We investigate the role played by the three-body $D\overline{D}\pi$ dynamics on the near-threshold resonance $X(3872)$ charmonium state, which is assumed to be formed by nonperturbative $D{\overline{D}}^{*}$ dynamics. It is demonstrated that, as compared to the naive static-pions approximation, the imaginary parts that originate from the inclusion of dynamical pions reduce substantially the width from the $D\overline{D}\pi$ intermediate state. In particular, for a resonance peaked at 0.5 MeV below the $D^0{\overline{D}}^{*0}$ threshold, this contribution to the width is reduced by about a factor of 2, and the effect of the pion dynamics on the width grows as long as the resonance is shifted towards the $D^0{\overline{D}}^0{\pi }^0$ threshold. Although the physical width of the $X$ is dominated by inelastic channels, our finding should still be of importance for the $X$ line shapes in the $D\overline{D}\pi$ channel below $D{\overline{D}}^{*}$ threshold. For example, in the scattering length approximation, the imaginary part of the scattering length includes effects of all the pion dynamics and does not only stem from the $D^{*}$ width. Meanwhile, we find that another important quantity for the $X$ phenomenology, the residue at the $X$ pole, is weakly sensitive to dynamical pions. In particular, we find that the binding energy dependence of this quantity from the full calculation is close to that found from a model with pointlike $D{\overline{D}}^{*}$ interactions only, consistent with earlier claims. Coupled-channel effects (inclusion of the charged $D{\overline{D}}^{*}$ channel) turn out to have a moderate impact on the results.

• Received 2 September 2011

DOI:https://doi.org/10.1103/PhysRevD.84.074029