Anisotropic Flow in UltraRelativistic HeavyIon Collisions

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Anisotropic Flow in Ultra-Relativistic
Heavy-Ion Collisions
Lake Louise Winter Institute

• Markus D. Oldenburg
• for the STAR Collaboration

February 20, 2004
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Outline

• Introduction to Heavy-Ion Collisions
• The STAR Experiment
• Anisotropic Flow
• Elliptic Flow for different particle species
• Directed Flow
• Flow of higher orders
• Summary and Outlook

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System Evolution of a Heavy-Ion Collision
QGP thermalized system with partonic degrees of
freedom
soft physics regime
hard (high-pT) probes
Chemical freeze-out (Tch Tc) inelastic
scattering ceases Kinetic freeze-out (Tfo ? Tch)
elastic scattering ceases
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The STAR Experiment at RHIC
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Anisotropic Flow

• Look at peripheral collisions
• Overlap region is not symmetric in coordinate
  space
• Almond shaped overlap region
• Larger pressure gradient in x-z plane than in
  y-direction

• Spatial anisotropy ? Momentum anisotropy
• Interactions among constituents transform the
  initial spatial anisotropy into an (observed)
  momentum anisotropy
• Process quenches itself ? sensitive to early
  times in the evolution of the system
• sensitive to the equation of state

• Perform a Fourier decomposition of the momentum
  space particle distributions in the x-y plane
• vn is the n-th harmonic Fourier coefficient of
  the distribution of particles with respect to the
  reaction plane
• v1 directed flow
• v2 elliptic flow

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Elliptic Flow v2 at low pt

• v2(pt) and mass dependence is reproduced by
  hydrodynamical model calculations
• Hydro model implicitly assumes local thermal
  equilibrium and is sensitive to the EOS

Pion, neutral Kaon and Proton data taken from
PHENIX Nucl. Phys. A715, 599 (2003) Hydro P.
Huovinen et al., Phys. Lett. B503, 58 (2001)
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Multi-strange baryon v2

• ? ?? show strong elliptic flow
• ? ?? show similar behavior

• ? v2(pt) follows ? evolution
• ? v2(pt) consistent with ? and ? v2(pt)
• assumed small hadronic cross-section of
  multi-strange baryons suggests partonic
  collectivity

Hydro P. Huovinen et al., Phys. Lett. B503, 58
(2001)
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Quark coalescence picture
Hadronization via quark coalescence v2 of a
hadron at a given pt is the partonic v2 at pt/n
scaled by the of quarks (n).

• Shown to work for K0s ?
• (PRL 92, 052302 (2004))
• Works also for ?
• ? v2s v2u,d 7

D. Molnar, S.A. Voloshin Phys. Rev. Lett. 91,
092301 (2003) V. Greco, C.M. Ko, P. Levai Phys.
Rev. C68, 034904 (2003) R.J. Fries, B. Muller,
C. Nonaka, S.A. Bass Phys. Rev. C68, 044902
(2003) Z. Lin, C.M. Ko Phys. Rev. Lett. 89,
202302 (2002)
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Directed Flow v1

• sizeable v1 only visible in the forward
  directions
• measurement of directed flow confirms v2gt0
  (elliptic flow is in-plane)

STAR data PRL 92, 062301 (2004) NA49 data PRC
68, 034903 (2003)
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Higher harmonics

• strong potential to constrain model calculations
• sensitive to the dynamical evolution of the
  system
• initial conditions of hydro influence magnitude
  and even the sign of vn

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v4(pt) and v6(pt)

• utilizing large elliptic flow to estimate the
  reaction plane
• sizeable v4
• v60 within errors

• v4 and v6 scale like v22 resp. v23

PRL 92, 062301 (2004)
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Summary and Outlook

• v2(pt) and mass dependence below 1 GeV/c are well
  reproduced by hydro calculations
• consistence with strong partonic reinteractions
  and early thermalization
• meson-baryon scaling of v2 at higher pt
• consistent with hadronization via quark
  coalescence
• indication of quark collectivity
• higher harmonics were measured and give further
  constraints on model calculations

• current RHIC run IV will increase the size of our
  data sample significantly
• statistical uncertainties will shrink
• Anisotropic flow for different particle species
  and harmonics will be studied in much more detail
  than ever before.

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Backup
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The Relativistic Heavy Ion Collider

• Two independent accelerator rings
• 3.83 km in circumference
• Accelerates everything from p to Au
• Running conditions
• Run I
• Run II
• Run III
• Run IV

Long Island
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One of the first Au on Au Events at CM Energy of
200 GeV?A
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