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Azimuthally-differential femtoscopic measurements, being sensitive tospatio-temporal characteristics of the source as well as to the collectivevelocity fields at freeze out, provide very important information on the natureand dynamics of the system evolution. While the HBT radii oscillations relativeto the second harmonic event plane measured recently reflect mostly the spatialgeometry of the source, model studies have shown that the HBT radiioscillations relative to the third harmonic event plane are predominantlydefined by the velocity fields. In this Letter, we present the first results onazimuthally-differential pion femtoscopy relative to the third harmonic eventplane as a function of the pion pair transverse momentum $k_{\rm T}$ fordifferent collision centralities in Pb-Pb collisions at $\sqrt{s_{\rmNN}}=2.76$ TeV. We find that the $R_{\rm side}$ and $R_{\rm out}$ radii, whichcharacterize the pion source size in the directions perpendicular and parallelto the pion transverse momentum, oscillate in phase relative to the thirdharmonic event plane, similar to the results from 3+1D hydrodynamicalcalculations. The observed radii oscillations unambiguously signal a collectiveexpansion and anisotropy in the velocity fields. A comparison of the measuredradii oscillations with the Blast-Wave model calculations indicate that theinitial state triangularity is washed-out at freeze out.

Azimuthally-differential pion femtoscopy relative to the third harmonic event plane in Pb-Pb collisions at $\sqrt{\textit{s}_{_{\rm NN}}}$ = 2.76 TeV