Femtoscopy with STAR

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Femtoscopy with STAR

• Zbigniew Chajecki
• for the Collaboration

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Outline

• Probing space and time in Heavy Ion Collisions
  with Femtoscopy
• Evolution and emission time
• Size and shape of the source
• Do we understand the transverse mass dependence
  of HBT radii?
• Underlying physics in AA dynamically-driven
  geometric substructure
• AuAu relative to pp (and dAu)
• Problem with baseline of correlation function for
  pp and dAu collisions
• Decomposition of correlation function in terms of
  spherical harmonics new approach of studying
  particle correlations
• Summary

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The sketch of Heavy Ion Collision
hadronic phase and freeze-out
QGP and hydrodynamic expansion
hadronization
initial state
pre-equilibrium
Emission time
temperature
Evolution time
Chemical freeze out
Thermal freeze out
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Hydro predictions Evolution and Emission time

• Long lifetime of the source
• as signal of the QGP formation
• Ro - Rs ? Emission time
• Rl ? Evolution time
• Hydrodynamic calculations reproduce momentum
  observables but fail to reproduce HBT results ?
  RHIC HBT puzzle

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Origin of HBT puzzle
STAR - AuAu_at_200GeV, PRC 71 (2005) 044906
Modified Sinyukov fit to RL
M. Herrmann and G.F. Bertsch, PRC 51 (1995) 328
in the absence of transverse flow
Short emission time (2fm/c) consistent with
RO/RS ratio
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Properties of the source at freeze-out
STAR, PRL93 012301 (2004)
STAR, PRL93 012301 (2004)
system changes the shape
system expands
eccentricity

• estimate ?initial from Glauber
• efinal from asHBT

• Size of homogeneity region scales with initial
  source size (increase with increase of
  centrality of the collision - b?0 )
• Oscillation phases out-of-plane extended
  source

PRC66 (2002) 044903
Source remains out-of-plane at freeze out
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Why do the HBT radii fall with increasing pT ?

• Decrease of HBT radii with pT
• usually attributed to collective flow
• RL ? longitudinal flow
• RT ? transverse flow

• Non-flow possibilities
• termally (not collectively) expanding source
• hot core surrounded by cool shell
• important ingredient of Buda-Lund hydro picture
  (Csorgo et al.)

t
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HBT in AuAu vs pp (dAu)
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AuAu like superposition of pp?

• AuAu N(strings in pp) ?
• AuAu N(blast-waves) ?
• NO! that would generate local space-momentum
  correlations but in AuAu we see global
  correlations although both scenarios would
  lead to the identical p-space observables

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Initial vs Final Size pp(dAu) vs AuAu

• AuAu mostly understood in terms of collective
  flow
• pp (dAu) show apparent global space momentum
  correlations, but no (or little) expansion
• Model-dependent approach of
• veryfication of expansion scenario
• RMS of initial size of the source calculated with
  Glauber model vs. experimental RSIDE for
  ltkTgt0.2 GeV/c

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Intriguing results ....Lets take a closer look
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Projection of 3D CF in dAu
STAR preliminary

• Gaussian source vs non-Gaussianess of CF
• It looks like fit describes the shape of
  correlation function

dAu central, kt 0.25-0.35 GeV/c
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Worst-case picture of dAu
STAR preliminary

• Not great fit
• One imagines one could do a better job, but fit
  must work in 3D we see tiny portion of space
• What is going on in the normalization
  region?(independent of Gaussian ansatz)
• Problem with projections
• cannot see systematics in 3D
• experiments w/same CF will have different
  projections

dAu peripheral, kt 0.15-0.60 GeV/c
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Decomposition of CF onto spherical harmonics
Z.Ch., Gutierrez, Lisa, Lopez-Noriega,
nucl-ex/0505009

• Cartesian-space (out-side-long) naturally encodes
  physics, but is poor/inefficient representation
• Recognize symmetries of Q-space -- decompose by
  spherical harmonics!
• Direct connection to source shapes
  Danielewicz,Pratt nucl-th/0501003
  decomposition of CF on cartesian harmonics
• immune to acceptance
• full information content at a glancethanks to
  symmetries

? 0,2p ? 0,p
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Some properties of Alm coefficients

• Alm 0 for l or m odd identical particle
  correlations
• (for non-id particles, odd l encodes shift
  information)
• A00(Q)