Hard Probes at RHICHard Probes at RHIC

Saskia Mioduszewski

Texas A&M University

Winter Workshop on Nuclear Dynamics

8 April, 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

Trigger Biases

Escaping Jet“Near Side”

Lost Jet“Far Side”

q• Photon trigger is sensitive to full medium

• True measure of the Energy (no energy loss for direct photon)

Renk and Eskola, hep-ph/0610059 Renk and Eskola, hep-ph/0610059

Single inclusive hadrons

Di-jets (hadron-hadron correlations)

Trigger particle

Trigger particle

“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?

8

-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 from 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

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

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

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

• Fragmentation photons – 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)

21

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!

22

Comparison of away-side yields

,assoc,assoc

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

23

IAA of 0

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

24

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

25

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

27

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 show fragmentation function in p+p vs. Au+Au