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.

PB talk: https://indico.cern.ch/event/286692/contributions/1639614/attachments/53...