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

We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb-Pb collisions at sNN=2.76 TeV as a function of collision centrality and the average transverse momentum of the pair kT. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with kT, 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 sNN, a decrease in the ratio Rout/Rside 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.

Note:

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

freeze-out
pi
impact parameter
transverse momentum
pair
model: hydrodynamics
heavy ion
strong coupling
charged particle
density

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

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