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{\small \it Quark Matter 2008, Feb. 4-10, 2008 - Jaipur - India}

\textbf{oral}
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\centerline{\bf 
FEMTOSCOPIC SIGNALS OF TRANSVERSE EXPANSION:
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\centerline{\bf 
LEARNING FROM LIGHT SYSTEMS
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\centerline{ {\bf M.~Kopytine}$^{\rm a}$ for the STAR Collaboration }

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\centerline{$^{\rm a}$Physics Department, Kent State University,
}\centerline{Kent, OH, 44242, USA, {\it kopytin@bnl.gov}}

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Decrease of HBT radii with $m_t$ has an established interpretation 
as a signal of transverse expansion believed to be a collective property 
of heavy ion collisions. Paradoxically such a decrease has also been 
seen in $e^+e^-$ collisions[1,2]. Correlations of non-identical mesons, 
being sensitive to a difference in space-time emission patterns of light 
and heavier particles, and thus to transverse expansion, cross-check the 
interpretations. STAR presents correlations of identical charged $\pi\pi$ 
and $K\pi$ in light systems (pp, dAu). We use spherical harmonics to 
capture the features of the two-particle correlation in the space of 
momentum differences. We use own and established models (such as HIJING) 
to associate longer range features with effects of energy-momentum 
conservation on the global and binary-collision scale. At lower momentum 
difference, $\pi\pi$ and $K\pi$ correlations due to HBT and/or Coulomb 
effects paint a qualitatively consistent if unexpected picture of 
transverse collectivity.

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\parindent=0pt \textbf{References}


[1] O.~Smirnova for DELPHI, ISMD XXVIII 

[2] OPAL, CERN-PH-EP/2007-025



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