PARTICLE RATIOS FROM CENTRAL PB+PB COLLISIONS AT THE CERN SPS

We will address the physics of K-/K+ and (p) over bar/p ratios measure d in 158 A GeV Pb + Pb collisions as a function of centrality and tran sverse momentum. We observe little significant centrality dependence i n neither K-/K+ nor (p) over bar/p ratios and they are almost constant as a function of P-T. In addition to the chemical freeze-out temperat ure T-ch. We extract the chemical potentials for both light and strang e quarks (mu(q) and mu(s),) by comparing the present data with simple model predictions. T-ch is greater than the thermal freeze-out tempera ture which is extracted from the transverse momentum distribution of c harged hadrons.