This thesis focuses on the study of the decay channel Bs → J/ψ φ → µ+ µ− K + K 0 to measure the width diﬀerence |∆Γs | between the light and heavy eigenstates of the 0 Bs meson. The analysis is performed using a data sample of proton-proton collisions √ at s = 7 TeV, corresponding to an integrated luminosity of 5 fb−1 and recorded by the CMS detector at the LHC. First, the reconstruction and selection of the Bs → J/ψ φ → µ+ µ− K + K − de 0 cay channel is presented. In this channel, the four-track candidates are displaced 0 from the primary vertex due to the decay of the Bs meson. The trigger selection requires di-muon events with the presence of a J/ψ candidate with a vertex displaced from the primary vertex. Monte Carlo samples are used to validate the theoreti- cal models, the contributions of the possible background processes, and the event selection techniques. The comparison of results of the diﬀerent possible triggers ob- tained from real data and from simulation studies conﬁrm that the triggering and selection methods used are appropriate for selecting the Bs → J/ψ φ → µ+ µ− K + K − 0 decay channel. After applying all selection cuts, 19 200 Bs → J/ψ φ → µ+ µ− K + K − 0 event candidates are obtained. A full untagged angular analysis is performed to 0 disentangle the mass eigenstates of the Bs meson and to extract the parameters τs = 1/Γs , |∆Γs |, |A0 |2 , |A⊥ |2 , and the strong phase δ|| . A ﬁtting technique is de- veloped based on the maximum-likelihood method. This thesis assumes a vanishing CP violating phase φs , predicted to be very small in the Standard Model.