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

The CP-violating asymmetry $a_{sl}^s$ is studied using semileptonic decays of $B_s^0$ and anti-$B_s^0$ mesons produced in pp collisions at a centre-of-mass energy of 7 TeV at the LHC, exploiting a data sample corresponding to an integrated luminosity of 1.0fb$^-1$. The reconstructed final states are $D_s^{\pm}\mu^{\mp}$, with the $D_s^{\pm}$ particle decaying in the $\phi\pi^{\pm}$ mode. The $D_s^{\pm}\mu^{\mp}$ yields are summed over anti-$B_s^0$ and $B_s^0$ initial states, and integrated with respect to decay time. Data-driven methods are used to measure efficiency ratios. We obtain $a_{sl}^s$ = (-0.06 $\pm$ 0.50 $\pm$ 0.36)%, where the first uncertainty is statistical and the second systematic.