#### Transcript Document

```Ahmed M. Hamed
Elliptic Flow Measurements
Introduction
of Inclusive Photons
Heavy-ion physics
and
STAR Detector
Neutral Pions Reconstruction
Physics at RHIC
Physics with STAR BEMC
oElliptic flow of inclusive photons.
oNeutral pions reconstruction.
Summary
1
Introduction:
Levels of structure
Atom is almost empty, Nucleus is crowdy !
eV
3.0
0
MeV
Na Atom
3.0
0
GeV
208Pb
Nucleus
0.3
0
Proton
Nuclear density 0 ~ 0.15nucleons/fm3
Specific volume  ~ 6fm3
Typical hadronic volume ~ 1-3 fm3
The average inter-nucleon distance in the nucleus ~ 1.8fm
2
Introduction:
Forces
-(r/a)

hc
e
F~ 2
r
Electromagnetic Force
Strong Force
=e/4hc
2
=g/4
hc
2
a=
a=10
-13
cm
The force itself is carried by elementary particles, namely, by the continuous
exchange of zero mass bosons between the two charges.
The macroscopic, classical force results from the average effect of the bosons
exchange.
3
Introduction:
Quantum Field Theories
Quantum Chromodynamics
Quantum Electrodynamics
(Q2)=
(Q02)
s(Q2)=
1 (Q2)
0 ln(Q2/Q2)
10
2
1
2
=
3
1
1 >0
1 <0
QED
QED
lim (Q2)=
Q2

QCD
=
s(Q02)
1 s(Q2)
0 ln(Q2/Q2)
10
2
2Nf - 11Nc
6
QCD
lim s(Q2)=0
Q2

QCD exhibits confinement and asymptotic freedom.
4
Heavy-ion physics:
Phases and D.o.F.
The exact order of phase transition is not known.
Smooth nature of the transition increases the experimental challenges.
QGP near Tc should not be regarded as an ideal gas of quarks and gluons.
5
Heavy-ion physics
Relativistic Heavy-Ion Collider
Design Parameters:
6
Heavy-ion physics
Space-time evolution
QGP characteristics:
Small size “ a few fermi in diameter”
Very short life time 5-10x10-23 s.
Scenarios of fireball formation:
Landau Model: Full Stopping.
Bjorken Model: Transparency.
7
Heavy-ion physics
Signatures of QGP
Small size and short-lived system.
Kinematical and hydrodynamical Probes.
Temperature, entropy, energy density, PT, dN/dy, dET/dy
Electromagnetic Probes
Dilepton productions.
Direct Photons.
Signatures from Deconfined phase.
Strangeness enhancement.
J/ Suppression.
Indication of Chiral Symmetry Restoration.
Hard QCD Probes.
Jet quenching.
Event-by-Event fluctuation.
+Large production rates
- Final State Interactions.
Electromagnetic Probes:
+Large mean-free path compared to the size of the fireball system.
8
STAR Detector
STAR Detector
Tracker detectors(slow), Trigger detectors(fast), and Calorimeters(fast).
9
STAR Detector
Barrel Electromagnetic Calorimeter
=-ln tan(/2)
Cross-section in 
Cross-section in 
10
Physics at RHIC:
Collectivity
Hydrodynamic calculations assume local thermal equilibrium in the early stage
t 1 fm/c to reproduce the magnitude of the observed 2 at RHIC.
11
Physics at RHIC:
High Pt suppression
The invariant cross section for 0production in pp collisions at s=200GeV
agrees with NLO PQCD predictions over the range 2.0Pt15GeV/C.
The suppression of 0,s and ,s is very similar which supports the conclusion
that the suppression occurs at the parton level.
The binary scaling of direct photons is strong evidence that the suppression is
not an initial state effect.
12
Physics with STAR BEMC Elliptic flow of inclusive photons
Elliptic Flow of Inclusive Photons
13
Physics with STAR BEMC:
Collective flow
Longitudinal expansion
Collective flow
Directed flow
Anisotropic transverse flow
Elliptic flow
Anisotropic flow  correlations
with respect to the reaction plane
Higher harmonics flow
=
=
dN
const.
d
y
V1=0
V1<0
V1>0
x
V2<
0
y
V2 >
0
V2 =
0
No symmetry in the configuration space
anisotropy in the momentum space
2 is a measure of interactions
in the system
14
x
Physics with STAR BEMC: Direct real photons
Non-Thermal photons:
Thermal photons:
1. Prompt photons.
1. QGP photons.
2. Pre-equilibrium photons.
Background Photons:Decayed photons.
Schematic Photon Spectrum
Bremsstrahlung
n
Low Pt:
A+A collisions:
Hard: 1/PT
Thermal: e-E /T
Photons don’t strongly interact with fireball.
High pT
Allow test of binary scaling for hard processes.
Important for interpretation of high-pT hadron suppression
at RHIC.
15
Physics with STAR BEMC: 2 of inclusive 
What is the origin of the binary scaling of direct photon?
* “Jet quenching” photons.
Bremsstrahlung
Enhancement
R.J.Fries, B. Muller and
D.K.Srivastava,
Phys.Rev. Lett. 90,132301
(2003)
2 of Direct photons:
Scattering
Suppression
* Thermal Photons.
Enhancement
B.G. Zakharov, JETP
Lett. 80, 1
(2004)
-Help to disentangle the different production mechanism of direct photon.
Coulomb: 2=0
Bremsstrahlung: 2<0
-Thermal photons reflect the dynamical evolution of the formed matter.
STAR BEMC can probe for further higher transverse energy.
2 of inclusive photon is the first step toward 2 of direct photon.
16
Physics with STAR BEMC: Analysis details and technique
Standard Method:
|z|<=25 cm, Number of fit points>15, 0<pt<2GeV/C ,
Event plane reconstruction
-1<<0 TPC, 2.6<<4.0 FTPC ,Number of tracks>10
TPC: 2 EP = atan2(i sin21 i+ i sin22 i , i cos21 i+ i cos22 i)
FTPC: 2EP = atan2(i sin2w1 i+ i sin2w2 i + i sin2E1 i+ i sin2E2 I
, i cos2w1 i+ i cos2w2 I + i cos2E1 i+ i cos2E2 i)
’s are the azimuthal angle of the tracks in the sub-events. W and E stand for the West and East sides of FTPC.
Remove the acceptance bias:
Event plane recentering:
sin2
sin2 - M sin2 / M 
cos2
cos2 - M cos2 / M 
M: number of tracks in each sub-event
Photons from BEMC:
2400 Towers 0.050.05 0<<1,
0<<2, E=0.16E
Each track is extrapolated to the BEMC face and the charged particles veto cut for the target tower is used.
Energy Threshold:
Minimum Bias: ET>0.1GeV High Tower: ET>3GeV
QA of BEMC: Check the uniformity of BEMC.
Correlating inclusive photon with event plane: 2
Observed
=  cos(2tower- 2 EP) 
Finite multiplicity: Correction for event plane resolution: 2 Real= 2 Observed / R
17
Physics with STAR BEMC: 2 of inclusive  result
2(Et) of inclusive photons integrated over unit pseudorapidity
 Au+Au MinBias
o Au+Au High Tower
Au+Au MinBias
Au+Au High Tower
Reaction plane from TPC
0<<1
Reaction plane from FTPC
At low and intermediate ET,the 2(ET) behavior of inclusive  is consistent with
that of other mesons which indicate the dominance of 0 in that range.
At high ET, although the statistical errors are large, it is clear that 2
decreases with Et but it is still finite.
18
Physics with STAR BEMC:
Non-flow effect
Scalar product method:
The scalar product is the same for all collision systems in the case of only “nonflow”.
The difference results from the collective motion and/or effects of medium modification.
 Au+Au MinBias 50%-80%
o Au+Au High Tower 50%-80%
p+p MinBias
p+p High Tower
 Au+Au MinBias 10%-50%
o Au+Au High Tower 10%-50%
p+p MinBias
p+p High Tower
 Au+Au MinBias 0%-10%
o Au+Au High Tower 0%-10%
p+p MinBias
p+p High Tower
There is a clear contribution from the so called “non-flow” in TPC.
The azimuthal correlations is dominated by nonflow effects at high Et.
19
Physics with STAR BEMC:
 Au+Au MinBias 50%-80%
o Au+Au High Tower 50%-80%
p+p MinBias
p+p High Tower
reducing non-flow effect
 Au+Au MinBias 10%-50%
o Au+Au High Tower 10%-50%
p+p MinBias
p+p High Tower
 Au+Au MinBias 0%-10%
o Au+Au High Tower 0%-10%
p+p MinBias
p+p High Tower
The non-flow contribution in FTPC is very negligible.
The behavior of the azimuthal correlations with centrality shows strong
evidence for elliptic flow at low and moderate Et.
The quantitative similarity between 2(Et) of inclusive photons with that of
hadrons is a promising sign for negligible amount of 2(Et) of direct photons.
20
Physics with STAR BEMC: Neutral pions reconstruction
Neutral Pions Reconstruction
21
Physics with STAR BEMC: Reasons for new cluster finder
[email protected]
Minbias
[email protected]
High tower1
simulation
[email protected]
Strong peak at low invariant mass region.
No 0 invariant mass peak is seen in the high multiplicity system.
No low invariant mass peak in simulation.
22
Physics with STAR BEMC:
Cluster splitting
New Cluster Finder:
Energy seedTower&&SMDs>=
0.25GeV && >=0.2GeV
Are the dots outside the band 0s ?
Certainly they aren’t 0s with pt between 5 and 6GeV/c
but they are 0 candidates for other pt bins.
Is the asymmetry cut efficient here?
It is obvious NO.
23
Physics with STAR BEMC:
Cluster splitting removal
No successful study for the EM shower shape so far using STAR BEMC.
No cluster splitting effect for the large opening angle decayed photons.
The distribution is peaked heavily around the minimum opening angle.
The energy threshold is already greater than energy of one of the decayed photon
in the asymmetric decay case for 0 with low Pt
Use the 0 decay kinematics to remove the cluster splitting.
Cluster Splitting Removal
24
Physics with STAR BEMC: Cluster splitting removal check
Accepted pairs
Rejected pairs
All Paris
All Paris
Rejected pairs
Rejected pairs
Accepted pairs
Accepted pairs
All pairs+  0.3
All pairs+  0.3
25
Physics with STAR BEMC: New cluster finder results
Extrapolate the track to the BEMC face and set the energy of the target tower to zero.
[email protected]
[email protected]
Simulation
All Paris
Rejected pairs
Accepted pairs
All pairs+  0.3
[email protected]
MinBias
[email protected]
High Tower 1
High tower1
[email protected]
High tower2
MinBias
26
Physics with STAR BEMC:
New cluster finder results
jkjh
cucu@
200GeV
MinBias
[email protected]
[email protected]
[email protected]
[email protected]
[email protected]
27
Summary
oFirst measurements of the inclusive photon elliptic flow up to ET 15GeV
have been done at RHIC energy with STAR BEMC.
The elliptic flow of inclusive photons decreases with transverse momentum at
high Pt but it is still finite up to Pt 15GeV.
The finite value of inclusive photons elliptic flow at high Pt is mainly due to the
nonflow effect.
oA new cluster algorithm for STAR BEMC has been developed.
The cluster splitting is highly suppressed in the new cluster finder.
The neutral pion invariant mass peak is clearly seen in all different collsion
systems at RHIC energy.
oTwo important steps toward the final results of direct photons elliptic flow