Directed flow in Au Au collisions at 62GeV

1
Directed flow in AuAu collisions at 62GeV
Gang Wang for STAR Collaboration

• Introduction
• ? Directed Flow
• ? Wiggle
• ? Previous result
• ? 3 Methods
• ? ZDC upgrade
• ? ZDC-SMD method
• Physics results
• ? 10 - 70
• ? Centrality dependence
• ? Pt dependence
• ? Integrated v1
• ? Limiting fragmentation
• Summary

2
Flow
Among different kinds of flow, such as radial
flow, directed flow, elliptic flow and higher
harmonics, directed flow is the first discovered
type of flow (20 years ago).
Radial Flow (isotropic)
Flow
Directed Flow
Anisotropic Flow
Elliptic Flow
Higher Harmonics
3
Directed Flow

• Directed flow the first harmonic (v1) in the
  Fourier expansion of the particle azimuthal
  anisotropy with respect to the reaction plane
• Its likely influenced by the the compression in
  the initial stages of the collision, and makes a
  probe for the early stage of the collision.

v1 changes with the collision energy.
J. -Y. Ollitrault, Nucl. Phys. A638, 195c (1998)
4
Wiggle
Models predicted that anti-flow/3rd flow
component, with QGP ? v1(y) flat or a wiggle
structure.
J. Brachmann et al. PRC 61, 024909 (2000)
L.P. Csernai, D. Rohrich PRL 458 (1999) 454
5
Wiggle
Models without QGP also predicted a wiggle
structure in peripheral or mid-peripheral events.
M. Bleicher and H. Stöcker, PLB 526, 309 (2002)
RQMD
Baryon stopping and positive space-momentum
correlation
uRQMD
R. Snellings, H. Sorge, S. Voloshin, F. Wang, N.
Xu, PRL 84, 2803 (2000)
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Previous result
STAR Collaboration, PRL 92, 062301 (2004)
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Experimental complications
At RHIC, most detectors cover the central
rapidity region where the directed signal is
small and the analysis procedures are easily
confused by non-flow effects (azimuthal
correlations not related to the reaction plane
orientation).

• We try to reduce sensitivity to non-flow effects
• v13
• the three-particle cumulant method
• by Aihong Tang
• v1EP1, EP2
• the event plane method with mixed harmonics
• by Markus Oldenburg, Ilya Selyuzhenkov
• v1ZDC-SMD
• the standard method with the 1st-order event
  plane reconstructed from neutral fragments of the
  incident beams
• the event plane is determined from the sideward
  deflection of spectator neutrons (bounce-off)
  measured in ZDC-SMD
• by Gang Wang

8
STAR ZDC

• Each of the RHIC experiments has a pair of Zero
  Degree Calorimeter

• Each of the RHIC experiments has a pair of Zero
  Degree Calorimeter detectors for beam monitoring,
  triggering, and locating interaction vertices.
• ZDCs detect neutrons emitted along beam
  directions and measure their total energy
  (multiplicity) .
• Baseline ZDCs have no transverse segmentation,
  which motivates upgrade.

9
STAR ZDC-SMD
SMD is an 8 channel by 7 channel hodoscope that
sits directly on the face of the 2nd ZDC module
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Raw event plane from ZDC-SMD
The raw event plane distributions are not flat.
We can use the inverse of the bin content as psi
weight in the analysis.
STAR Preliminary
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Flatten event plane distribution
E877 Collaboration, Phys. Rev. C 56, 3254 (1997)
STAR Preliminary
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Comparison between 2 analysis
The analysis with psi weight gives the same
result as the one with flat psi.
50 -- 60
STAR Preliminary
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Systematic study 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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Systematic study Corrections
East and west correction
When east and west ZDC-SMD have different RP
resolutions, we have to use a 3rd 1st-order event
plane ( ) as an independent reference to
correct this effect.
E877 Collaboration, Phys. Rev. C 55, 1420 (1997)
They are inverse of each other
where denotes another 1st-order event
plane, e.g. from FTPC
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Systematic study Corrections
X - Y correction (from Sergei V. and Ilya S.)
When detectors like TPC and FTPC dont have the
same sensitivity in X and Y direction, we have
to replace the 2 in the numerator with
for X
direction or for Y direction.
For example
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Systematic study Comparison
After corrections applied, the 4 separate terms
get much closer to each other.
50 -- 60
STAR Preliminary
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Systematic study Comparison
The average of the 4 separate terms almost
coincides with the result of the
full-event-plane analysis.
50 -- 60
STAR Preliminary
18
Charged hadrons Eta dependence
STAR Preliminary
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Charged hadrons Eta dependence
Centrality by Centrality
STAR Preliminary
20
Charged hadrons Pt dependence
The magnitude of v1 is observed to increase with
particle transverse momentum and then appears to
saturate.
STAR Preliminary
21
Charged hadrons Integrated v1

• v1 is integrated over
• 0.15 GeV lt pt lt 2 GeV.
• The magnitude of v1 decreases with centrality.
• v1 in the more forward pseudorapidity region
  varies more strongly with centrality than in the
  region closer to midrapidity.

22
Limiting Fragmentation
It has been observed that particle emission(both
spectra and flow) as a function of rapidity in
the vicinity of beam rapidity appears unchanged
over a wide range of beam energies, a pattern
known as limiting fragmentation.
STAR Preliminary
The hypothesis holds here.
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Summary

• We present the first measurements of charged
  particle directed flow in AuAu collisions at
  62GeV.
• The analysis has been performed using three
  different methods and the results agree very well
  with each other.
• STAR ZDC-SMD now allows to determine the reaction
  plane from the bounce-off of fragmentation
  neutrons, the first measurement of this type at
  RHIC.
• Charged particles in the pseudorapidity region
  covered by the STAR TPC and FTPCs (up to eta
  4.0) flow in the opposite direction to the
  fragmentation nucleons.
• Over the pseudorapidity range studied, the v1 for
  charged particles is found to vary monotonically
  with pseudorapidity at all centralities.
• Theres no evidence so far for pion wiggle. We
  are working on v1 of identified particles.
• Our data provide further support for the limiting
  fragmentation picture.

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• Backup Slides

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Event Plane Resolution for ZDC-SMD
STAR Preliminary
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EOS data pion flow