Study of $CP$ Asymmetry in $B^0\text{−}{\overline{B}}^0$ Mixing with Inclusive Dilepton Events

We present a measurement of the asymmetry $A_{CP}$ between same-sign inclusive dilepton samples ${\ell }^{+}{\ell }^{+}$ and ${\ell }^{-}{\ell }^{-}$ ($\ell =e$, $\mu$) from semileptonic $B$ decays in $\mathrm{\Upsilon }(4S)\to B\overline{B}$ events, using the complete data set recorded by the BABAR experiment near the $\mathrm{\Upsilon }(4S)$ resonance, corresponding to $471\times 10^6$ $B\overline{B}$ pairs. The asymmetry $A_{CP}$ allows comparison between the mixing probabilities $\mathcal{P}({\overline{B}}^0\to B^0)$ and $\mathcal{P}(B^0\to {\overline{B}}^0)$, and therefore probes $CP$ and $T$ violation. The result, $A_{CP}=[-3.9\pm 3.5(\text{stat})\pm 1.9(\text{syst})]\times 10^{-3}$, is consistent with the standard model expectation.

Figure 1

Signal probability distributions from the dilepton multivariate algorithm for (a) the same-sign sample and (b) the opposite-sign sample; all lepton flavors are combined. Points are continuum-subtracted data; shaded regions from bottom to top are for signal, $B\overline{B}$ background with $\ge 1$ misidentified lepton, and $B\overline{B}$ background with both real leptons. Hatched region is rejected. Data/MC simulation ratios are shown in inset plots. Regions below 0.45 are not shown.

Figure 2

$A_{CP}$ of the six data-taking periods (dots), the first four and the last two periods (squares), and the four flavor subsamples (rhombuses). The horizontal band is the $\pm 1\sigma$ region of the final fit result. All error bars are statistical only.

Figure 3

Measurements of $CP$ asymmetry in neutral $B$ mixing, including this measurement (red square), recent LHCb result [8] (teal rhombus), Refs. [4, 5, 6, 7] ($B^0$, green triangles), Refs. [21, 22] ($B_s^0$, blue dots), and Ref. [9] ($B^0$, $B_s^0$ mixture, magenta contour). The vertical band is the average of Refs. [4, 5, 6, 7] and several other older measurements (not shown). The horizontal band is the average of Refs. [21, 22]. The 2014 average “HFAG Spring ’14” [3] (excluding LHCb [8] and this result) is also shown (orange contour).