$D_{s0}^{\ast }(2317)$ and $DK$ scattering in $B$ decays from BaBar and LHCb data

We study the experimental $DK$ invariant mass spectra of the reactions $B^+ \to \bar{D}^0 D^0 K^+$,$B^0 \to D^- D^0 K^+$ (measured by the BaBar Collaboration) and $B_s\to {\pi }^{+}{\bar{D}}^0{K}^{-}$ (measured by the LHCb Collaboration), where an enhancement right above the threshold is seen. We show that this enhancement is due to the presence of $D_{s0}^{\ast }(2317)$, which is a $DK$ bound state in the $I(J^P)=0(0^{+})$ sector. We employ a unitarized amplitude with an interaction potential fixed by heavy meson chiral perturbation theory. We obtain a mass $M_{D^\ast_{s0}} = 2315^{+12}{-17}
^{+10}{-5}\ $\text{MeV}$$, and we also show, by means of the Weinberg compositeness condition, that the $DK$ component in the wave function of this state is $P_{DK}=70_{-6}^{+4}{}_{-8}^{+4}\mathrm{\%}$, where the first (second) error is statistical (systematic).