Centrality dependence of pion freeze-out radii in Pb-Pb collisions at √s_NN = 2.76  TeV

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at √s_NN = 2.76 TeV as a function of collision centrality and the average transverse momentum of the pair k_T. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one- dimensional radii are extracted. The radii decrease with k_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 ⟨dNch/dη⟩^1/3. This behavior is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller √s_NN, a decrease in the ratio R_out/R_side is seen, which is in qualitative agreement with a specific prediction from hydrodynamic models: a change from inside-out to outside-in freeze-out configuration. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the CERN Large Hadron Collider.