Particle ratios from central Pb + Pb collisions at the CERN SPS

We will address the physics of K^-/K⁺ and p̄/p ratios measured in 158 A GeV Pb + Pb collisions as a function of centrality and transverse momentum. We observe little significant centrality dependence in neither K^-/K⁺ nor p̄/p ratios and they are almost constant as a function of P_T. In addition to the chemical freeze-out temperature T_ch, we extract the chemical potentials for both light and strange quarks (μ_q and μ_s) by comparing the present data with simple model predictions. T_ch is greater than the thermal freeze-out temperature which is extracted from the transverse momentum distribution of charged hadrons.