Progress in the determination of theJ/ψ−πcross section

Improving previous calculations, we compute the $J/\psi \overrightarrow{\pi }\mathrm{charmed}\mathrm{}\mathrm{meson}$ cross section using QCD sum rules. Our sum rules for the $J/\psi \overrightarrow{\pi }\overline{D}{D}^{*},$ $D{D}^{*},$ $D^{*}{D}^{*},$ and $\overline{D}D$ hadronic matrix elements are constructed by using vacuum-pion correlation functions, and we work up to twist-4 in the soft-pion limit. Our results suggest that using meson exchange models is perfectly acceptable, provided that they include form factors and that they respect chiral symmetry. After evaluating a thermal average we get $〈{\sigma }^{\pi J/\psi }v〉\sim 0.3\mathrm{mb}$ at $T=150\mathrm{}\mathrm{MeV}.$