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

Abstract

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ 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 ${⟨\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta ⟩}^{1/3}$. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller $\sqrt{s_{\mathrm{NN}}}$, a decrease in the $R_{\mathrm{out}}/R_{\mathrm{side}}$ 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.

28 pages, 9 captioned figures, 1 table, authors from page 23, published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/1816

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