K$^0_S$ and $\Lambda$ production in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV

The ALICE measurement of K$^0_S$ and $\Lambda$ production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 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 $\Lambda$) to 12 GeV/c. The $p_T$ dependence of the $\Lambda/K^0_S$ 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 pT 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 $\Lambda/K^0_S$ ratio.