$B_d$ and $B_s$ mixing at LHCb

The performances of the re-optimized LHCb detector is studied. In the first part, the reconstruction and selection efficiencies are given for different channels which will be used to estimate the sensitivities to the mixing parameters $\Delta_m$ and $\phi$. The high statisics of LHCb will allow to reduce the uncertainty on $\Delta m_d$ and $\phi_d$, and determine $\Delta m_s$ and $\phi_s$.