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The STAR Experiment
High-pT physics at LHC 2009,
4-7th February, Prague, Czech Republic
Texas A&M University
A. M. Hamed for the STAR collaboration
Table of Contents and Disclaimer
Table of Contents:
The Road Behind
Disclaimer: The road behind is personal view, so
biases and mistakes are expected.
The Road Behind
Why study high-pT particles?
High-pT particles are produced from the hard
scattering processes.
Hard Scattering:
At mid-rapidity
Power law
Heavy-ion collisions
Hard processes take place at early time of collisions (0.1 fm/c). V~5 fm3 and ~10 fm/c
The Road Behind
How to use high-pT particles in heavy-ion collisions?
“ An interesting signature may be events in which the hard collision occurs near the edge of the
overlap region, with one jet escaping without absorption and the other fully absorbed”
J.D.Bjorken 1982 “elastic scattering?”
1. Jet reconstruction
Detection efficiency, quark- versus gluon-jet properties, Jet-mass effects.
The jet modify the medium as well as the medium quench the jet.
2. Inclusive single particle spectra- Leading particle method “RAA”
Glauber model uncertainties, Parton distribution functions.
dAu control experiment
3. Associated particle yield, Fragmentation Function, dijet events “IAA”.
Better to shed insight on the underlying physics and also reduces many unwanted effects! 4
The Road Behind
Experimental results “RAA, IAA” overview
PRL 98 (2007)192301
Hadron RAA is pt independent as expected by the radiative energy loss (LPM).
Direct photons follow the binary scaling. Number of binary scaling works!
Unexpected level of suppression for the heavy quarks. Equark,m=0  Equark,m>0
No sign for the color factor effect on energy loss. E CR Egluon  Equark
Similar near-side and strongly suppressed away-side in Au+Au relative to p+p and d+Au.
Away-side yield strongly suppressed to level of RAA
Surface bias free probe is needed
An access to the parton initial energy is required in order to better quantify the medium effect
The Road Behind
Theoretical models “energy loss” overview
On the jet quenching parameter q
The four major models use pQCD framework to estimate energy loss.
Different assumptions in various models lead to similar descriptions of the
light quark suppression with different model-dependent parameters.
Hierarchy of scales.
Modeling the medium evolution/structure.
Energy loss
<E> 
“Static medium”
q k2=2/
Scattering power of the medium
ASW and GLV: Similar models different
AMY and Higher twist: Different models same
The Road Behind
On the jet quenching parameter q
The Baier plot
Ideal QGP
Pion gas
Cold nuclear matter
radiative energy loss
If s(T) were weak…
q extracted via comparison with RHIC data is larger…
^q  8 GeV2/fm Armesto,Cauiari Hirono Salgado
3 GeV2/fm
Zhang Owens Wang Wong
q1 GeV2/fm
8-19 GeV2/fm
4-14 GeV2/fm
Baier Schiff
PHENIX; at 2, neglecting
theoretical uncertainties
Dainese Loizives Palc
Strong coupling calculation of q is required
! Nonperturbative calculation is needed !
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.
The Road Behind
Why study direct photon – hadron azimuthal correlation?
Fast Detector
Fast Detector
zero near-side yield
for direct photons
Leading particle
Due to fragmentation full jet reconstruction
is required to access the initial parton energy
get the initial parton energy
with a powerful alternative method:
“Direct -hadron azimuthal correlations”
Heavy ion collision
Direct photon is a surface bias free probe.
Jet-energy is calibrated by “Direct ”
The Road Behind
Direct photon production mechanisms
Direct photon: photons unaccompanied by additional hadrons
Direct photon production provides an insight into the dynamics of hadronic constituents which is
not obscured by their fragmentation.
High-pt direct photons are produced at a rate comparable to that of
single particles: perform high-statistics measurements with practical
Photon Bremsstrahlung and effects due to intrinsic motion
The photon takes only a fraction of the parton’s momentum.
More texts are to be added here about this topic
Examples of Bremsstrahlung
Inclusive spectra of direct photon in heavy ion collisions:
Sources of suppression and enhancement of direct photons yield.
RAA saturates , pt-independent (LPM)
RAA follows binary scaling
The Road Behind
Direct photon-jet production mechanisms
Both mechanisms yield associated photons recoiling against a gluon or quark jet
depending on the value of xT
In the approximation that parton kT effects can be neglected:
Direct photon-hadron correlations
Direct photon energy balances the outgoing parton, module negligible
correction from initial state radiation.
Calibrated probe of the QGP – at LO.
No Surface Bias
Hard process
Possible candidate for quark/gluon jet discrimination.
Challengeable measurements!
0 is suppressed at high pT by a factor of ~5 in central
AuAu collisions.
Analysis technique
Build correlation function for neutral “triggers”
with “associated” charged particles
Use transverse shower profile to distinguish 2-photon “0”
from single-photon showers “rich”
Comparison of 0 – triggered yields with previously measured
h triggered yields.
Extract the yields associated with direct photon
triggers using the fact that direct photon
has no near side yield.
Systematic Study!
STAR and the azimuthal correlation measurements
Correlate photon candidate “triggers” with “associated tracks”
pT,trig > 8 GeV/c
One tower out of 4800
towers (0.05 x 0.05)
Charged hadrons
No track with
p > 3 GeV/c points
to the trigger tower
Offline trigger:
Etower > 6 GeV,
Ecluster > 8GeV,
Esmd > 0.5 GeV,
Cluster is away from
the tower edge
Eγ = Eparton
Online trigger:
Etower > 5.76 GeV,
Ecluster > 7.44 GeV
“cluster =2 towers
out of 3x3 towers”
Use  triggers
to explore
fragmentation functions
in p+p and Au+Au
Associated charged
particles “3 <pT< 16 GeV/c”
BEMC: Barrel
Electro-Magnetic Calorimeter
Track quality, eff.
TPC: Time Projection Chamber
Full azimuthal coverage
|TPC| < 1.5
|bemc| < 1
Event general QA
How to distinguish between 0/ ?
STAR and the 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
i : strip energy
The tower energy asymmetry cut to purify the rich sample
ri : distance relative
for the case of photons across the module
to energy maxima
 7 RM
Use the shower-shape analysis to separate the two closeby photons
shower from one photon shower.
Frag. Photons, asymmetric decay of pi0, and eta?
STAR and the Shower Shape Analysis
Trigger photons-charged particles azimuthal correlations
535 ub-1 of Au+Au run 7 tagged as L2gamma trigger using the full azimuth coverage of BEMC
pp data
STAR Preliminary
Place holder for
Pt associated effect
Same centrality, same
triggers bin but different
Pt figure
Clear dijet structure is seen for inclusive gamma – charged hadron azimuthal correlation at STAR
Background level increases with centrality as expected
Both near side yield and away side increase with trigger energy as the initial parton energy
Near side is suppressed with centrality which might due to the increase of /0
ratio .
Results: Effect of shower-shape cut
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
-sample is not pure direct  ! How about the 0 ?
0 results compared to h results
Place holder for texts
0 results compared to h results
0-triggered yields to charged-hadron triggered yields
Completely different data set from different RHIC runs, different detectors were
involved in the analysis too.
This analysis
Associated yields per trigger
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.
0 sample is pure.
Method of extract direct  associated yield
Extraction of direct away-side yields
Assume no near-side yield
for direct 
then the away-side yields per
trigger obey
This procedure removes correlations due to contamination (asymmetric
decay photons+fragmentation photons) with assumption that correlation
is similar to 0 – triggered correlation at the same pT.
Direct  compared to 0 results
Place holder for text
Direct  compared to 0 results
Fragmentation function of direct  triggers and 0 triggers
Direct 
Associated yields per trigger
Differences between 
and 0 triggers
0 -triggers are resulted from
higher parton energy than
0 -triggers are surface
Color factor effect.
The away-side yield per trigger of direct  triggers shows smaller value
compared to 0 triggers.
Direct  compared to  results
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
8 < pT < 16 GeV/c
pT > 3 GeV/c
Strong medium effect
7 < pT < 9 GeV/c
 Icp agrees with theoretical predictions.
More precision is needed for the measurements to distinguish
between different color charge densities.
Within the current uncertainty in the scaling the
Icp of direct  and 0 are similar.
Systematic Study
Systematic study
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
10% of inclusive  at intermediate pT in p+p
~30-40% of direct  at PT > 8 GeV/c.
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
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.