The $CP$ asymmetry in the mixing of $B_s^0$ and $\overline{ B}{ }_s^0$ mesons is measured in proton-proton collision data corresponding to an integrated luminosity of 3.0 fb$^{ -1}$, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Semileptonic $B_s^0$ and $\overline{ B}{ }_s^0$ decays are studied in the inclusive mode $D_s^{ \mp} \mu^{ \pm} \nu_{ \mu} X$ with the $D_s^{ \mp}$ mesons reconstructed in the $K^+ K^- \pi^{ \mp}$ final state. Correcting the observed charge asymmetry for detection and background effects, the $CP$ asymmetry is found to be $a_{ \text{ sl}}^s = (0.39 \pm 0.26 \pm 0.20) \% $, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $a_{ \text{ sl}}^s$ to date. It is consistent with the prediction from the Standard Model and will constrain new models of particle physics.
Categories: hep-ex
