First observation of a doubly charged tetraquark and its neutral partner

A combined amplitude analysis is performed for the decays $B^0\to {\bar{D}}^0{D}_s^{+}{\pi }^{-}$ and $B^{+}\to D^{-}{D}_s^{+}{\pi }^{+}$, which are related by isospin symmetry. The analysis is based on data collected by the LHCb detector in proton-proton collisions at center-of-mass energies of 7, 8 and 13$\mathrm{T}\mathrm{e}\mathrm{V}$. The full data sample corresponds to an integrated luminosity of 9$\mathrm{f}{\mathrm{b}}^{-1}$. Two new resonant states with masses of $2.908\pm 0.011\pm 0.020\mathrm{G}\mathrm{e}\mathrm{V}$ and widths of $0.136\pm 0.023\pm 0.011\mathrm{G}\mathrm{e}\mathrm{V}$ are observed, which decay to $D_s^{+}{\pi }^{+}$ and $D_s^{+}{\pi }^{-}$ respectively. The former state indicates the first observation of a doubly charged open-charm tetraquark state with minimal quark content $[c\bar{s}u\bar{d}]$, and the latter state is a neutral tetraquark composed of $[c\bar{s}\bar{u}d]$ quarks. Both states are found to have spin-parity $0^{+}$, and their resonant parameters are consistent with each other, which suggests that they belong to an isospin triplet.