Transcript Incident-energy and system-size dependence of directed flow  Detectors: ZDC-SMD, (F)TPC

Incident-energy and system-size
dependence of directed flow
Gang Wang (UCLA)
for STAR Collaboration
Introduction to directed flow
Detectors: ZDC-SMD, (F)TPC
Cross check with PHOBOS
Comparison between 4 systems
Summary
pt
  r
Anisotropic flow
Directed flow (v1)
Elliptic flow (v2)
Higher harmonics
<px> or directed flow

d 3N
1 d 2N 

E 3 
1   2vn cos[ n(  r )] 
rapidity
d p 2 pt dp
n 1

t dy 
vn  cos[ n(  r )]
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Non-flow effects
Flow fluctuations
Low statistics
Harder!
Easier to
measure
RHIC
J. -Y. Ollitrault, Nucl. Phys. A638, 195c (1998).
STAR Collaboration, Phys. Rev. Lett. 92, 062301 (2004)
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STAR Main Detector
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STAR ZDC-SMD
• New knowledge of the direction of the impact parameter vector
• Minimal, if any, non-flow effects
• Worse resolution than from TPC, but that disadvantage is minor
SMD is 8 horizontal slats &
7 vertical slats located at
1/3 of the depth of the ZDC
ZDC side view
Scintillator slats of
Shower Max Detector
Transverse
plane of
ZDC
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Cross check with PHOBOS
PHOBOS Collaboration, Phys. Rev. Lett. 97, 012301 (2006)
5% - 40%
Within errors, STAR v1 of charged hadrons agrees with PHOBOS’ results.
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Comparisons of charged hadron v1
between 4 collision systems:
200 GeV Au +Au
200 GeV Cu +Cu
62.4 GeV Au +Au
62.4 GeV Cu +Cu
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Resolution of 1st-order event plane
from STAR ZDC-SMD
Full EP resolution  2 cos( a  b ) depends on spectator v1 and
the number of hits falling on ZDC-SMDs.
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Best case: 200 GeV Au + Au
Charged hadron v1 is in the direction opposite to that of spectators.
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Best case: 200 GeV Au + Au
In mid-rapidity, v1(pt) crosses zero at
pt above 1 GeV/c in the central and
mid-central collisions.
Not seen in the forward region.
Hydro model predicts the zerocrossing in v1(pt), but the prediction
also applies to the forward region.
If we assume that pions and protons
flow oppositely, then the change in
their relative abundances leads to the
zero-crossing.
With the yields of (anti)proton and
pion, we can fit the v1(pt) points
assuming v1(pt) for pions and
(anti)protons are straight lines.
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v1(η): system-size dependence
AMPT
System size doesn’t seem to influence v1(η).
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v1(η): incident-energy dependence
v1 magnitude
The
hypothesis
is of
bigger
limiting
at 62.4
fragmentation
GeV than atis200
supported.
GeV.
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v1(pt)
At 200 GeV, v1(pt)shows
similarity in Au +Au and
Cu +Cu collision systems.
At 62.4 GeV, this is true
for low pt. More statistics
is needed in higher pt for
Cu +Cu.
For all 4 cases, v1 (pt) goes
to zero when pt is zero,
and increases in magnitude
with pt until reaching the
maximum and turning
back.
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Integrated v1
At 200 GeV, integrated v1
shows similarity in Au +Au
and Cu +Cu collision systems.
At 62.4 GeV, this is seemingly
true. More statistics is needed
for Cu +Cu.
For all 4 cases, integrated v1
goes to zero in central
collisions, and increases in
magnitude from central to
peripheral collisions.
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Summary
• Best case: 200 GeV Au + Au
– Charged particle v1(η) tends to turn back towards zero at
forward pseudo-rapidities in central collisions.
– v1(pt) crosses zero at pt above 1 GeV/c in central and midcentral collisions in mid-rapidities.
• v1 shows similiarities between AuAu and CuCu
as functions of η, pt and centrality at both 200
GeV and seemingly at 62.4 GeV.
• AMPT model predicts this effect at 200 GeV.
• Limiting fragmentation hypothesis is supported
by AuAu data, and seemingly by CuCu data.
• Results of v1{ZDC-SMD} will be cross-checked
by other methods.
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Backup Slides
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Flow study with ZDC-SMD: 4 terms
In analysis:
To systematically study the method with ZDC-SMD, we can use the sub
event plane from only east or west ZDC-SMD, instead of the full event
plane, and even break down the correlation into X and Y direction.
For example:
Definitions of 4 terms:
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Identified particles : v1 in 62 GeV Au +Au
STAR Collaboration, Phys. Rev. C 73, 034903 (2006)
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