Investigating S-wave bound states composed of two pseudoscalar mesons

In this study, we systematically investigated two-pseudoscalar meson systems with the Bethe-Salpeter equation in the ladder and instantaneous approximations. By solving the Bethe-Salpeter equation numerically with the kernel containing the one-particle exchange diagrams, we found that the \begin{document}$ K\bar{K} $\end{document}, \begin{document}$ DK $\end{document}, \begin{document}$ B\bar{K} $\end{document}, \begin{document}$ D\bar{D} $\end{document}, \begin{document}$ B\bar{B} $\end{document}, \begin{document}$ BD $\end{document}, \begin{document}$ D\bar{K} $\end{document}, \begin{document}$ BK $\end{document}, and \begin{document}$ B\bar{D} $\end{document} systems with \begin{document}$ I=0 $\end{document} can exist as bound states. We also studied the contributions from heavy meson (\begin{document}$ J/\psi $\end{document} and \begin{document}$\Upsilon $\end{document}) exchanges and found that the contributions from heavy meson exchanges cannot be ignored.