First observation and branching fraction measurement of the ${\mathrm{\Lambda }}_b^0\to D_s^{-}p$ decay

The first observation of the ${\mathrm{\Lambda }}_b^0\to D_s^{-}p$ decay is presented using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=13\text{TeV}$, corresponding to a total integrated luminosity of $6{\text{fb}}^{-1}$. Using the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$ decay as the normalisation mode, the branching fraction of the ${\mathrm{\Lambda }}_b^0\to D_s^{-}p$ decay is measured to be $\mathcal{B}({\mathrm{\Lambda }}_b^0\to D_s^{-}p)=(12.6\pm 0.5\pm 0.3\pm 1.2)\times {10}^{-6}$, where the first uncertainty is statistical, the second systematic and the third due to uncertainties in the branching fractions of the ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$, $D_s^{-}\to K^{-}{K}^{+}{\pi }^{-}$ and ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decays.