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 theoretical models, the contributions of the possible background processes, and the event selection techniques. The comparison of results of the diﬀerent possible triggers obtained 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 developed based on the maximum-likelihood method. This thesis assumes a vanishing CP violating phase φs , predicted to be very small in the Standard Model.