photon-hadron correlations at rhic saskia mioduszewski texas a&m university e-m workshop of...
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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?
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-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
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# 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)
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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.
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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).
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
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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
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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