${\mathrm{\Psi }}^{\mathrm{\prime }}\mathrm{/}\mathrm{\Psi }$ ratio in Nucleus-Nucleus Collisions : a Measure for the Chiral Symmetry Restoration Temperature ?

We argue that a decrease of the chiral scalar meson mass is responsible for re-creation of ${\mathrm{\Psi }}^{\mathrm{\prime }}$ from $J\mathrm{/}\mathrm{\Psi }$ in ultrarelativistic nucleus-nucleus collisions. This causes the charmonium yields to freeze out at temperatures close to the chiral symmetry restoration temperature $T_c$. As a result ${\mathrm{\Psi }}^{\mathrm{\prime }}\mathrm{/}\mathrm{\Psi }$ may serve as a thermometer for $T_c$ itself. Results in a detailed reaction model support the conjecture. They show good agreement with recent data of NA38 and NA50 for $J\mathrm{/}\mathrm{\Psi }$ and ${\mathrm{\Psi }}^{\mathrm{\prime }}$ production in S on U and Pb on Pb collisions.