ψ′/ψ 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 recreation of ψ′ from J/ψ 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 the ratio ψ′/ψ 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/ψ and ψ′ production in S on U and Pb on Pb collisions.