Centrality dependence of hadronization and chemical freeze-out conditions in heavy ion collisions at $\sqrt{s_{NN}}=2.76$ TeV

We present an analysis of hadronic multiplicities measured in Pb-Pb collisions at ${\sqrt{s}}_{NN}=2.76$ TeV as a function of the collision centrality within the statistical hadronization model. Evidence is found of a dependence of the chemical freeze-out temperature as a function of centrality, with a slow rise from central to peripheral collisions, which we interpret as an effect of posthadronization inelastic scatterings. Using correction factors calculated by means of a simulation based on the urqmd model, we are able to obtain a significant improvement in the statistical model fit quality and to reconstruct the primordial chemical equilibrium configuration. This is characterized by a nearly constant temperature of about 164 MeV, which we interpret as the actual hadronization temperature.

Figure 1

Ratio between antiproton and negative pion yields in relativistic heavy ion collisions as a function of centrality at different energies.

Figure 2

Ratio between ${\overline{\Xi }}^{+}$ and negative pion yields in relativistic heavy ion collisions as a function of centrality at different energies. At the largest energy (a), a structure can be clearly seen. We interpret the bump in midperipheral events as the combination of two effects: increased baryon annihilation owing to larger multiplicity at LHC and the corona effect in very peripheral. Also shown is the prediction of coronaless urqmd calculations normalized to the most central bin.

Figure 3

Modification factors (see text for definition) for ${\pi }^{+}$, proton, and ${\Xi }^{-}$ as a function of centrality at ${\sqrt{s}}_{NN}=2.76$ TeV calculated with urqmd. The error bars are statistical.

Figure 4

Temperature as a function of the impact parameter $b$ (central values corresponding to centralities measured by ALICE). Black dots, chemical freeze-out temperature; red dots, LCEP (see text) temperature obtained by including urqmdmodification factors.

Figure 5

${\chi }^2$ of the SHM fits with and without afterburning corrections as a function of the impact parameter $b$ (central values corresponding to centralities measured by ALICE). The fitted parameters being in this case $T,{\gamma }_S$, and the normalization; the number of degrees of freedom is 7.

Figure 6

Difference between the corrected temperature and the chemical freeze-out temperature as a function of the impact parameter $b$ (central values corresponding to centralities measured by ALICE). The error bar has been estimated by taking a 100% correlation between the errors on $T$ in the two fits.