Photon-Jet Correlations at RHIC

Saskia Mioduszewski

Texas A&M University

18 July, 2007

Outline

• What can be learned from What can be learned from -jet -jet measurements?measurements?

• How to measure?How to measure?

• Current Status of Measurement at RHICCurrent Status of Measurement at RHIC

Why -jet?q

qg

γ q

q g

γ

Compton Annihilation

q

q

q

q

g

g

g

γγ

Bremsstrahlung

“Golden Probe” of energy loss - calibrated probe of jet energy - X.-N. Wang et al.

Gluon jet vs. quark jet energy loss – compare -jet to 0-jet (in -jet, the jet will mostly be from quarks) - need to understand the surface bias to interpret data !

Dominant

What can be learned?

• Ultimate goal is to understand energy loss mechanism

• Complementary to single particle spectra and di-jets

• No surface bias for trigger particle, different surface bias for -jet coincidence

Single-Particle Spectra – What have we learned?

Hadrons are suppressed by factor ~5, Photons are not

Single-Particle Spectra – What have we learned?

Hadrons are suppressed by factor ~5, Photons are not

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

PHENIX 0 Spectrum

Fractional “effective” energy loss Sloss ~ 20% in most central Au+Au collisions

PHENIX preliminary

pedestal and flow subtracted

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

Di-Jets through Hadron-Hadron Correlations

“Disappearance of away-side jet” in central Au+Au collisions

0-5%

Escaping Jet“Near Side”

Lost Jet“Far Side”

STAR, PRL 90 (2003) 082302

“Reappearance of away-side jet”

With increasing trigger pT, away-side jet correlation reappears

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

Higher pT Jet“Near Side”

“Punch-through” Jet?

“Reappearance of away-side jet”

With increasing trigger pT, away-side jet correlation reappears

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

Tangential emission?

Surface Bias of Di-Jets?

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

8 < pT,trig< 15 GeV/c

STAR, Phys. Rev. Lett. 97 (2006) 162301Renk and Eskola, hep-ph/0610059

BDMPS-style energy loss + evolving bulk system

What can be learned?

• Di-jets suffer surface bias of trigger jet, punch-through of away-side (or only tangential emission)

• -Jet trigger photon has no bias, but existence of away-side jet creates a (different) bias

Measurement of % transmission as a function of true jet energy

q

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 from inclusive signal

• Need large-statistics data set for -jet measurement

Statistical measurement of -jet yields

• Measure inclusive -jet correlation function

• Measure 0-jet (-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

Direct -h YieldsRun 5 p+p @ 200 GeV

5-7 X 1-2 GeV/c 7-9 X 1-2 GeV/c

5-7 X 2-5 GeV/c 7-9 X 2-5 GeV/c 9-12 X 2-5 GeV/c

9-12 X 1-2 GeV/c

• Signature small near-side correlation signal apparent

• Yield sensitive to contribution at the near-side

• New corrected results consistent with QM result within systematic error

• Still room for some fragmentation contribution

M. Nguyen, APS 07

PHENIX -jet in Au+Au collisions

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

1/N

trigd

N/d

(rad)

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

per-trigger jet pair yields: systematic from R

systematic from subtraction method

A. Adare, WWND 2007

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

• 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 direct-jet “rich”

• Estimate remaining contamination from 0-jet

• Measure and subtract 0-jet away-side yield

Distinguishing single photons from 2 decay photons in STAR

Shower-max detector

()~(0.007,0.007)

A. Hamed, Collective Dynamics 2007

STAR -jet in p+pSubhasis Chattopadhyay, QM 2006

-- Improved 0/ separation using shower profile in SMD

0

0 bin

mixed bin

photon bin

STAR -jet in p+p

Subhasis Chattopadhyay, QM 2006

EEtttrigger trigger from 8.5GeV/c to 10.5 GeVfrom 8.5GeV/c to 10.5 GeV

Comparison to HIJING p+p –

verification of method

Next Steps

• Do similar analysis for Au+Au collisions as was done in p+p and d+Au

• Challenge within large-multiplicity background

• Run-7 results coming soon

• Results must be interpreted together with di-jet results and/or within model to take surface bias into account

Summary

• Di-Jets’ surface bias results indicate jets are either transmitted with negligible energy loss or absorbed completely

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

• A significant (statistically and systematically) -jet Measurement in Au+Au is needed

• Need a great deal of statistics!• Stay tuned for the results from Run-7….

Increase in statistics over Run-4 Au+Au > factor of 10