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

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb–Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV as a function of collision centrality and the average transverse momentum of the pair $k_{\mathrm{T}}$. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with $k_{\mathrm{T}}$, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with ${<\mathrm{d}{N}_{\mathrm{c}\mathrm{h}}\mathrm{/}\mathrm{d}\eta >}^{1\mathrm{/}3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\mathrm{N}\mathrm{N}}}$, a decrease in the $R_{\mathrm{o}\mathrm{u}\mathrm{t}}\mathrm{/}{R}_{\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}}$ ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.