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Particle Ratios in Heavy-Ion Collisions

In framework of statistical models, different particle ratios at energies ranging from $3.5 $ to $200 $GeV are calculated. Assuming that the particle production takes place along the freeze-out curve, we study the sharp peak in $K^+/π^+$ ratio observed at SPS energy. We study the responsibility of non-equilibrium quark occupancy of phase space $γ_i$ for particle production. Allowing $γ_i$ to take values other than that of equilibrium, we got a very well description for $K^+/π^+$ ratio at all energies. Using the resulting parameter set, we analyzed the $K^-/π^-$, $Λ/π^+$ and \hbox{$Λ/<π>$} ratios. We found that the corresponding peaks all are located at the same value of energy, $\sqrt{s_{NN}}^{(c)}\simeq7.5 $GeV. At this energy, the entropy per particle is singular. The saddle-point in entropy per particle likely refers to critical phenomenon and change in the phase space.