Evolution of charm-meson ratios in an expanding hadron gas

We study the time evolution of the numbers of charm mesons after the kinetic freeze-out of the hadron gas produced by a central heavy-ion collision. The $\pi D^{\ast }\to \pi D^{\ast }$ reaction rates have $t$-channel singularities that give contributions inversely proportional to the thermal width of the $D$. The ratio of the $D^0$ and $D^{+}$ production rates can differ significantly from those predicted using the measured $D^{\ast }$ branching fractions.