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Abstract

In this thesis the measurement of the Bs-Bsbar oscillation frequency, Delta ms, in the decay Bs -> Ds Pi is presented. It is an important step to prove that the LHCb experiment is able to resolve the fast Bs-Bsbar oscillation and, thus, can perform precision measurements in the Bs system to search for physics beyond the Standard Model. Examples for such searches are the measurement of the weak mixing phase, phis, in the decays Bs -> J/Psi Phi and Bs -> J/Psi Pi Pi and the CKM-angle, gamma, in the decay Bs -> Ds K. For the measurement of Delta ms presented in this thesis a data set, accumulated by the LHCb experiment at the CERN LHC in 2011, is used. The data sample corresponds to an integrated luminosity of 1 fb^{-1} and about 34,000 signal Bs candidates are reconstructed with an average decay time resolution of 45 fs. To determine the production flavour of the Bs candidates both opposite and same side kaon flavour tagging algorithms are used with effective tagging efficiencies epsilonOST = 2.4 +- 0.4 % and epsilonSST = 1.2 +- 0.3 %. Both, the excellent decay time resolution and performance by the flavour tagging algorithms are prerequisits for a precision measurement of Delta ms. The oscillation frequency is measured to be Delta ms = 17.768 +- 0.023 \stat +- 0.006 ps^{-1}, which is the world's best measurement of this quantity.