Measurement of the branching ratio of B${}_s^0$ → K${}_S^0$K${}_S^0$ decays with the LHCb experiment

The presented thesis deals with the branching ratio measurement of the decay channel B${}_s^0$ → K${}_S^0$K${}_S^0$ reconstructed as two charged pion pairs. The data correspond to an integrated luminosity of 5 fb${}^{-1}$ collected by the LHCb experiment in proton-proton collisions at the centre-of-mass energies of $\sqrt{s}$ = 7 TeV (1 fb${}^{-1}$), 8 TeV (2 fb${}^{-1}$) and 13 TeV (2 fb${}^{-1}$). As normalisation channel the decay B${}^0$ → ϕK${}_S^0$ is used in which the ϕ meson is reconstructed as two charged kaons. The branching ratio is determined to be BR(B${}_s^0$ → K${}_S^0$K${}_S^0$) = [9.5 ± 1.9 (stat) ± 0.9 (syst) ± 0.9 (BR(B${}^0$ → ϕK${}_S^0$) ± 0.5 ($f_d$/$f_s$)] × 10${}^{-6}$, whereby the B${}_s^0$ → K${}_S^0$K${}_S^0$ component has a significance of 7.24σ. The result is consistent with measurements of the BELLE collaboration within less than one standard deviation and compatible with Standard Model predictions. Furthermore, the analysis represents the first study of b hadrons decaying exclusively into long lived V${}^0$ particles at the LHCb experiment.