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

TL;DR

This paper reports on the measurement of strange particle production in Pb-Pb collisions at 2.76 TeV, revealing a centrality-dependent baryon enhancement at intermediate transverse momentum, with implications for understanding quark-gluon plasma dynamics.

Contribution

It provides detailed $p_T$ spectra and $ m ext{Lambda}$/K$^0_{ m S}$ ratios in Pb-Pb collisions, highlighting the evolution of baryon enhancement with collision centrality and $p_T$, and compares results with hydrodynamical and medium-modified fragmentation models.

Findings

Baryon-to-meson ratio peaks near 3 GeV/$c$ and shifts with centrality.

Maximum baryon enhancement increases significantly from peripheral to central collisions.

High-$p_T$ spectra deviate from hydrodynamical predictions but align qualitatively with medium-influenced fragmentation models.

Abstract

The ALICE measurement of K$^0_{\rm S}$ and $\rm\Lambda$ production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV is presented. The transverse momentum ($p_{\rm T}$) spectra are shown for several collision centrality intervals and in the $p_{\rm T}$ range from 0.4 GeV/$c$ (0.6 GeV/$c$ for $\rm\Lambda$) to 12 GeV/$c$. The $p_{\rm T}$ dependence of the $\rm \Lambda$/K$^0_{\rm S}$ ratios exhibits maxima in the vicinity of 3 GeV/$c$, and the positions of the maxima shift towards higher $p_{\rm T}$ 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 $p_{\rm T}$ 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 $p_{\rm T}$ spectra above 2 GeV/$c$ progressively decouple from hydrodynamical-model calculations. For higher values of $p_{\rm T}$, models that incorporate the influence of the medium on the fragmentation and hadronization processes describe qualitatively the $p_{\rm T}$ dependence of the $\rm\Lambda$/K$^0_{\rm S}$ ratio.