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The STAR Experiment
Texas A&M University A. M. Hamed for the STAR collaboration
d2NAA/dydpT
TAA . d2/dydpT Single
-particle spectra RAA
PRL. 96, 202301 (2006)
PRL 98 (2007)192301
STAR QM08
RAA of light quarks is pt independent as expected by the radiative energy loss.
But
Unexpected level of suppression for the heavy quarks.
No sign for the color factor effect on energy loss.
There is no single commonly accepted calculation of the underlying physics to describe
in-medium energy loss for different quark generations as well as for the gluon.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
More differentiable observable is needed
Fragmentation function in vacuum / medium
Fragmentation Function : particles distribution in fractional energy.
Factorizations
Compare
Hard Scattering in
vacuum-QCD
p+p or
peripheral
Au+Au
versus
Hard Scattering in
the medium
Central
Au+Au
Gluon radiation is
induced
by multiple scattering
.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.

A particle distribution in
fractional energy is
softened
in the medium
3
Di-hadron azimuthal correlations
conservation of linear momentum”
momentum
Jet-like azimuthal correlations “conservation
E
 E parton
trigger
hadron
Central Au+Au
How much
energy is
lost in here?
PRL. 91, 072304 (2003)
Trigger
4 < pT,trig < 6 GeV/c
2 < pT,assoc < pT,trig
Background is subtracted
In the near-side p+p, d+Au, and Au+Au are similar while in the away-side
“back-to-back” Au+Au is strongly suppressed relative to p+p and d+Au.
A. Hamed
An access to the parton initial energy is
Texas A&M University
required to quantify the energy lost
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
4
Jet-energy calibration “Direct ”
“Mid-rapidity”
zero near-side yield
for direct photons
Fast Detector
“Calorimeter”
Leading particle
“trigger”
Background
0
P

Color charge density?
How much energy is lost in the medium?
Due to fragmentation full jet reconstruction
is required to access the initial parton energy
OR
xP
xP
Direct
photons
No
access
to the
escape
from
the
parton
initial
energy
medium without any
further
interactions
FF is softened
P
in the
medium
Associated
particles
get the initial parton energy
with a powerful alternative method:
“Direct -hadron azimuthal correlations”
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
How to measure direct -hadron azimuthal
correlations?
5
Analysis technique
Build correlation function for neutral “triggers”
with “associated” charged particles
Use transverse shower profile to distinguish 2-photon
from single-photon showers
Comparison of 0 – triggered yields with previously measured
charged-hadrons- triggered yields.
Extract the yields associated with direct
photon triggers
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
6
Build correlations function
Correlate photon candidate “triggers” with “associated tracks”
pT,trig > 8 GeV/c
Charged hadrons
One tower out of 4800
towers (0.05 x 0.05)
2
0
No track with
p > 3 GeV/c points
to the trigger tower
BEMC: Barrel
Electro-Magnetic Calorimeter
TPC: Time Projection Chamber
TPC
Eγ = Eparton
Use  triggers
to explore
fragmentation functions
in p+p and Au+Au
Associated
charged
particles
“3 <pT,assoc
< 8 GeV/c”
Full azimuthal coverage
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
How to distinguish between 0/ ?
7
Shower Shape Analysis
The two photons originated from 0 hit the same tower at pT>8GeV/c
STAR Shower Maximum Detector is embedded at ~ 5x0 between the lead-scintillator layers
“BEMC”
i : strip energy
ri : distance relative
to energy maxima
 7 RM
Use the shower-shape analysis to separate
the two close photons shower from one photon shower.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
8
Trigger photons-charged particles azimuthal correlations
STAR Preliminary
Near side is suppressed with centrality which might
due to the increase of /0 ratio .
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
9
Effect of shower-shape cut
Centrality
Centrality
Background is not subtracted
Vacuum QCD
Medium effect
oThe away-side correlation strength is suppressed compared to pp and peripheral
Au+Au. oThe -rich sample has lower near-side yield than 0 but not
A. Hamed zero.
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
-sample is not pure direct  ! How about the 0 ?
10
Comparison of 0-triggered yields to charged-hadron triggered yields
Completely different data set from different RHIC runs, different detectors were
involved in the analysis too.
Associated yields per trigger
This analysis
Surface bias
PRL 97
162301 (2006).
Central Au+Au
Near side: Yields are similar for p+p and central Au+Au
Away side: Yields show big difference between p+p and central Au+Au
0-charged and charged-charged results are consistent.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
0 sample is pure.
11
Method of extract direct  associated yield
O(αs2α(1/αs+g))
Extraction of direct away-side yields
near
near
R=Y-rich+h/Y0+h
Assume no near-side yield
for direct 
then the away-side yields per
trigger obey
away
away
Y+h = (Y-rich+h - RY0+h )/(1-R)
0

This procedure removes correlations due to contamination (asymmetric
decay photons+fragmentation photons)
with assumption that correlation is similar to
A. Hamed
0 – triggered correlation at the same pT.
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
12
Fragmentation function of direct  triggers and 0 triggers
Direct 
Associated yields per trigger
0
Differences between 
and 0 triggers
0 -triggers are resulted from
higher parton energy than
-triggers.
0 -triggers are surface
biased.
Color factor effect.
The away-side yield per trigger of direct  triggers shows smaller value
compared to 0 triggers which is consistent with
partons loose energy “dense medium” and then fragment.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
What is the medium color charge density?
13
Medium effect on fragmentation function
Icp(zT) =
Icp(zT) = 1
D0-10% (zT)
D40-80% (zT)
IAA(zT) =
DAA (zT)
Dpp (zT)
If there is no medium effect
Data points
trig
8 < pT < 16 GeV/c
assoc
pT > 3 GeV/c
Strong medium effec
trig
7 < pT < 9 GeV/c
STAR
STARPreliminary
Preliminary
 Icp agrees with theoretical predictions.
More precision is needed for the measurements to distinguish between different
color charge densities.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
Within the current uncertainty in the scaling the Icp of
direct  and 0 are similar.
14
Summary and Outlook
First result of -jet azimuthal correlations and fragmentation
function D(zT) in AuAu at RHIC energy is reported.
Away-side yield for direct photons is significantly suppressed in
heavy ion events. Suppression level agrees with theoretical
expectations.
All results of 0’s near and away-side associated particle yields
shows consistency with that of charged hadron triggers.
Large luminosity at RHIC enables these measurements. Expect
reduced uncertainties from further analysis and future runs.
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
15
Thank you for your
attention
and
many thanks to
all STAR collaborators
16
Limitations of the shower shape cut
Shower Shape Cuts:
Reject most of the 0’s.
But do not reject photons from:
highly asymmetric 0 decay. 10% of all 0 with pT > 8 GeV/c
’s - similar level of background as asymmetric 0
fragmentation photons
A. Hamed
Texas A&M University
Hot Quark 2008 Estes Park,
CO August 18th - 23th.
10% of inclusive  at intermediate pT in p+p
~30-40% of direct  at PT > 8 GeV/c.
17