Short-range dynamics of $X(3872)$ and $T_{cc}(3875{)}^{+}$ in a coupled-channel framework

Abstract

We study the short-range dynamics in the coupled-channel description of the $S$-wave $D{\bar{D}}^{\ast }$ and $D{D}^{\ast }$ systems for $X(3872)$ and $T_{cc}(3875{)}^{+}$, respectively. Contact interactions are related through scalar- and vector-meson exchange saturation, while pion exchange is included explicitly with its local term subtracted. The coupled Lippmann-Schwinger equations incorporate the self-energy of the unstable $D^{\ast }$ mesons, thereby accounting for the three-body $D\bar{D}\pi$ ($DD\pi$) dynamics. Fixing the contact interaction from the $T_{cc}(3875{)}^{+}$ pole, we trace the predicted $X(3872)$ pole as the local magnetic dipole term from vector-meson exchange is varied. Retaining this term in full produces the shallowest poles, which remain approximately $5$--$12\mathrm{MeV}$ below the observed $X(3872)$ mass. It is found that the pion exchange has a moderate effect on the binding energy and a stronger effect on the isospin violation for the $X(3872)$ state. The residual tension between the pure-molecular bound descriptions of $X(3872)$ and $T_{cc}(3875{)}^{+}$ is analyzed with the Feshbach projection. This analysis indicates a $D{D}^{\ast }$ interaction weaker than that implied by the purely molecular fits, and shows that the $T_{cc}(3875{)}^{+}$ state instead arises from the mixing of the $D{D}^{\ast }$ channel with a compact $cc\bar{u}\bar{d}$ state. We further examine the spatial structure using Born--Oppenheimer effective field theory. For separation radii of $0.5$--$1.5\mathrm{fm}$, the combined charmonium and short-distance tetraquark weight in $X(3872)$ is $3.0$--$9.7\%$, while the short-distance weight in $T_{cc}(3875{)}^{+}$ is $1.3$--$14.4\%$. These results favor retaining the local vector-exchange contribution within the saturation framework and motivate a joint treatment of molecular and compact dynamics for the $T_{cc}(3875{)}^{+}$.

arXiv comment17 pages, 8 figures, 3 tables