Charm-beauty meson bound states from $B(B^{*})D(D^{*})$ and $B(B^{*})\overline{D}({\overline{D}}^{*})$ interaction

We evaluate the $s$-wave interaction of pseudoscalar and vector mesons with both charm and beauty to investigate the possible existence of molecular $BD$, $B^{*}D$, $B{D}^{*}$, $B^{*}{D}^{*}$, $B\overline{D}$, $B^{*}\overline{D}$, $B{\overline{D}}^{*}$, or $B^{*}{\overline{D}}^{*}$ meson states. The scattering amplitude is obtained implementing unitarity starting from a tree level potential accounting for the dominant vector meson exchange. The diagrams are evaluated using suitable extensions to the heavy flavor sector of the hidden gauge symmetry Lagrangians involving vector and pseudoscalar mesons, respecting heavy quark spin symmetry. We obtain bound states at energies above 7 GeV for $BD$ ($J^P=0^{+}$), $B^{*}D$ ($1^{+}$), $B{D}^{*}$ ($1^{+}$), and $B^{*}{D}^{*}$ ($0^{+}$, $1^{+}$, $2^{+}$), all in isospin 0. For $B\overline{D}$ ($0^{+}$), $B^{*}\overline{D}$ ($1^{+}$), $B{\overline{D}}^{*}$ ($1^{+}$), and $B^{*}{\overline{D}}^{*}$ ($0^{+}$, $1^{+}$, $2^{+}$) we also find similar bound states in $I=0$, but much less bound, which would correspond to exotic meson states with $\overline{b}$ and $\overline{c}$ quarks, and for the $I=1$ we find a repulsive interaction. We also evaluate the scattering lengths in all cases, which can be tested in current investigations of lattice QCD.

Figure 1

$BD$ interaction via vector-meson exchange.

Figure 2

Elementary isospin $I=1$ $BD$ and $B\overline{D}$ diagrams at quark level which show why the interaction is zero for $BD$ and not for $B\overline{D}$.

Figure 3

Vector meson exchange contribution for $BD$ interaction in isospin $I=0$.

Figure 4

$t_{BD\to BD}^{(I=0,s\text{-wave})}$, $t_{B\overline{D}\to B\overline{D}}^{(I=0,s\text{-wave})}$, and $t_{B\overline{D}\to B\overline{D}}^{(I=1,s\text{-wave})}$ as functions of $\sqrt{s}$. These kernels are dimensionless.