Exotic mesons in the core-molecular hybrid model

Hadrons that are difficult to explain as ordinary hadrons are called exotic hadrons. Since the $X(3872)$ was reported by the Belle experiment in 2003, exotic hadrons containing charm quarks have been reported one after another. Exotic hadrons are believed to have a more complex structure than ordinary hadrons, and various structures are being studied, such as hadron molecular states where multiple hadrons are loosely bound like deuteron, and compact states where all the constituent quarks are gathered together, but no conclusion has been reached yet. $X(3872)$ is one of the best-known exotic hadrons and has been reported in various experiments. $X(3872)$ has $J^{PC}=1^{++}$, and its mass is very close to the threshold of $D^{0}\bar{D}^{*0}$, with a mass difference of 0.04 MeV. Also, the mass of $X(3872)$ is close to the mass of $χ_{c1}(2P)$ of 3953 MeV predicted from the Godfrey-Isgur relativized quark model. In this study, $X(3872)$ is analyzed as a superposition state of the $D\bar{D}^*$ hadron molecule, the $D^*\bar{D}^*$ hadron molecule, and the core of the charmonium $χ_{c1}(2P)$. We consider meson exchange and core-molecule mixing potential as interactions. We solve the Schrödinger equation using the Gaussian expansion method (GEM). We use the results of $X(3872)$ to analyze the structure of other exotic hadrons.