$B_{s}\to J/\psi ~\phi$ at LHCb

CP violation in the interference between mixing and decay of $B_{s}\to J/\psi ~\phi$ is characterised by the weak phase $\Phi$ which is predicted to be $-2\beta_s$ in the SM. A deviation in $\Phi$ from $-2\beta_s$ is an indicator of new physics entering into $B_{s}/\bar{B}_{s}$ mixing. LHCb will yield a competitive measurement of the phase $\Phi$ in less than one year of data taking.