$K_S^0$ and $\Lambda$ Production in Pb-Pb Collisions at $\sqrt{s_{NN}}\mathbf{=}2.76\mathrm{TeV}$

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

Figure 1

$K_S^0$ and $\Lambda$ $p_T$ spectra for different event centrality intervals. The curves represent results of blast-wave fits [17].

Figure 2

Left: $\Lambda /K_S^0$ ratios as a function of $p_T$ for different event centrality intervals in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ and $pp$ collisions at $\sqrt{s}=0.9$ [11] and 7 TeV [20]. Right: selected $\Lambda /K_S^0$ ratios as a function of $p_T$ compared with $\Lambda /K_S^0$ and $\overline{\Lambda }/K_S^0$ ratios measured in Au-Au collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ [23]. The solid, dashed, and dot-dashed lines show the corresponding ratios from a hydrodynamical model [24, 25, 26], a recombination model [28] and the EPOS model [29], respectively.