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 $K^+/\pi^+$ ratio observed at SPS energy. We study the responsibility of non-equilibrium quark occupancy of phase space $\gamma_i$ for particle production. Allowing $\gamma_i$ to take values other than that of equilibrium, we got a very well description for $K^+/\pi^+$ ratio at all energies. Using the resulting parameter set, we analyzed the $K^-/\pi^-$, $\Lambda/\pi^+$ and \hbox{$\Lambda/<\pi>$} 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.