Speaker
Description
Motivated by the recent HAL QCD lattice results on the $D$ - $D^*$ potential at nearly physical point $m_\pi=146.4$ MeV, we theoretically examine effects from the two-pion exchange in (semi-)long-range parts of the $D$ - $D^*$ system. We employ the framework of heavy-meson chiral perturbation theory with heavy-quark spin symmetry. The pion exchanges up to next-to-next-to-leading order (N$^2$LO) are taken into account. In order to separate the (semi-)long-range contributions, we apply the dispersion-relation method to one-loop diagrams mediated by two-pion exchanges. As a result, we find that the lattice data on the $D$ - $D^*$ potential in a range of $0.5\, {\rm fm} \lesssim r\lesssim 2.0\, {\rm fm}$ is beautifully reproduced by adjusting only three higher-order couplings. In particular, it turns out that contributions from isospin-independent N$^2$LO triangle diagrams play a central role in reproducing the lattice data of the form $V_{DD^*}\sim \left({\rm e}^{-m_\pi r}/r\right)^2$ in $1.0\, {\rm fm} \lesssim r\lesssim 2.0\, {\rm fm}$. Our findings provide useful information on the (semi)long-range regime of the $D$ - $D^*$ potential focusing on two-pion exchanges.