadron and looking for near-side direct photon partners one can measure the fragmentation photon yield directly

- Measure hadron - inclusive and hadron - decay correlations

- Decay corr’s are made by tagging 0 and by invariant mass

- Must know tagging efficiency and false tagging rate precisely dominant source of systematic uncertainty

2.5 < pT, < 3.5

M. Nguyen, QM2008

h- Correlations IIM. Nguyen, QM2008

Nfrag/Ninc ~0.1

Distinguishing direct photons from 0

pT (GeV/c)

Op

enin

g a

ng

le (

rad

ian

s)

Opening angle of 0 as a function of pT

• Segmentation of calorimeter towers in STAR () = (0.05,0.05) 2 decay photons hit same tower starting pT ~ 5 or 6 GeV/c Use SMD to discriminate

• Segmentation in PHENIX ~ 0.01 radians

Distinguishing direct photons from 0

• Reject clusters that look like 2 close photons (from 0 decay) from shower shape

• Retain sample of correlations that are rich-jet

• Estimate remaining contamination from 0-jet from near-side yields

• Measure and subtract 0-jet away-side yield

Backgrounds

• Photons from asymmetric 0 decays (~10% of 0 for pT>8 GeV/c)

• decays (~5% contribution in central Au+Au collisions, as much as 25% in p+p)

• Fragmentation photons – PHENIX measures ~10% of inclusive photons in p+p collisions

- subtracted to the extent that correlations are similar to 0-triggered correlations

Distinguishing single photons from 2 decay photons in STAR

Shower-max detector

()~(0.007,0.007)

22

Comparison of Near-side yields (0-charged versus charged-charged PRL 97(2006)162301)

,assoc

Similar near-side yields for p+p, d+Au and Au+Au

Agreement between charged-charged and 0-charged correlations

Important cross-check that shower-shape PID for 0 is good!

23

Comparison of away-side yields

,assoc,assoc

Smaller away-side yield in Au+Au compared to pp and d+Au.

24

IAA of 0

IAA of the away-side in 0-trigger shows similar suppression for all pT associated bins (3-8 GeV/c).

25

Extraction of direct Away-Side Yields

R=Y-rich+h/Y0+h

near near

Y+h = (Y-rich+h - RY0+h )/1-Raway away

Assume no near-side yield for direct

 then the away-side yields per trigger is calculated as

26

Direct- Away-Side Yields

The away-side yield of the associated particle per trigger in -jet is suppressed.

Suppression similar level to inclusives in central collisions

First measure of away-side IAA for -h

T. Renk and K. Eskola PRC75:054910,2007

Good agreement between theory and measurement

Ejet = E = E trig

A. Hamed, QM2008

collisions pp

collisions AA

YieldAssociated

YieldAssociated

)E,(zD

)E,(zDI trig

TTpp

trigTTAA

AA

28

Away-side Yields Relative to Peripheral Au+Au

Icp of -jet exhibits same suppression on the away-side yield per trigger of the associated particles (3-8GeV/c).

A. Hamed, QM2008

AAPeripheral

AACentral

YieldAssociated

YieldAssociated

)E,(zD

)E,(zDI trig

TTAAeripheral

trigTTAA entral

CP

P

C

Peripheral Au+Au ~ p+p = vacuum

Summary

• Correlation measurements provide more differential probe of energy loss than single-particle pT spectra

• Di-Jets (those that are observed) may have less surface bias

• Photon-Jet Measurement will complement the di-jet for more complete probe

Summary

• Both STAR and PHENIX have measured a correlation function for direct +jet

• Preliminary associated-hadron yields have been measured

• Large statistics necessary

• Next step is to extract fragmentation function in p+p vs. Au+Au