Charmonium resonances and Fano line shapes

Abstract

Anomalous line shapes of quarkonia are explained naturally as an interference effect of a $c\bar c$ confined closed channel with the surrounding continua, well established in other fields of physics as Fano-resonances. We discuss a quark model coupled-channel analysis describing quarkonium as a mixing of closed $Q\bar Q$ and molecular-like $D\bar D$ open channels. The asymmetric line shapes observed in $\psi(3770)$ production cross sections in $e^+e^-$ annihilation to $D^0\bar{D}^0$ and $D^+ D^-$, respectively, are described very well. The method allows to extract directly from the data the amount of $Q\bar Q \leftrightarrow D\bar D$ configuration mixing.