Transcript 2_draft_QM09_gamma_jet_Peter

The STAR Experiment
Quark Matter 2009
Knoxville, TN
Texas A&M University
A. M. Hamed for the STAR collaboration
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Outline
Motivation
Analysis Technique
 Status QM08 and HP08
 New Results
 d+Au
 enhanced kinematic reach
 Discussion and summary
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Energy loss via gamma-jet coincidence
oDirect photon energy balances the outgoing parton, modulo correction from
constrain kinematics.
initial state radiation.
Motivation:
oAccurate measurements of energy dependence of energy loss (-jet in AuAu and pp)
oPossible measurements of path length dependence of energy loss (-jet vs 0-jet)
h
h
0
LO production mechanisms:

Away side yield per direct photon vs. per pi0
O(αs)
1. Geometrical bias
Photon is electromagnetically interacting
particle, then 0 -triggers are surface biased
compared to direct photon.
On average the away side of pi0 trigger suffer from Compton
longer path length unless it is tangential emission
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
Annihilation
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Energy loss via gamma-jet coincidence
2. Away side parton type
Away side of direct photon trigger in the dominant
channel is always quark vs. quark/gluon mix in
pi0 trigger
a. Gluon jet fragment softer than quark jet at the same energy
b. Gluon is bi-colored (CA=3, CF=4/3) “color factor”
3. Initial parton energy
0 -triggers are resulted from higher parton energy than -triggers.
Associated away side yield per pi0 trigger is higher than this from gamma trigger at
the same energy
More away side yield is associated with pi0 trigger than this from direct gamma trigger
Higher order processes:
kT
Photon Bremsstrahlung
Ahmed M. Hamed
O(αs2)
Example of Bremsstrahlung
diagrams
QM2009 Knoxville, Tennessee
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Analysis: Analysis technique
Build correlation function for neutral “triggers”
with “associated” charged particles
0
rich
Use transverse shower profile to distinguish 2-photon “0”
from single-photon showers “rich”
The fact that direct
photon has zero near side yield
: away side yield per trigger associated with direct photon
: away side yield per trigger associated with pi0
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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Away side results
Y+h
Y0+h
Shown results
Path length
dependence
of energy loss
oY+h
oY0+h
pp, run’06 11 pb-1
AuAu, run’07 535 ub-1
dAu, run’08
Energy dependence
of energy loss
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Status QM’08 and HP’08
Direct gamma – charged hadron coincidence at high pT
The associated yield with direct gamma trigger agrees with theoretical model predictions.

h
0 
h


h h
0

h

Similar suppression in IAA , IAA, IAA, RAA and RAA
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : neutral pions yield
Y0+h
D(zT) = 0
N
The first measurements
at RHIC energy for FF
cover this wide range of
kinematics in pp and dAu
of 0-hadron coincidence
Good agreement between
data from different runs
and different systems
at STAR detector
Yields show
significant medium effect
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : neutral pions IAA
The per trigger yield
suppression is
final state effect.
Quantifying the
suppression
-jet coincidence
measurements
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : Direct photon yield
Y+h
D(zT) =
N
The first measurements
at RHIC energy for FF
cover this wide range of
kinematics in pp and dAu
of -hadron coincidence
Yields show
significant medium effect
FF of -jet coincidence is
in well described by theory
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : Direct photon in dAu
Good agreement between
pp and dAu data for  and 0 results
The modification in the FF of
-jet coincidence
measurements is
final state effect
Associated yield for direct
photons is significantly
suppressed compared to
that of 0 due to the
difference
in the parton
initial energy.
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : IAA for  and 0 theory vs. data
No agreement bewteen different theoretical calculations and no strong raise on IAA  at ZT <
0.6 as expected by one of the theoretical calculation.
No differences between IAA  and 0 results which indicate that the surface bias is driven by the
near side trigger only. See STAR talk on Full jet reconstruction
Path length energy loss dependence is better to be explored by -jet in- and out- of
the reaction zone.
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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New Results : Direct photon IAA
Theory describe the data within the measured uncertainties.
Other centralities show similar IAA(pt) within current uncertainties which contradict
the picture of volume and surface emission.
Ahmed M. Hamed
QM2009 Knoxville, Tennessee
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Summary and Discussion
An accurate measurement for the energy dependence of energy loss of the
propagating parton through the medium is shown via -jet coincidence measurements.
Continuous effort is currently undertaking to improve the uncertainties levels.
The similarity between -jet and 0-jet indicate the free geometrical bias of the
recoil parton.
-jet coincidence measurements are needed to explore the path length dependence
of the energy loss.
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