Measurement of the decay $B^{0}_{s} \to \phi\mu^{+}\mu^{-}$ at LHCb

The measurements of two branching fraction ratios are reported in this thesis. The first one is a measurement of the ratio of the branching fractions for the decays $B^0_s\rightarrow\psi^'\phi$ and $B^0_s\rightarrow J/\psi\phi$. It is based on approximately $36\mbox{pb}^{-1}$ of data collected by the LHCb detector at $\sqrt{s}=7~\rm{TeV}$ during 2010. The result is $0.68\pm0.10\mathrm{(stat)}\pm0.09\mathrm{(syst)}\pm0.07({\cal B})$, which is compatible with previous measurements from experiments at the Tevatron. The second measurement is the ratio of the branching fractions for the decays $B^0_s\rightarrow\phi\mu^+ \mu^-$ and $B^0_s\rightarrow J/\psi\phi$ as a function of the dimuon invariant mass. It is based on $1\mbox{fb}^{-1}$ of data collected by the LHCb detector at $\sqrt{s}=7~\rm{TeV}$ during 2011. The ratio of branching fractions is measured to be $(0.558\pm0.070\mathrm{(stat)}\pm0.043\mathrm{(syst)}\pm0.006({\cal B}))\times10^{-3}$. Using the measured branching fraction of the decay $J/\psi \rightarrow \mu^+ \mu^-$ gives a branching fraction of ${\cal B}(B^0_s\rightarrow\phi\mu^+ \mu^-)=(0.78\pm0.10\mathrm{(stat)}\pm0.06\mathrm{(syst)}\pm0.28({\cal B}))\times10^{-6}$. This is the most precise measurement of ${\cal B}(B^0_s\rightarrow\phi\mu^+ \mu^-)$ and is in agreement with the Standard Model expectation.