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 $K^{+}\mathrm{/}{\pi }^{+}$ 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 $K^{+}\mathrm{/}{\pi }^{+}$ ratio at all energies. Using the resulting parameter set, we analyzed the $K^{-}\mathrm{/}{\pi }^{-}$, $\mathrm{\Lambda }\mathrm{/}{\pi }^{+}$ and \hbox{$\mathrm{\Lambda }\mathrm{/}<\pi >$} ratios. We found that the corresponding peaks all are located at the same value of energy, ${\sqrt{s_{NN}}}^{(c)}\simeq 7.5$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.