Measurement of the branching fraction Br(B0s → D(*)s D(*)s) using the D0 detector at Fermilab

This thesis describes a measurement of the branching fraction Br(B0s → D(*)s D(*)s) made using a data sample collected from proton-antiproton collisions at a centre-of-mass energy of 1.96 TeV, corresponding to approximately 1.3 fb-1 of integrated luminosity collected in 2002--2006 by the D0 detector at the Fermilab Tevatron Collider. One D(*)s meson was partially reconstructed in the decay Ds → φμν, and the other D(*)s meson was identified using the decay Ds → φπ where no attempt was made to distinguish Ds and D*s states. The resulting measurement is Br(B0s → D(*)s D(*)s) = 0.039+0.019-0.017(stat)+0.016-0.015(syst). This was subsequently used to estimate the width difference ΔΓCPs in the B0s-(anti B)0s system: ΔΓCPs/Γs = 0.079+0.038-0.035(stat)+0.0310.030(syst), and is currently one of the most precise estimates of this quantity and consistent with the Standard Model.