First observation of the $B^{+}\to D_s^{+}{D}_s^{-}{K}^{+}$ decay

The $B^{+}\to D_s^{+}{D}_s^{-}{K}^{+}$ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at centre-of-mass energies of $7$, $8$ and $13\text{TeV}$, corresponding to an integrated luminosity of $9{\text{fb}}^{-1}$. Its branching fraction relative to that of the $B^{+}\to D^{+}{D}^{-}{K}^{+}$ decay is measured to be $$\frac{B(B^{+}\to D_s^{+}{D}_s^{-}{K}^{+})}{B(B^{+}\to D^{+}{D}^{-}{K}^{+})}=0.525\pm 0.033\pm 0.027\pm 0.034,$$ where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the $D_s^{\pm }\to K^{\mp }{K}^{\pm }{\pi }^{\pm }$ and $D^{\pm }\to K^{\mp }{\pi }^{\pm }{\pi }^{\pm }$ decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo$-$favoured $\bar{b}\to \bar{c}c\bar{s}$ transitions and opens the path for unique studies of spectroscopy in future.