Measurement of the Semileptonic Charge Asymmetry using $B_s^0\to D_s\mu X$ Decays

We present a measurement of the time-integrated flavor-specific semileptonic charge asymmetry in the decays of $B_s^0$ mesons that have undergone flavor mixing, $a_{\mathrm{sl}}^s$, using $B_s^0({\overline{B}}_s^0)\to D_s^{\mp }{\mu }^{\pm }X$ decays, with $D_s^{\mp }\to \varphi {\pi }^{\mp }$ and $\varphi \to K^{+}{K}^{-}$, using $10.4{\mathrm{fb}}^{-1}$ of proton-antiproton collisions collected by the D0 detector during Run II at the Fermilab Tevatron Collider. A fit to the difference between the time-integrated $D_s^{-}$ and $D_s^{+}$ mass distributions of the $B_s^0$ and ${\overline{B}}_s^0$ candidates yields the flavor-specific asymmetry $a_{\mathrm{sl}}^s=[-1.12\pm 0.74(\mathrm{stat})\pm 0.17(\mathrm{syst})]\%$, which is the most precise measurement and in agreement with the standard model prediction.

Figure 1

The weighted $K^{+}{K}^{-}{\pi }^{\mp }$ invariant mass distribution for the ${\mu }^{\pm }\varphi {\pi }^{\mp }$ sample with the solid line representing the signal fit and the dashed line showing the background fit. The lower mass peak is due to the decay $D^{\mp }\to \varphi {\pi }^{\mp }$, and the second peak is due to the $D_s^{\mp }$ meson decay. Note the zero suppression on the vertical axis.

Figure 2

The fit to the difference distribution for the data (for clarity, the data have been rebinned).

Figure 3

A combination of this result with two measurements of $A_{\mathrm{sl}}^b$ with different muon impact parameter selections made using like-sign dimuons [3] and the average of $a_{\mathrm{sl}}^d$ measurements from $B$ factories [16]. The error bands represent the $\pm 1$ standard deviation uncertainties on each individual measurement. The ellipses represent the 1, 2, 3, and 4 standard deviation two-dimensional C.L. regions, respectively, in the $a_{\mathrm{sl}}^s$ and $a_{\mathrm{sl}}^d$ plane.