Azimuthally differential pion femtoscopy in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV

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2.76 TeV
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We present the first azimuthally differential measurements of the pion source size in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV relative to the second
harmonic event plane.  We find that at all analyzed centralities 0-50%, and at all pion transverse momenta, $0.2 <k_{T}< 0.7$ GeV/c, $R_{side}$ and
$R_{out}$ radii oscillate out of phase, similar to what was observed at RHIC. The relative amplitude of $R_{side}^{2}$ oscillation decreases in more
central collisions, but always stays positive --an indication of the out-of-plane extended source even after in-plane expansion. We compare our
estimates of the final state eccentricity with the initial state eccentricity derived from a Monte Carlo Glauber model as well as
to existing 3+1D hydrodynamic calculations.

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