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 B0s→ψ'Ø and
B0s→J/ψØ. It is based on approximately 36 pb⁻¹ of data collected by the LHCb
detector at √s = 7TeV during 2010. The result is 0.68 ± 0.10(stat) ± 0.09(syst) ± 0.07(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 B0s→Øμ⁺μ⁻
and B0s→J/ψØ as a function of the dimuon invariant mass. It is based on 1 fb⁻¹ of
data collected by the LHCb detector at
√s= 7TeV during 2011. The ratio of branching
fractions is measured to be (0.558 ± 0.070(stat) ± 0.043(syst) ± 0.006(B)) x 10⁻³. Using
the measured branching fraction of the decay J/ψ→μ⁺μ⁻ gives a branching fraction of
B(B0s→Øμ⁺μ⁻) = (0.78 ± 0.10(stat) ± 0.06(syst) ± 0.28(B)) x 10⁻⁶. This is the most
precise measurement of B(B0s→Øμ⁺μ⁻) and is in agreement with the Standard Model
expectation.