Improving previous calculations, we compute the $J/\psi \pi\to { charmed mesons}$ cross section using QCD sum rules. Our sum rules for the $J/\psi \pi\to \bar{ D} D^*$, $D \bar{ D}^*$, ${ \bar D}^* D^*$ and ${ \bar D} D$ hadronic matrix elements are constructed by using vaccum-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 doing a thermal average we get $<\sigma^{ \pi J/\psi} v>\sim 0.3$ mb at $T=150\MeV$.
Categories: nucl-th hep-ph