K⁰ S and Λ production in Pb-Pb collisions at √s NN = 2.76 TeV

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The ALICE measurement of K⁰ S and Λ production at mid-rapidity in Pb-Pb collisions at √s NN = 2.76 TeV is presented. The transverse momentum (p T) spectra are shown for several collision centrality intervals and in the p T range from 0.4 GeV/c (0.6 GeV/c for Λ) to 12 GeV/c. The p T dependence of the Λ/K⁰ S ratios exhibits maxima in the vicinity of 3 GeV/c, and the positions of the maxima shift towards higher p T with increasing collision centrality. The magnitude of these maxima increases by almost a factor of three between most peripheral and most central Pb-Pb collisions. This baryon excess at intermediate p T is not observed in pp interactions at sqrt(s) = 0.9 TeV and at sqrt(s) = 7 TeV. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured p T spectra above 2 GeV/c progressively decouple from hydrodynamical-model calculations. For higher values of p T, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the p T dependence of the Λ/K⁰ S ratio.