Exotic Hadrons from Scattering in the Diabatic Dynamical Diquark Model

The diabatic framework generalizes the adiabatic approximation built into the Born-Oppenheimer (BO) formalism, and is devised to rigorously incorporate the mixing of BO-approximation eigenstates with two-particle thresholds. We recently applied this framework in a bound-state approximation to the mixing of hidden-charm dynamical-diquark tetraquark states with open-charm di-meson thresholds. Since almost all of these states are observed as above-threshold resonances, we here implement the corresponding scattering formalism to allow for a study of exotic tetraquark resonances within the diabatic framework. We calculate elastic open-charm di-meson cross sections (in channels with zero, open, and hidden strangeness) as functions of center-of-mass energy, and observe the development of true resonances, near resonances, and various threshold cusp effects. As an example, $\chi_{c1}(3872)$ can originate in the $1^{++}$ channel as a diquark-antidiquark state enhanced by the $D^0 \overline{D}^{*0}$ threshold, with or without an additional contribution from the conventional charmonium $\chi_{c1}(2P)$ state.