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

Understand

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 $k_{\rm T}$.

• Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted.
• The radii decrease with $k_{\rm T}$, following a power-law behavior.
• This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations.