Measurement of the differential branching fraction of the decay $\Lambda_b^0 \rightarrow \Lambda\mu^+\mu^-$

The differential branching fraction of the decay $\Lambda_b^0\rightarrow\Lambda\mu^+\mu^-$ is measured as a function of the square of the dimuon invariant mass, $q^2$. A yield of $78\pm12$ $\Lambda_b^0\rightarrow\Lambda\mu^+\mu^-$ 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 $J/\psi$ 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 $J/\psi$ and $\psi(2S)$ regions, gives a branching fraction of $B(\Lambda_b^0\rightarrow\Lambda\mu^+\mu^-)=(0.96\pm 0.16(stat)\pm 0.13(syst)\pm 0.21 (\mathrm{norm}))\times 10^{-6}$, where the uncertainties are statistical, systematic and due to the normalisation mode, $\Lambda_b^0\rightarrow J/\psi\Lambda$, respectively.