Measurement of the ratio of branching fractions for $B ^0_s \rightarrow \phi\mu\mu$ and $B ^0_s \rightarrow J/\psi \phi$

A measurement of the ratio of the branching fractions for $B ^0_s \rightarrow \phi\mu\mu$ and $B ^0_s \rightarrow J /\psi \phi$ as a function of the dimuon invariant mass with $1\rm{fb}^{-1}$ of data collected by the LHCb detector at $\sqrt{s}=7 \,\rm{TeV}$ during the 2011 run is reported. The ratio of branching fractions is measured to be $(0.558\pm0.070\rm{(stat)}\pm0.043\rm{(syst)})\times10^{-3}$, leading to a branching fraction of ${\cal B}(B ^0_s \rightarrow \phi\mu\mu)=(0.78\pm0.10\rm{(stat)}\pm0.06\rm{(syst)}\pm0.28({\cal B}))\times10^{-6}$.