contribution from mixing with S-wave states, such as the nearby $\pp(2^3S_1)$ and to a lesser extent with $J/\psi(1^3S_1)$ [@Rich] and $n \ge 3$ S-wave states above 4 GeV/$c^2$. Early calculations of this mixing based on contributions from intermediate real and virtual states of charmed particle pairs [@Eetal] predicted a $\ppp$ contribution to the $e^+ e^- \to D \bar D$ cross section which indicated the utility of this state as a “charm factory” and predicted its leptonic width quite well.[^5] It was later found that mixing due to a tensor force based on perturbative QCD also was adequate to explain the observed leptonic width [@MR]. Probably both perturbative and non-perturbative (e.g., coupled-channel) effects are present.

   Mass (MeV/$c^2$)   $\Gamma_{\rm tot}$ (MeV)   $\Gamma_{ee}$ (keV)   ${\cal B}(D^0 \od)$   ${\cal B}(D^+ D^-)$
  ------------------ -------------------------- --------------------- --------------------- ---------------------
   $3769.9 \pm 2.5$        $23.6 \pm 2.7$          $0.26 \pm 0.04$             58%                   42%

  : Properties of the $\ppp = \psi(3770)$

The mixing of the $\ppp$ with other states can affect both its decays and those of the other states. In Section II we discuss a simplified model for $\pp$–$\ppp$ mixing and its implications for leptonic and radiative partial decay rates of these states. The ratio $\Gamma(\ppp \to \gamma +
\chi_{c2})/ 