Measurement of the differential branching fraction of the decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\mu }^{+}{\mu }^{-}$

The differential branching fraction of the decay ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\mu }^{+}{\mu }^{-}$ is measured as a function of the square of the dimuon invariant mass, $q^2$. A yield of $78\pm 12$ ${\mathrm{\Lambda }}_b^0\to \mathrm{\Lambda }{\mu }^{+}{\mu }^{-}$ decays is observed using data, corresponding to an integrated luminosity of 1.0,fb${}^{-1}$, collected by the LHCb experiment at a centre-of-mass energy of 7,TeV. A significant signal is found in the $q^2$ region above the square of the $J\mathrm{/}\psi$ mass, while at lower-$q^2$ values upper limits are set on the differential branching fraction. Integrating the differential branching fraction over $q^2$, while excluding the $J\mathrm{/}\psi$ and $\psi (2S)$ regions, gives a branching fraction of $\text{BF}($\Lambda_b^0\rightarrow\Lambda\mu^+\mu^-$)=(0.96\pm 0.16\stat\pm 0.13\syst\pm 0.21 ($\mathrm{n}\mathrm{o}\mathrm{r}\mathrm{m}$))\times 10^{-6}$, where the uncertainties are statistical, systematic and due to the normalisation mode, $${\mathrm{\Lambda }}_b^0\to J\mathrm{/}psi\mathrm{\Lambda }$, respectively.