Physics and detector performance measurements with the decays $B_s^0 \to J/ \psi \phi$ and $B_d^0 \to J/ \psi K^{0*}$ with early data

The decay processes $B_s^0 \to J/ \psi \phi$ and $B_d^0 \to J/ \psi K^{0*}$ are expected to be observed in large numbers with the ATLAS experiment. During the early data taking period, with an integrated luminosity of around ~10-150 pb$^{-1}$, it will be possible to measure the masses and proper lifetimes for these decays with sufficient precision to allow them to be used for detector performance checks. Methods for the determination of the mass and lifetime %5appropriate when the performance of the detector and reconstruction software will not be fully understood are presented. A powerful simultaneous fitting technique is used. %tudies of the jet charge tagger for \Bd and \Bs decays are also presented. Understanding the potential for flavour tagging methods will be one of the important goals for \Bmes-physics with early data. The performance of the jet charge tagger for the self-calibrating decay, $B_d^0 \to J/ \psi K^{0*}$, is presented. The implications for the jet charge tag in the $B_s^0 \to J/ \psi \phi$ decay are also discussed.