Hard Probes at RHIC

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Hard Probes at RHIC

• Saskia Mioduszewski
• Texas AM University
• Winter Workshop on Nuclear Dynamics
• 8 April, 2008

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Outline

• Review of High pT Probes
• Motivation for g-jet Measurement
• Experimental Methods
• Current Status of Measurement

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Single-Particle Spectra
High-pT suppression comparison to theory
GLV ? dNg/dy 1000 I. Vitev
and M. Gyulassy, PRL 89 (2002) 252301
? Initial energy density e0 15 GeV/fm3
Hadrons are suppressed, photons are not photons
serve as the control experiment
PHENIX, Phys. Rev. Lett. 96, 202301 (2006)
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From Single-Particle Spectra to 2-Particle
Correlations
Single inclusive hadrons
Di-jets (hadron-hadron correlations)
Suppression quantified by RAA
YieldAA/(Yieldpp ltNbinarygtpp) 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
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Trigger Biases
Di-jets (hadron-hadron correlations)
Single inclusive hadrons
Renk and Eskola, hep-ph/0610059
Renk and Eskola, hep-ph/0610059
Trigger particle
Trigger particle

• Photon trigger is sensitive to full medium
• True measure of the Energy (no energy loss for
  direct photon)

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Reappearance of away-side jet
With increasing trigger pT, away-side jet
correlation reappears
STAR, Phys. Rev. Lett. 97 (2006) 162301
4 lt pT,triglt 6 GeV/c, 2lt pT,assoclt pT,trig
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Surface Bias of Di-Jets?
STAR, Phys. Rev. Lett. 97 (2006) 162301
8 lt pT,triglt 15 GeV/c
Renk and Eskola, hep-ph/0610059
8 lt pT,triglt 15 , 4lt pT,assoclt 6 GeV/c
Tangential emission?
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?-Jet Golden Probe of Energy Loss
Wang et al., Phys.Rev.Lett. 77 (1996) 231-234
?
?
q
g
q
QCD analog of Compton Scattering
h

• ? 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)

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Challenging Measurement

• Trigger photon measured with electro-magnetic
  calorimeter
• Large background from hadronic decays (p0, h)

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

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

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g-jet Analysis in AuAu collisions
Thomas Dietel, Quark Matter 2005
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Challenging Measurement

• Trigger photon measured with electro-magnetic
  calorimeter
• Large background from hadronic decays (p0, h)
• Background signal is di-jet (p0-jet)
• Subtract from true signal
• Need large-statistics data set for g-jet
  measurement

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Statistical measurement of g-jet yields

• Measure inclusive g-jet correlation function
• Measure p0-jet correlation function
• Construct resulting gdecay-jet correlation taking
  into account decay kinematics
• Subtract gdecay-jet from ginclusive-jet
• ? Result is gdirect-jet

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PHENIX g-jet in pp
J.Jin, QM 2006
PHENIX Preliminary
PHENIX Preliminary
Comparison to PYTHIA verification of method
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PHENIX g-jet in AuAu collisions
Black AuAu Red pp
J.Jin, QM 2006
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Away-side integrated yields
J. Jin, QM 2006

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

• pp shows a systematic trend of having higher
  yields in the
• away side than AuAu!

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CuCu gdirect-h/- vs. p0-h/- jet yields
A. Adare, WWND 2007
per-trigger jet pair yields
systematic from Rg systematic from subtraction
method
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Distinguishing direct photons from p0

• Segmentation of calorimeter towers in STAR
  (Dj,Dh) (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

Opening angle of p0 as a function of pT
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Distinguishing direct photons from p0

• Reject clusters that look like 2 close photons
  (from p0 decay) from shower shape
• Retain sample of correlations that are grich-jet
  
• Estimate remaining contamination from p0-jet from
  near-side yields
• Measure and subtract p0-jet away-side yield

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Backgrounds

• Asymmetric p0 decays (10 of p0 for pTgt8 GeV/c)
• h decays (5 contribution in central AuAu
  collision, as much as 25 in pp)
• Fragmentation photons subtracted to the extent
  that correlations are similar to p0-triggered
  correlations

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Distinguishing single photons from 2 decay
photons in STAR
Shower-max detector
(Dh,DF)(0.007,0.007)
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Comparison of Near-side yields (?0-charged versus
charged-charged PRL 97(2006)162301)
Similar near-side yields for pp, dAu and AuAu
Agreement between charged-charged and
p0-charged correlations
,assoc
Important cross-check that shower-shape PID for
p0 is good!
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Comparison of away-side yields
Smaller away-side yield in AuAu compared to pp
and dAu.
,assoc
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IAA of ?0
IAA of the away-side in ?0-trigger shows similar
suppression for all pT associated bins (3-8
GeV/c).
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Extraction of direct?? Away-Side Yields
Assume no near-side yield for direct ?  then
the away-side yields per trigger is calculated as
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Direct-? Away-Side Yields
The away-side yield of the associated particle
per trigger in ?-jet is suppressed.
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First measure of away-side IAA for g-h
A. Hamed, QM2008
Ejet E? E trig
Good agreement between theory and measurement
T. Renk and K. Eskola PRC75054910,2007
Suppression similar level to inclusives in
central collisions
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Away-side Yields Relative to Peripheral AuAu
A. Hamed, QM2008
Peripheral AuAu pp vacuum
Icp of ?-jet exhibits same suppression on the
away-side yield per trigger of the associated
particles (3-8GeV/c).
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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

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Summary

• Both STAR and PHENIX have measured a correlation
  function for direct gjet
• Preliminary associated-hadron yields have been
  measured
• Large statistics necessary
• Next step is to show fragmentation function in
  pp vs. AuAu