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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