Centrality dependence of pion freeze-out radii in Pb-Pb collisions at $\sqrt{\mathbf{s_{NN}}}$=2.76 TeV

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2.76 TeV
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We report on the results of femtoscopic analysis of pairs of
identical-charge pions measured in Pb--Pb collisions at
\rootsNN=2.76~TeV as a function of collision centrality and the
average transverse momentum of the pair \ktns. Three-dimensional sizes
of the system (femtoscopic radii) are extracted. They decrease with
\ktns, following a power-law behavior. This is qualitatively
consistent with expectations from a collectively expanding system,
produced in hydrodynamic calculations. Independently the sizes scale
linearly with  \dndens. This behaviour is compared to world data on
femtoscopic radii in heavy-ion collisions. While the dependence is
qualitatively similar, a distinct decrease in the $R_{\rm out}/R_{\rm
  side}$ ratio to values consistent with unity is seen, in agreement with
specific predictions from hydrodynamic models. The results confirm the
production of a collective, strongly coupled system
in heavy-ion collisions at the LHC.