K S 0 and Λ Production in Pb-Pb Collisions at s N N = 2.76 TeV

The ALICE measurement of ${K}_{S}^{0}$ and $\ensuremath{\Lambda}$ production at midrapidity in Pb-Pb collisions at $\sqrt{{s}_{NN}}=2.76\text{ }\text{ }\mathrm{TeV}$ is presented. The transverse momentum (${p}_{T}$) spectra are shown for several collision centrality intervals and in the ${p}_{T}$ range from $0.4\text{ }\text{ }\mathrm{GeV}/c$ ($0.6\text{ }\text{ }\mathrm{GeV}/c$ for $\ensuremath{\Lambda}$) to $12\text{ }\text{ }\mathrm{GeV}/c$. The ${p}_{T}$ dependence of the $\ensuremath{\Lambda}/{K}_{S}^{0}$ ratios exhibits maxima in the vicinity of $3\text{ }\text{ }\mathrm{GeV}/c$, 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 $\sqrt{s}=0.9\text{ }\text{ }\mathrm{TeV}$ and at $\sqrt{s}=7\text{ }\text{ }\mathrm{TeV}$. Qualitatively, the baryon enhancement in heavy-ion collisions is expected from radial flow. However, the measured ${p}_{T}$ spectra above $2\text{ }\text{ }\mathrm{GeV}/c$ 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 $\ensuremath{\Lambda}/{K}_{S}^{0}$ ratio.