Measurement of CP violation in $B^{0}-\overline{B^{0}}$ mixing using semileptonic decays at LHCb

This thesis presents the measurement of the CP-violating asymmetry in $B^0-\overline{B}^0$ mixing using data corresponding to an integrated luminosity of 3.0fb$^{-1}$ collected at the LHCb experiment in proton-proton collisions at the center-of-mass energies of 7 TeV and 8 TeV. This analysis uses untagged, semileptonic $B^0\rightarrow D^-\mu^+\nu_{\mu}$ and $B^0\rightarrow D^{*-}\mu^+\nu_{\mu}$ decays, where the $D^-$ decays into $K^+\pi^-\pi^-$, and the $D^{*-}$ decays into ${\overline D}^0(\rightarrow K^+\pi^-)\pi^-$. The neutrino in the semileptonic $B$ decays is not reconstructed. A decay time dependent fit allows to disentangle the CP asymmetry from the possible $B^0-\overline{B}^0$ production asymmetry. Detection and reconstruction asymmetries are calibrated using promptly produced Cabibbo-favored $D^+$ decays, and inclusive secondary $J/\psi$ decays. The CP-violating asymmetry is measured to be \begin{equation} a_{\rm sl}^{d} = (-0.02 \pm 0.19 ({\rm stat})\pm 0.30({\rm syst})\% \ . \nonumber \end{equation} This result is consistent with the Standard Model prediction, and it is the most precise measurement from a single experiment to date. This measurement is published in Phys. Rev. Lett. 114 (2015) 041601.