Measurement of the flavour-specific CP violating asymmetry $a_{sl}^s$ in $\bar{B}_s^0$ decays

The CP violating asymmetry $a_{sl}^s$ is studied using samples of $\bar{B}_s^0$ and $B_s^0$ semileptonic decays in pp collisions at a centre-of-mass energy of 7 TeV using an integrated luminosity of 1/fb provided by the LHC. The detected final states are $D_s^\pm\mu^\mp$ and $D_s^-\mu^+$ with $D_s^\pm$ reconstructed in the $\phi\pi^\pm$ mode. The analysis is summed over $\bar{B}_s^0$ and $B_s^0$ initial states, so that flavour tagging is unnecessary, and integrated with respect to decay time. Data driven methods have been developed to measure all the efficiency ratios needed to determine $a_{sl}^s$ . We obtain $a_{sl}^s = (-0.24 \pm 0.54 \pm 0.33)$%, where the first uncertainty is statistical and the second systematic.